Global Positioning System (GPS) Standard Positioning Service (SPS) Performance Standard

Edition

April 2020

Integrity - Service - Excellence

GLOBAL POSITIONING SYSTEM

STANDARD POSITIONING SERVICE

PERFORMANCE STANDARD

Distribution Statement A: Approved for public release; distribution is unlimited

April 2020 GPS SPS PS

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FOREWORD

This document defines the levels of performance the U.S. Government makes available to users

of the Global Positioning System (GPS) Standard Positioning Service (SPS). The GPS SPS

Performance Standard underscores the U.S. commitment to cooperate with other global

navigation satellite systems and augmentation system providers to ensure compatibility and

interoperability of GPS with emerging systems for peaceful, civilian worldwide use. Please refer

any questions or comments, in writing, to:

Office of the Department of Defense

Chief Information Officer

Attn: Assistant for GPS, Positioning and Navigation

6000 Defense Pentagon

Washington, DC 20301-6000

Frederick D. Moorefield, Jr.

Deputy Chief Information Officer for

Command, Control, and Communications

MOOREFIELD.FRE

DERICK.D.1229141

143

Digitally signed by

MOOREFIELD.FREDERICK.D.12

29141143

Date: 2020.04.15 12:05:55 -04'00'

April 2020 GPS SPS PS

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April 2020 GPS SPS PS

Edition Page i

TABLE OF CONTENTS

List of Figures… ......................................................................................................... iii

List of Tables.. ............................................................................................................ iii

List of Appendices… .................................................................................................. iv

Executive Summary… ................................................................................................ v

SECTION 1.0 The GPS Standard Positioning Service ............................................. 1

1.1 Purpose.. ........................................................................................................................................ 1

1.2 Scope ............................................................................................................................................. 2

1.3 GPS SPS Definition ....................................................................................................................... 2

1.4 Backward Compatibility.................................................................................................................. 3

1.5 Key Terms and Definitions ............................................................................................................. 4

1.6 Global Positioning System Overview ............................................................................................. 4

1.6.1 GPS Space Segment (SS) .................................................................................................... 4

1.6.2 GPS Control Segment (CS) ................................................................................................... 7

SECTION 2.0 SPS SIS Characteristics and Minimum

Usage Assumptions ............................................................................ 9

2.1 SPS SIS Interface Specification Requirements ............................................................................. 9

2.2 Overview of SPS SIS Interface Characteristics ............................................................................. 9

2.2.1 SPS SIS RF Characteristics .................................................................................................. 9

2.2.2 GPS NAV Message Characteristics ...................................................................................... 11

2.2.3 GPS SPS SIS Component Combinations ............................................................................. 14

2.3 Overview of SPS SIS Performance Characteristics ....................................................................... 15

2.3.1 SPS SIS Availability ............................................................................................................... 15

2.3.2 SPS SIS Health ..................................................................................................................... 16

2.3.3 SPS SIS Accuracy ................................................................................................................. 24

2.3.4 SPS SIS Integrity ................................................................................................................... 25

2.3.5 SPS SIS Continuity ................................................................................................................ 29

2.3.6 SPS SIS UTC(USNO) Accuracy ............................................................................................ 29

2.4 Usage Assumptions for SPS Performance Standards .................................................................. 30

2.4.1 SPS User ............................................................................................................................... 30

2.4.2 SPS SIS Configuration .......................................................................................................... 30

2.4.3 Single-/Dual-/Triple-Frequency Operation ............................................................................. 30

2.4.4 SPS SIS Health ..................................................................................................................... 30

2.4.5 Excluded Errors ..................................................................................................................... 32

2.4.6 “Dummy” Satellites ................................................................................................................ 33

SECTION 3.0 SPS SIS Performance Standards ....................................................... 35

3.1 Overview ........................................................................................................................................ 35

3.1.1 SPS SIS Performance Standards Organization .................................................................... 36

3.1.2 CM-Code, CL-Code, I5-Code, and Q5-Code SIS Performance Standards .......................... 36

3.2 24-Slot Constellation Definitions .................................................................................................... 37

3.3 SPS SIS Coverage ........................................................................................................................ 41

3.3.1 Per Satellite Coverage ........................................................................................................... 41

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3.3.2 Baseline/Expandable 24-Slot Constellation Coverage .......................................................... 42

3.4 SPS SIS Accuracy ......................................................................................................................... 43

3.4.1 SPS SIS URE Accuracy Standards ....................................................................................... 44

3.4.2 SPS SIS URRE Accuracy Standards .................................................................................... 45

3.4.3 SPS SIS URAE Accuracy Standards .................................................................................... 45

3.4.4 SPS SIS UTCOE Accuracy Standards .................................................................................. 46

3.5 SPS SIS Integrity ........................................................................................................................... 47

3.5.1 SPS SIS Instantaneous URE Integrity Standards ................................................................. 47

3.5.2 SPS SIS Instantaneous URRE Integrity Standards .............................................................. 48

3.5.3 SPS SIS Instantaneous URAE Integrity Standards ............................................................... 49

3.5.4 SPS SIS Instantaneous UTCOE Integrity Standards ............................................................ 49

3.5.5 P

sat

and P

const

Standards ........................................................................................................ 50

3.6 SPS SIS Continuity ........................................................................................................................ 51

3.6.1 SPS SIS Continuity Standards – Unscheduled Failure Interruptions .................................... 51

3.6.2 SPS SIS Continuity Standards – Unscheduled Maintenance Interruptions .......................... 51

3.6.3 SPS Status and Problem Reporting Standards ..................................................................... 52

3.7 SPS SIS Availability ....................................................................................................................... 53

3.7.1 SPS SIS Per-Slot Availability Standards ............................................................................... 53

3.7.2 SPS SIS Constellation Availability Standards ....................................................................... 54

3.7.3 Operational Satellite Count Standards .................................................................................. 55

3.8 SPS Position/Time Domain Standards .......................................................................................... 56

3.8.1 PDOP Availability Standards ................................................................................................. 57

3.8.2 SPS Position Availability Standards ...................................................................................... 57

3.8.3 SPS Position/Velocity/Time Accuracy Standards .................................................................. 58

SECTION 4.0 References ........................................................................................... 59

4.1 Government Documents ................................................................................................................ 60

4.2 Non-Government Documents ........................................................................................................ 62

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List of Figures

Figure 1.6-1 Block II Series Satellite GPS SIS Generation and Transmission..... .................................... 5

Figure 1.6-2 The GPS Control Segment (CS)… ....................................................................................... 8

Figure 2.2-1 LNAV Message Content and Format Overview ................................................................... 12

List of Tables

Table 1.4-1 SPS SIS Backward Compatibility .......................................................................................... 3

Table 1.6-1 GPS SIS Taxonomy ............................................................................................................... 5

Table 1.6-2 GPS SPS SIS Broadcast Configurations ............................................................................... 6

Table 2.2-1 CNAV Message Types/Content vs LNAV Subframes/Content ............................................. 13

Table 2.2-2 SPS SIS Component Combinations Covered by this Edition of the SPS PS ....................... 14

Table 3.1-1 SPS SIS Performance Standards vs. SPS SIS Signals ........................................................ 37

Table 3.2-1 Baseline 24-Slot Constellation Slot Assignments as of the Defined Epoch .......................... 38

Table 3.2-2 Expandable 24-Slot Constellation Slot Assignments as of the Defined Epoch ..................... 39

Table 3.2-3 Reference Orbit Parameters .................................................................................................. 40

Table 3.3-1 SPS SIS Per-Satellite Coverage Standards .......................................................................... 41

Table 3.3-2 SPS SIS Constellation Coverage Standards ......................................................................... 42

Table 3.4-1 SPS SIS URE Accuracy Standards ....................................................................................... 44

Table 3.4-2 SPS SIS URRE Accuracy Standards .................................................................................... 45

Table 3.4-3 SPS SIS URAE Accuracy Standards .................................................................................... 46

Table 3.4-4 SPS SIS UTCOE Accuracy Standards .................................................................................. 46

Table 3.5-1 SPS SIS Instantaneous URE Integrity Standards ................................................................. 47

Table 3.5-2 SPS SIS Instantaneous URRE Integrity Standards ............................................................... 48

Table 3.5-3 SPS SIS Instantaneous URAE Integrity Standards ............................................................... 49

Table 3.5-4 SPS SIS Instantaneous UTCOE Integrity Standards ............................................................ 49

Table 3.5-5 SPS SIS Instantaneous P

sat

and P

const

Standards ................................................................. 50

Table 3.6-1 SPS SIS Unscheduled Failure Interruption Continuity Standards ......................................... 51

Table 3.6-2 SPS SIS Unscheduled Maintenance Interruption Continuity Standards ............................... 52

Table 3.6-3 SPS Status and Problem Reporting Standards ..................................................................... 52

Table 3.7-1 SPS SIS Per-Slot Availability Standards ............................................................................... 53

Table 3.7-2 SPS SIS Constellation Availability Standards ....................................................................... 54

Table 3.7-3 Operational Satellite Count Standards .................................................................................. 55

Table 3.8-1 PDOP Availability Standards ................................................................................................. 57

Table 3.8-2 SPS Position Availability Standards ...................................................................................... 57

Table 3.8-3 SPS Position/Velocity/Time Accuracy Standards .................................................................. 58

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List of Appendices

Appendix A: SPS Signal-In-Space (SIS) Background Information

Appendix B: SPS Position, Velocity, and Time (PVT) Performance Expectations

Appendix C: Key Terms, Definitions, Abbreviations and Acronyms

Note: A Table of Contents is contained within each respective Appendix.

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Executive Summary

The U.S. Global Positioning System (GPS) Standard Positioning Service (SPS) consists of space-

based positioning, navigation, and timing (PNT) signals delivered free of direct user fees for

peaceful civil, commercial, and scientific uses worldwide. This SPS Performance Standard (SPS

PS) specifies the levels of SPS performance in terms of broadcast signal parameters and GPS

constellation design. The U.S. Government is committed to meeting and exceeding the minimum

levels of service specified in this SPS PS.

Since GPS initial operational capability (IOC) was first declared in 1993, actual GPS performance

has continuously met and exceeded minimum performance levels specified in the SPS PS and

users can generally expect improved performance over the minimum levels described here. For

example, with current (2018) Signal-in-Space (SIS) accuracy, well-designed GPS receivers have

been achieving horizontal accuracy of 3 meters or better and vertical accuracy of 5 meters or

better 95% of the time. A number of U.S. agencies continually monitor actual GPS SPS

performance, including the Federal Aviation Administration (FAA) which publishes quarterly

Performance Analysis Reports at its National Satellite Test Bed (NSTB) web site

(http://www.nstb.tc.faa.gov/). Interested readers are encouraged to refer to this and other sources

for updated GPS performance.

The performance specifications in this edition of the SPS PS apply to the L1 (1575.42 MHz)

Coarse/Acquisition (C/A) signal. Information is also provided regarding characteristics of planned

L2C (1227.60 MHz) Civil Moderate Code (CM) and Civil Long Code (CL) signals, and the L5

(1176.45 MHz) In-Phase Code (I5) and Quadrature Phase (Q5) signals. The L2C and L5 signals

are pre-operational and their use is at the users’ own risk. No commitment of signal availability

for L2C or L5 will be made until the signals are declared fully operational by the DoD and available

for users. The “Baseline 24-Slot” GPS constellation definition remains unchanged from the

previous edition of the SPS PS while the “Expandable 24-Slot” GPS constellation definition has

improved slightly (see Section 3.2).

The SPS PS is updated periodically as GPS modernizes its SPS signal structure. This edition of

the SPS PS revises and supersedes the previous edition, published September 2008, and meets

or surpasses all performance commitments of the previous edition. Significant changes in this

update include the addition of information regarding the pre-operational L2C and L5 signals, and

expanded capabilities which allow the total size of operational constellation to grow beyond the

previous maximum of 32 Navstar satellites. The L1C signal, the newest GPS signal designed to

improve interoperability, will be fielded over time as GPS III satellites populate the GPS

constellation and is not addressed in this edition of the SPS PS.

It is important to distinguish the operational status of the signals defined in this document. Full

Operational Capability (FOC) has been declared for the L1 C/A signal, and it will continue to be

provided in accordance with the Federal Radionavigation Plan (FRP) for the foreseeable future.

Any use of the pre-operational signals L2C and L5 is at the users’ own risk until those signals are

declared fully operational by the DoD and incorporated as operational signals in future editions of

the SPS PS, in accordance with the FRP.

In addition to specifying GPS minimum performance parameters, the SPS PS serves as a

complement to the GPS SPS SIS Interface Specifications (IS-GPS-200 and IS-GPS-705), which

provide relevant information for design and fabrication of GPS civil receiver equipment. Readers

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interested in GPS tutorial information are encouraged to refer to the wide range of reference

material available on the subject.

Finally, in line with the U.S. Space-Based PNT Policy (http://www.gps.gov/policy/), the SPS PS

underscores the U.S. commitment to cooperate with other GNSS and augmentation system

providers to ensure compatibility and interoperability of GPS with emerging systems for peaceful,

civilian worldwide use.

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SECTION 1.0 The GPS Standard Positioning Service

The Navstar Global Positioning System, hereafter referred to as GPS, is a space-based

radionavigation system owned by the United States Government (USG) and operated by the

United States Space Force (USSF). GPS has provided positioning, navigation, and timing

services to military and civilian users on a continuous worldwide basis since initial operational

capability was declared in 1993. An unlimited number of users with a civil or military GPS receiver

can determine accurate time and location, in any weather, day or night, anywhere in the world.

The USSF is responsible for the design, development, procurement, operation, sustainment, and

modernization of the system. The 2

Space Operations Squadron (2 SOPS) maintains the health

and status of the operational constellation at facilities located at Schriever Air Force Base,

Colorado through a network of dedicated ground antennas and monitor stations located worldwide

to ensure GPS performance and reliability meet or exceed the needs of both military and civilian

users. The Space and Control Segments are acquired by the Space and Missile Systems Center

(SMC) at Los Angeles Air Force Base, California.

GPS has grown into a global utility whose multi-use services are integral to U.S. and global

security, economic growth, transportation safety, and are an essential element of the worldwide

economic infrastructure. In an effort to ensure beneficial services are available to the greatest

number of users without degrading security interests, two GPS services are provided. The

Precise Positioning Service (PPS) is available primarily to the military of the United States and its

allies for users properly equipped with PPS receivers. The Standard Positioning Service (SPS),

as initially described in the SPS Signal Specification, was originally designed to provide civil users

with a less accurate positioning capability than PPS, through a feature known as Selective

Availability (SA). The use of SA has been discontinued.

The SPS Performance Standard serves as a companion document to the PPS Performance

Standard for the “dual use” (SPS or PPS) system. This update to the SPS Performance Standard

(5th Edition) is part of the evolution of the performance standards toward the overall goal of

providing users -- civil and military alike -- complete, consistent, and appropriate performance

standards for both the SPS SIS and PPS SIS.

1.1 Purpose

This 5th Edition of the SPS Performance Standard (SPS PS) defines the levels of Signal In Space

(SIS) performance to be provided by the USG to the SPS user community. In addition to providing

general information to the SPS user community, it is established to provide a basis for certification

of SPS receivers for use in aviation Instrument Flight Rules (IFR) and to establish a minimum

performance level which the GPS constellation must sustain. As additional capabilities are

realized on future GPS space, control and user segments, the standards in this SPS PS will be

updated. Its performance metrics and assumptions should therefore not be used as the sole

basis for estimates of utility for future civil applications. Performance standards described in this

document lie between original design parameters and maximum constellation capability. GPS

constellation operations are conducted by 2 SOPS in a manner that balances system

performance and operational tempo so as to assure the most consistent and sustainable GPS

performance to all users. The performance standards presented in this document are supported

by 2 SOPS operational procedures, and are tempered with technical and operational margin.

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This SPS PS consists of a main body and three appendices. The SPS PS first provides an

overview of the GPS program plus an overview of the SPS SIS and how it is used. It then provides

the performance standards for the SPS SIS. It concludes with the relevant reference documents.

The appendices provide additional information that quantifies and illustrates SPS SIS

performance. Provided below is a definition of each appendix's purpose.

 Appendix A: SPS Signal-In-Space (SIS) Background Information. This appendix

provides further background information on the SPS SIS and its performance standards.

 Appendix B: SPS Position, Velocity, and Time (PVT) Performance Expectations.

This appendix describes examples of how to translate the SPS SIS performance

standards into end user position, velocity, and time (PVT) statistical performance

expectations. These are only examples because user equipment (UE) performance

characteristics vary significantly based upon user applications. UE performance

specifications are beyond the scope of this SPS PS.

 Appendix C: Definitions. This appendix provides a list of key terms, definitions,

abbreviations and acronyms used in this SPS PS.

1.2 Scope

This SPS PS defines standards for the GPS SPS SIS performance. Section 3 specifies the

performance standards for the SPS SIS from a global perspective, in terms of performance

metrics the USG uses to specify system performance. Appendix B describes the PVT

performance an end user can expect to achieve using those same performance metrics. GPS

users need to be aware that GPS is not optimized to support any specific user group, except

potentially in time of emergency or national need. The USG reserves the right to optimize

performance to support high priority mission needs over an area of operations (AOO). See the

Concept of Operations for the Global Positioning System (“GPS CONOPS”) for additional details.

Any such optimization will not degrade GPS SPS SIS performance beyond the standards defined

in this SPS PS.

This SPS PS employs standard definitions and relationships between the performance

parameters such as availability, continuity, integrity, and accuracy. The standard definitions in

this SPS PS represent the performance attributes of a space-based positioning and time transfer

system. Refer to Appendix B for a more comprehensive discussion of the relationships between

SPS SIS performance and end user PVT expectations.

This SPS PS only applies to the SPS SIS as it exists on the publication date of this document.

This document does not address P(Y)-code, M-code, or L1C which is being or will be broadcast

by the latest satellites.

1.3 GPS SPS Definition

The GPS Standard Positioning Service (SPS) is defined as follows:

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1.4 Backward Compatibility

The SPS is, and will continue to be, backward compatible. Even though the SPS SIS evolves

over time, existing SPS receiving equipment continues to obtain performance that is the same, or

is better, than received in the past.

The effectivity milestone for backward compatibility is the declaration of initial operational

capability (IOC). IOC for the L1 C/A-code portion of the SPS SIS was declared on 8 December

1993. The first edition of this SPS PS was issued that same day to define the initial operational

capabilities of the C/A-code portion of the SPS SIS. SPS receiving equipment which was

compliant with the relevant SIS Interface Control Document / Interface Specification (ICD/IS) in

effect on that date was provided with C/A-code signals which met the performance commitments

in that first edition of the SPS PS. If any SPS receiving equipment from 1993 is still in operation

today, that equipment is still receiving ICD-compliant C/A-code signals which meet or exceed the

performance commitments in that first edition of the SPS PS.

The historical nature of SPS SIS backward compatibility is shown in Table 1.4-1. The milestones

corresponding to each edition of this SPS PS are included for reference purposes.

Table 1.4-1. SPS SIS Backward Compatibility

Date

SIS Definition

SIS Performance Specification

Milestone

1993

ICD-GPS-200C

Edition of SPS Signal Specification

C/A-Code IOC

1995

Edition of SPS Signal Specification

C/A-Code FOC

2001

Edition of SPS Performance Standard

SA Discontinued

2004

IS-GPS-200D

2008

Edition of SPS Performance Standard

GPS Modernization

2010

IS-GPS-200E

2011

IS-GPS-200F

2012

IS-GPS-200G

2013

IS-GPS-200H

2018

Edition of SPS Performance Standard

CM, CL, I5, Q5 Introduced

The SPS is a positioning and timing service that is available for peaceful civil, commercial, and

scientific use. It includes the C/A-code signal, the CM/CL-code signals, and the I5-code/Q5-

code signals. The C/A-code signal is transmitted by all satellites and comprises an L1 carrier

modulated by a coarse/acquisition (C/A) code ranging signal with a legacy navigation (LNAV)

data message. The CM-code and CL-code signals are transmitted by some satellites and

comprise an L2 carrier modulated by both a civil moderate length (CM) code ranging signal with

a civil navigation (CNAV) data message and a civil long length (CL) code ranging signal without

a data message. The I5-code and Q5-code signals are transmitted by some satellites and

comprise an L5 carrier modulated by both a civil in-phase (I5) code ranging signal with a CNAV

data message and a civil quadrature-phase (Q5) code ranging signal without a data message.

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Note that this edition of the SPS PS is not associated with either an IOC or full operational

capability (FOC) declaration for the CM-code, CL-code, I5-code, or Q5-code signals. Future

editions are, however, expected in response to these milestones.

1.5 Key Terms and Definitions

Terms and definitions which are key to understanding the scope of the GPS SPS SIS are provided

in Appendix C. A list of abbreviations and acronyms is also provided in Appendix C.

1.6 Global Positioning System Overview

Sufficient information is provided below to promote a common understanding of the GPS baseline

for the purposes of this document. The GPS baseline herein is comprised of the segments owned

by the USG: the Control and Space Segments. The Control and Space Segments provide two

types of service, the SPS SIS and the PPS SIS. This document covers the SPS SIS. For further

information on the PPS SIS, refer to the PPS PS.

The two GPS system segments are described below. The SPS SIS interface is described later

in Section 2.

1.6.1 GPS Space Segment (SS)

The GPS constellation nominally consists of 24, properly geometrically spaced slots, where each

slot contains at least one operational satellite (see Section 3.2).

The GPS SIS generation and transmission process for an objective Block II series satellite is

illustrated in Figure 1.6-1. The atomic frequency standard generates the stable time base for the

satellite. The synthesized 10.23 MHz clock signal and synchronized code/carrier timing signals

are distributed by the frequency synthesizer to other payload subsystems. The mission computer

receives the uploaded navigation (NAV) data from the Control Segment (CS) through the

Telemetry, Tracking, and Command (TT&C) subsystem. The Navigation Baseband generates

the pseudorandom noise (PRN) ranging codes and adds the NAV message data to the PRN

ranging codes. The L-Band subsystem modulates the resulting binary sequences onto the L1

(1575.42 MHz), L2 (1227.6 MHz), and L5 (1176.45 MHz) carriers which are then broadcast by

the helix array antenna.

Each satellite broadcasts a minimum of two PRN ranging codes: (1) the precision (P) code, which

is the principal ranging code or the Y-code which is used in place of the P-code whenever the

anti-spoofing mode of operation is activated; and (2) the coarse/acquisition (C/A) code which is

used for acquisition of the P (or Y) code (denoted as P(Y)) and as the primary civil ranging signal.

The legacy NAV (LNAV) message based upon data periodically uploaded from the Control

Segment is provided by adding the LNAV data to both the 10.23 MHz P(Y)-code sequence and

the 1.023 MHz C/A-code sequence. The satellite modulates the two resulting code-plus-data

sequences onto the L1 carrier, and modulates just the 10.23 MHz code-plus-data sequence onto

the L2 carrier; and then both modulated carriers are broadcast to the user community. Depending

on the particular satellite configuration, the following additional PRN ranging codes and NAV

message data may also be broadcast by a satellite: (a) the military (M) codes and military NAV

(MNAV) messages on the L1 and L2 carriers, (b) the second civil (L2C) codes and civil NAV

April 2020 GPS SPS PS

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(CNAV) messages on the L2 carrier, and (c) the third civil (L5) codes and CNAV message on the

L5 carrier.

Figure 1.6-1. Block II Series Satellite GPS SIS Generation and Transmission

The set of P(Y), M, and C/A code-plus-NAV-data sequences, and carriers is referred to as the PPS SIS.

The set of C/A-code-plus-NAV-data sequences, CM-code-plus-NAV-data sequences, CL-code sequences,

I5-code-plus-NAV-data sequences, Q5-code sequence, and carriers is referred to as the SPS SIS.

Collectively, the PPS SIS and the SPS SIS are known as the satellite's navigation signals (or navigation

SIS or GPS SIS). This GPS SIS taxonomy is illustrated in Table 1.6-1.

Table 1.6-1. GPS SIS Taxonomy

GPS SIS

Carrier Frequency

PPS SIS

SPS SIS

P(Y)-code + LNAV Data*

M-code + MNAV Data

C/A-code + LNAV Data*

P(Y)-code + LNAV Data*

M-code + MNAV Data

CM-code + CNAV Data

CL-code

I5-code + CNAV Data

Q5-code

* SIS components marked by an asterisk are broadcast by all Navstar satellites

Helix Array

Antenna

 10.23 MHz synthesized

digital clock

 Generation of synchronized

code and carrier timing

 NAV & control

data checks

 NAV data

generation

 Synchronized ranging

codes generation

 Modulo-2 addition of

codes and NAV data

 Spread spectrum modulation

of 1575.42 MHz, 1227.6 MHz,

and 1176.45 MHz carriers

GPS SIS

Right-Hand Circularly Polarized

1575.42 MHz, 1227.6 MHz, 1176.45 MHz

FREQUENCY SYNTHESIZER

MISSION

COMPUTER

NAVIGATION

BASEBAND

GENERATOR

L-BAND

SUBSYSTEM

NAV DATA

UPLOAD

FROM CS

TT&C

SUBSYSTEM

ATOMIC

FREQUENCY

STANDARD

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The SIS broadcast configuration of the satellites occupying each GPS constellation slot has

evolved in the past and will continue to evolve in the future. This evolution is designed to occur

in a backward compatible manner so as to sustain the existing SIS components while concurrently

deploying new SIS components. Table 1.6-2 defines four SPS SIS broadcast configurations

across three carriers addressed in the current edition of this SPS PS along with representative

Block II series satellite examples. The definitions of the objective, intermediate, and baseline

SPS SIS broadcast configurations are expected to further evolve in future editions of this SPS

PS. The definition of the minimum SPS SIS broadcast configuration is expected to remain

unchanged in the future.

Table 1.6-2. GPS SPS SIS Broadcast Configurations

Configuration Name

L1 Carrier

L2 Carrier***

L5 Carrier***

Objective (e.g., IIF)

C/A-code + LNAV Data

CM-code + CNAV Data

CL-code

I5-code + CNAV Data

Q5-code

Intermediate*

(e.g., IIR-M,

down-configured IIF)

C/A-code + LNAV Data

CM-code**

CL-code

I5-code**

Q5-code

C/A-code + LNAV Data

I5-code + CNAV Data

Q5-code

C/A-code + LNAV Data

I5-code**

Q5-code

C/A-code + LNAV Data

CM-code + CNAV Data

CL-code

C/A-code + LNAV Data

CM-code** + CL-code

Minimum (e.g., IIR/IIA)

C/A-code + LNAV Data

* Only selected examples of the intermediate SPS SIS broadcast configurations shown for illustration

** Includes the intentional broadcast of default CNAV data

*** Refer to the configuration code broadcast in the NAV message for each particular satellite

Regardless of their particular SIS broadcast configuration, all satellites are designed to provide

reliable service over their design lives through a combination of space qualified parts, multiple

redundancies for critical subsystems, and internal diagnostic logic. The satellites require minimal

interaction with the ground and allow all but a few maintenance activities to be conducted without

interruption to the broadcast SIS. Periodic uploads of NAV message data are designed to cause

no interruption to the SIS.

An important line of defense against loss-of-availability SIS anomalies comes from the ability of

certain satellites to operate in degraded broadcast configurations which still satisfy the SPS SIS

minimum broadcast configuration defined in Table 1.6-2. A satellite may be able to survive certain

subsystem failures and continue to operate by having its SIS broadcast configuration intentionally

downgraded to a less demanding configuration. As long as the minimum SPS SIS broadcast

configuration is still satisfied by the satellite, there is no loss-of-availability SIS anomaly. The

effects of a downgrade which does not satisfy the minimum SPS SIS broadcast configuration are

covered in Section 3 of this SPS PS.

The first line of defense against loss-of-integrity SIS anomalies are the satellites themselves. The

satellites automatically remove themselves from service whenever they experience any of a

number of different kinds of on-board failures that could result in loss-of-integrity SIS anomalies.

This removal from service is accomplished by the satellite switching from broadcasting its normal

navigation signals to instead broadcasting signals with non-standard PRN code sequences and/or

default NAV message data.

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1.6.2 GPS Control Segment (CS)

The Control Segment (CS) is comprised of four major subsystems: a Master Control Station

(MCS), an Alternate Master Control Station (AMCS), a network of four ground antennas (GAs),

and a network of globally-distributed monitor stations (MSs). An overview of the CS is provided

in Figure 1.6-2.

The MCS is located at Schriever Air Force Base, Colorado, and is the central control node for the

GPS satellite constellation. Operations are maintained 24 hours a day, seven days a week

throughout the year by highly skilled 2 SOPS personnel. The MCS is responsible for all aspects

of constellation command and control, to include:

 Routine satellite bus and payload status monitoring

 Satellite maintenance and subsystem anomaly resolution

 Satellite commissioning, decommissioning, and disposal support

 Management of GPS SIS performance to comply with all performance standards (SPS PS

and PPS PS)

 NAV message upload operations as required to sustain performance in accordance with

accuracy and integrity performance standards

 Detecting and responding to GPS SIS anomalies

 Communicating with military GPS users

In the event of a prolonged MCS outage, GPS operations can be moved to the AMCS.

The DoD does not currently monitor and assess SPS SIS performance in real time. The DoD

does monitor the P(Y)-code signals and LNAV data on both the L1 and L2 carriers for all satellites

in view of CS monitor stations in near-real time, to ensure they are meeting their PPS performance

standards. Due to the way that satellites generate their broadcast SIS, monitoring the PPS SIS

has been an adequate surrogate for monitoring the SPS SIS under the minimum SPS SIS

broadcast configuration (i.e., C/A-code + LNAV data).

The CS's four GAs provide a near real-time TT&C interface between the satellites and the MCS.

The MSs provide near real-time satellite pseudorange measurement data and received NAV

messages to the MCS and support continuous monitoring of constellation performance. The

current CS monitor stations provide 100% global coverage with the inclusion of National

Geospatial-Intelligence Agency (NGA) MSs.

The CS provides the second line of defense against GPS SIS anomalies. (The first line of defense

are the satellites themselves.) When a GPS SIS anomaly occurs that is not covered by the

satellite’s automatic removal capability, the CS will respond to the failure by manually removing

the satellite from service in a prompt manner, subject to MS visibility, GA visibility, and CS

equipment and communications reliability constraints. For details on both automatic and manual

removal from service, see the SPS SIS integrity alarms listed in paragraph 2.3.4 and the related

SPS SIS integrity performance standards given in Section 3.5.

When a MS is tracking a satellite's GPS SIS and the MCS is receiving the L-band measurements

in near-real time, the MCS monitors the following GPS SIS metrics (among others) from that

satellite:

a. pseudorange error, and

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b. pseudorange rate error (i.e., the first time derivative of the pseudorange error, also known

as the pseudorange "velocity" error).

Figure 1.6-2. The GPS Control Segment (CS)

The MCS does not directly monitor the pseudorange acceleration error (i.e., the second time

derivative of the pseudorange error, also known as the pseudorange rate rate error).

The pseudorange error and the pseudorange rate error for each GPS SIS are used internally by

the MCS to determine how to manage each satellite to ensure its GPS SIS meets the performance

standards (particularly the integrity standards). There are three primary options: (1) if the

satellite's pseudorange error is small enough and growing slowly enough, then no action needs

to be taken until the next regularly scheduled upload of NAV message data to that satellite; (2) if

the satellite's pseudorange error is large enough or is growing quickly enough, then an

unscheduled contingency upload may be performed to refresh the satellite's NAV message data

and restore the accuracy/integrity of the GPS SIS; or (3) in extreme cases, if the satellite's

pseudorange error is very large or is growing so rapidly that the satellite is at risk of exceeding its

integrity tolerance, then the MCS may need to manually remove the satellite from service.

 Commands

 Telemetry

 NAV Message Uploads

 Ground Antenna Control

GROUND ANTENNAS

TT&C

Link

GPS

SIS

MONITOR STATIONS

MASTER CONTROL STATION

 Received NAV Messages

 Ranging Measurement Data

 Monitor Station Control

 Monitor Station Status

Positioning & Timing Function

 Ranging Measurement Processing

 Satellite State Estimation/Prediction

 NAV Message Generation

Satellite Control Function

 Command Generation

 Telemetry Processing

 Upload Generation & Control

Performance Assess Function

 Check performance against

specifications

 Alarm if failure detected

Network Control Function

 Data Traffic Management

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SECTION 2.0 SPS SIS Characteristics and

Minimum Usage Assumptions

This section provides an overview of the SPS SIS interface characteristics, SPS SIS performance

characteristics, and the assumptions made to arrive at the performance standards in Section 3.0.

The representative receiver characteristics are used to provide a framework for defining the SPS

performance standards. They are not intended to impose any minimum requirements on receiver

manufacturers or integrators, although they are necessary attributes to achieve the SPS

performance described in this document. Receiver characteristics used in this standard are

required in order to establish a frame of reference in which the SPS SIS performance can be

described.

2.1 SPS SIS Interface Specification (IS) Requirements

The SPS SIS shall comply with the technical requirements related to the interface between the

Space Segment and the SPS receivers as established by the current revision of IS-GPS-200 and

the current revision of IS-GPS-705. In the event of conflict between the SPS SIS interface

characteristics described in this document and the ISs, defer to the ISs.

2.2 Overview of SPS SIS Interface Characteristics

This section provides an overview of the SPS SIS interface characteristics. SPS SIS interface

characteristics are allocated to three categories: (1) carrier and modulation radio frequency (RF)

characteristics, (2) the structure, protocols, and contents of the NAV messages, and (3) the

combining of SPS SIS components.

2.2.1 SPS SIS RF Characteristics

For a particular Space Vehicle (SV), all transmitted signal elements (carriers, codes and data) are

coherently derived from the same on-board frequency source.

2.2.1.1 L1 Signals

All satellites transmit right-hand circularly polarized (RHCP) signals at 1575.42 MHz as specified

in IS-GPS-200. The L1 signals are transmitted with enough power to ensure the minimum

received signal power level of -158.5 dBW for the L1 C/A-code under the conditions defined in IS-

GPS-200.

The L1 carrier consists of two carrier components which are in phase quadrature with each other.

Each carrier component is phase shift key (PSK) modulated by separate bit trains. The in-phase

bit train includes the Modulo-2 sum of the P(Y)-code and the LNAV data clocked at 50 bits per

second (bps), while the quadrature-phase bit train includes the Modulo-2 sum of the C/A-code

and the LNAV data also clocked at 50 bps. Each satellite’s unique C/A-code is 1,023 chips long

and is clocked at a rate of 1.023 megachips per second (Mcps). At L1, the SPS SIS specifically

includes – and is specifically limited to – each satellite’s broadcast C/A-code and the associated

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LNAV data. See IS-GPS-200 for the detailed definition of the C/A-codes and the LNAV data. For

convenience, a summary description of the LNAV data is provided in Section 2.2.2.1.

2.2.1.2 L2 Signals

All satellites transmit RHCP signals at 1227.60 MHz as specified in IS-GPS-200. When the L2C

signal is present, the L2 signals are transmitted with enough power to ensure the minimum

received signal power level of at least -160.0 dBW for the composite L2C-code (i.e., the chip-by-

chip multiplex combination of the CM-code plus the CL-code) under the conditions defined in IS-

GPS-200.

When the L2C signal is present, the L2 carrier consists of two carrier components which are in

phase quadrature with each other. Each carrier component is PSK modulated by separate bit

trains. The in-phase bit train includes the Modulo-2 sum of the P(Y)-code and the LNAV data

clocked at 50 bps, while the quadrature-phase bit train normally includes the chip-by-chip

multiplex combination of the Modulo-2 sum of the CM-code plus CNAV data clocked at 25 bps

and the CL-code. Each satellite’s unique composite L2C-code is 1,534,500 chips long and is

clocked at a rate of 1.023 Mcps where the underlying CM-code and CL-codes are each clocked

at a rate of 0.5115 Mcps. At L2, the SPS SIS specifically includes – and is specifically limited to

– each satellite’s broadcast L2C-code and the associated CNAV data. See IS-GPS-200 for the

detailed definition of the L2C-codes and the CNAV data. For convenience, a summary description

of the CNAV data is provided in Section 2.2.2.2.

Notes:

1. Because the CNAV data is encoded in a rate ½ convolutional encoder before broadcast, a 25 bps

source rate for CNAV data on CM-code results in the 50 symbol per second (sps) broadcast rate

for the CM-code plus CNAV data train.

2. Some GPS satellites may occasionally employ an alternate configuration with the L2C-code signal

broadcast on the in-phase carrier component. See IS-GPS-200 for details.

2.2.1.3 L5 Signals

Some satellites transmit RHCP signals at 1176.45 MHz as specified in IS-GPS-705. When the

L5 signals are present, the L5 signals are transmitted with enough power to ensure the minimum

received signal power levels of at least -157.9 dBW for I5-code and -157.9 dBW for Q5-code

under the conditions defined in IS-GPS-705.

When the L5 signal is present, the L5 carrier consists of two carrier components which are in

phase quadrature with each other. Each carrier component is PSK modulated by separate bit

trains. The in-phase bit train includes the Modulo-2 sum of the I5-code and the CNAV data

clocked at 50 bps plus a 10-bit Neuman-Hofman (NH) overlay code clocked at 1 Kbps, while the

quadrature-phase bit train includes the Q5-code plus a 20-bit NH overlay code clocked at 1 Kbps.

Each satellite’s unique I5-code is 10,230 chips long and is clocked at a rate of 10.23 Mcps.

Likewise, each satellite’s unique Q5-code is also 10,230 chips long and is also clocked at a rate

of 10.23 Mcps. At L5, the SPS SIS specifically includes – and is specifically limited to – each

satellite’s broadcast I5-code, 10-bit NH-code, and the associated CNAV data; plus each satellite’s

broadcast Q5-code and 20-bit NH-code. See IS-GPS-705 for the detailed definition of the I5-

codes, Q5-codes, NH-codes, and CNAV data. For convenience, a summary description of the

CNAV data is provided in Section 2.2.2.2. Since the same CNAV data is carried by the I5-code

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as by the CM-code, the summary description of the CNAV data in Section 2.2.2.2 applies equally

to the CM-code signals and the I5-code signals.

Notes:

1. Because the CNAV data is encoded in a rate ½ convolutional encoder before broadcast, a 50 bps

source rate for CNAV data on I5 results in the 100 sps broadcast rate for the I5 CNAV data train.

This is twice as fast as the 50 sps broadcast rate for the CNAV data train on the CM-code signal.

2. The (I5-code + 10-bit NH-code + CNAV data) and (Q5-code + 20-bit NH-code) are separate signals

that share the same L5 carrier in quadrature, in the same way that P(Y)-code and C/A-code are

separate signals that share the same L1 carrier. See IS-GPS-705 for details.

2.2.2 GPS NAV Message Characteristics

2.2.2.1 Legacy NAV (LNAV) Message

Each satellite broadcasts LNAV message data to support the GPS receiver's PVT determination

process. Figure 2.2-1 provides an overview of the data contents and “fixed frame” structure within

the LNAV data stream. It shows each master frame consists of 25 data frames and each data

frame consists of 5 subframes. Subframe 4 and 5 data is different (paged) for each data frame

within a master frame. Each subframe begins with a telemetry word (TLM) and handover word

(HOW) pair. The rest of the data in each subframe includes information required to determine

the following:

 Satellite time-of-transmission

 Satellite position

 Satellite (SIS) health

 Satellite clock correction

 Single-frequency (SF) ionospheric delay model corrections

 Time transfer to Coordinated Universal Time (UTC) as kept by the U.S. Naval Observatory

(USNO)

 Constellation status

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The same LNAV message data is broadcast via the SPS SIS and the PPS SIS to all GPS

receivers. See IS-GPS-200 for further information on LNAV message development.

Figure 2.2-1. LNAV Message Content and Format Overview

2.2.2.2 Civil NAV (CNAV) Message

Some satellites also broadcast CNAV message data to support the GPS receiver's PVT

determination process. Table 2.2-1 provides an overview of the data contents and “flexible

message” structure within the CNAV data stream and compares them to the LNAV data contents

and “fixed frame” structure. Unlike LNAV’s rigid structure where every subframe and every page

of a subframe is broadcast by every satellite at exactly the same time each week, CNAV’s fluid

“flexible message” structure allows different messages to be broadcast by different satellites at

different times each week. It further allows the possibility for new message types to be added

into the sequence (or old message types omitted from the sequence) of CNAV messages

broadcast by each satellite.

Significant Subframe Contents

SUBFRAME 1

SUBFRAME 2

Frame

SUBFRAME 3 (30 sec

repeat)

SUBFRAME 4

SUBFRAME 5

TLM

HOW

GPS Week Number, User Range Accuracy (URA), Real-Time

Health, and Satellite Clock Correction Terms

TLM

HOW

Ephemeris Parameters

TLM

HOW

Ephemeris Parameters

TLM

HOW

Almanac and Long-Term Health for Satellites 1-24, and

Almanac Reference Time and Week Number

TLM

HOW

Almanac and Long-Term Health for Satellites 25-32, Special

Messages, Configuration Flags, Ionospheric and UTC Data

Pages

1-25

Pages

1-25

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Table 2.2-1. CNAV Message Types/Content vs LNAV Subframes/Content

CNAV MESSAGES

LNAV SUBFRAMES

Message

Type

Contents

Subframe

(Page)

Contents

Every

Satellite Time-of-Transmission

All

Satellite Time-of-Transmission

Satellite Position (half of ephemeris)

Satellite Position (rest of ephemeris)

Satellite (SIS) Health

10, 3x

Satellite User Range Accuracy (URA)

Satellite Clock Correction

Satellite Delays & Ionospheric Delay

4(18)

Constellation Ionospheric Delay

Constellation UTC(USNO) Offset

4(18)

Constellation UTC(USNO) Offset

4 & 5

Constellation Status (full almanacs)

Constellation Status (midi almanacs)

–

31 or 12

Constellation Status (mini almanacs)

–

Constellation Earth Orientation

–

34 or

13/14

Constellation Differential Corrections

4(13)

Constellation Differential Corrections

Constellation GPS-GNSS Time Offset

–

36 or 15

Constellation Text Messages

4(17)

Constellation Text Messages

Default CNAV Data (alternating 1s/0s)

Any

Default LNAV Data (alternating 1s/0s)

Notes:

1. CNAV messages with content designated as “satellite” are transmitted by each particular satellite

for its own L2C or L5 SIS. CNAV messages with content designated as “constellation” are common

across the constellation and may not be transmitted by a particular satellite on its L2C or L5 SIS.

2. The designator “Message Type 3x” or “MT-3x” means all of the CNAV messages in the Message

Type 30 series (i.e., MT-30, MT-31, MT-32, MT-33, etcetera).

3. See the individual GPS SPS SIS Interface Specifications (IS-GPS-200 and IS-GPS-705) for further

details, particularly the message broadcast intervals tables.

2.2.2.3 Clock, Ephemeris, Integrity (CEI) Data Sets

A “Clock, Ephemeris, Integrity” (CEI) data set is the collection of satellite-specific clock correction

parameters, ephemeris parameters, and related parameters (e.g., health flags, URA parameters,

time tags) needed to use the satellite’s broadcast signal(s) for accurate positioning or timing. The

performance standards in this document only apply to current and consistent CEI data sets within

the curve fit interval for that CEI data set.

For LNAV messages, the CEI data set comprises most of Subframes 1, 2, and 3. The Subframe

1, 2, and 3 messages which make up a consistent CEI data set share the same broadcast Index

of Data Ephemeris (IODE) value which further matches the 8 least significant bits (LSBs) of the

broadcast Index of Data Clock (IODC) value. See IS-GPS-200 for an explicit listing of the contents

of an LNAV CEI data set.

For CNAV messages, the CEI data set comprises MT-10, MT-11, and the front portion of MT-3x.

The MT-10, -11, and -3x messages which make up a consistent CEI data set all share the same

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broadcast data propagation time of week (t

) value. See IS-GPS-200 and IS-GPS-705 for an

explicit listing of the contents of a CNAV CEI data set.

2.2.3 SPS SIS Component Combinations

The SPS SIS component combinations covered by this edition of the SPS PS are identified in

Table 2.2-2. These component combinations are grouped by the number of carrier frequencies

involved and the corresponding user operating mode supported: one carrier and ‘single frequency’

(SF), two carriers and ‘dual frequency’ (DF), plus three carriers and ‘triple frequency’ (TF). SPS

SIS component combinations not explicitly identified in Table 2.2-2 are not covered by this edition

of the SPS PS and have no SPS performance standards.

Table 2.2-2. SPS SIS Component Combinations Covered by this Edition of the SPS PS

One Carrier,

Single Frequency (SF)

Two Carriers,

Dual Frequency (DF)

Three Carriers,

Triple Frequency (TF)

C/A-code + LNAV Data

CM-code + CNAV Data

CL-code + CNAV Data

(CM+CL)-codes + CNAV Data

I5-code + CNAV Data

Q5-code + CNAV Data

(I5+Q5)-codes + CNAV Data

(C/A + CM)-codes + CNAV Data

(C/A + CL)-codes + CNAV Data

(C/A+CM+CL)-codes + CNAV Data

(C/A + I5)-codes + CNAV Data

(C/A + Q5)-codes + CNAV Data

(C/A + I5+Q5)-codes + CNAV Data

(C/A + CM + I5)-codes +CNAV Data

(C/A + CL + I5)-codes +CNAV Data

(C/A + CM+CL + I5)-codes +CNAV Data

(C/A + CM + Q5)-codes +CNAV Data

(C/A + CL + Q5)-codes +CNAV Data

(C/A+CM+CL+Q5)-codes +CNAV Data

(C/A+CM+CL+I5+Q5)-codes +CNAV Data

Notes:

1. Proper use of CM-code, CL-code, I5-code, and Q5-code requires the application of the current

inter-signal correction (ISC) values which are provided in the CNAV data stream. Use of CL-code

in SF mode by itself or in DF mode with C/A-code imposes additional effort to obtain the required

ISC values. Use of Q5-code in SF mode by itself or in DF mode with C/A-code likewise also

imposes additional effort to obtain the required ISC values.

2. The CM-code and the CL-code, as well as the I5-code and the Q5-code can be used separately

(e.g., independently or sequentially) or jointly (e.g., concurrently).

3. DF operation with either of the L2 signals and either of the L5 signals is not covered – and is not

recommended – by this edition of the SPS PS. The L2 and L5 frequencies are only separated by

51 MHz which is not considered adequate for reliable DF operation. The CNAV integrity-related

information does not account for the excessive inaccuracies of L2/L5 DF operation.

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2.3 Overview of SPS SIS Performance Characteristics

The SPS SIS performance characteristics are described below. The SPS PS performance

characteristics are availability, health, accuracy, integrity, continuity, and UTC(USNO) accuracy.

This overview of the SPS SIS performance characteristics follows a logical progression relative

to the output of the SPS SIS from a satellite. Most fundamentally, a satellite’s SPS SIS is

considered either trackable or untrackable. A trackable SIS is a SIS which can be preprocessed

by an SPS receiver sufficiently to be categorized as either healthy, unhealthy, or marginal. Note

that only a trackable and healthy SPS SIS has performance standards for accuracy, integrity, and

continuity. The last characteristic in this section relating GPS time to UTC(USNO) applies at the

system level and is independent of the output of the SPS SIS from any particular satellite.

2.3.1 SPS SIS Availability

The SPS SIS availability is the probability that the slots in the GPS constellation will be occupied

by satellites transmitting a trackable and healthy SPS SIS. For this SPS PS, there are two

components of availability as follows:

Per-Slot Availability. The fraction of time that a slot in the GPS constellation will be

occupied by a satellite that is transmitting a trackable and healthy SPS SIS.

Constellation Availability. The fraction of time that a specified number of slots in the

GPS constellation are occupied by satellites that are transmitting a trackable and healthy

SPS SIS.

Both components of availability apply to each SPS SIS signal individually. Thus, there are

separate per-slot availability characteristics and separate constellation availability characteristics

for:

C/A-Code Signal Availability. Applies to C/A-code + LNAV data.

CM-Code Signal Availability. Applies to CM-code + CNAV data.

CL-Code Signal Availability. Applies to CL-code + CNAV data.

I5-Code Signal Availability. Applies to I5-code + 10-bit NH-code + CNAV data.

Q5-Code Signal Availability. Applies to Q5-code + 20-bit NH-code.

Notes:

1. The C/A-code signal (C/A-code + LNAV data) from a satellite is either both trackable and healthy

or else the L1 C/A-code signal is unavailable from that satellite. The CM-code signal (CM-code +

CNAV data) from a satellite is either both trackable and healthy or else the CM-code signal is

unavailable from that satellite. The CL-code signal from a satellite is either both trackable and

healthy or else the CL-code signal is unavailable from that satellite. The I5-code signal (I5-code +

10-bit NH-code + CNAV data) from a satellite is either both trackable and healthy or else the L5 I5-

code signal is unavailable from that satellite. The Q5-code signal (Q5-code + 20-bit NH-code) from

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a satellite is either both trackable and healthy or else the Q5-code signal is unavailable from that

satellite.

2. In this edition of the SPS PS, the availability performance standards in Section 3.7 only apply to

the C/A-code signals. There are no availability performance standards yet for the CM-code signals,

the CL-code signals the I5-code signals, or the Q5-code signals,

Not all satellites occupy a slot in the GPS constellation. Satellites that are not occupying a slot in

the GPS constellation are considered "auxiliary" satellites. The SPS SIS from an auxiliary satellite

is available if that satellite is transmitting a trackable and healthy SPS SIS (which is not the case

for SVN-49 for example). The SPS SIS signals from auxiliary satellites always improve the

geometric dilution of precision (DOP) provided by the constellation and therefore contribute to

accuracy. However, the SPS SIS signals from auxiliary satellites do not count towards either the

per-slot availability or the constellation availability.

Notes:

1. The term "spare satellite" has certain connotations which do not apply to an "auxiliary satellite." In

the past, there were 3 spare satellites in the previous 18+3-satellite and 21+3-satellite constellation

baselines. Each of the 3 spare satellites had a pre-defined slot and the operating plan was to

launch a new satellite to fill those slots when they were unoccupied. In contrast, the current

baseline is a 24-slot constellation, not a 24+3-slot constellation. Auxiliary satellites do not have

pre-defined slots, and there are no a priori plans to replace auxiliary satellites when they fail.

2. Auxiliary satellites are required to meet the performance standards for accuracy and integrity in this

standard but not continuity or availability. They may be newly launched satellites not yet assigned

to a constellation slot, or older satellites nearing retirement that previously occupied a constellation

slot. Some auxiliary satellites may broadcast fewer SPS signals than their original design due to

subsystem failures and/or intentional reconfiguration commanded by the Control Segment. If so,

the configuration code in the broadcast NAV message data will reflect the residual, rather than the

designed capabilities of the satellite, such that a GPS receiver tracking the satellite need not search

for signals that are no longer being broadcast. A satellite must have the minimum ability to

broadcast C/A code to remain in service.

2.3.2 SPS SIS Health

The SPS SIS health is the status given by the real-time health-related information broadcast by

each satellite as an integral part of each SPS SIS signal. The SPS SIS health is also sometimes

referred to as "satellite health", “space vehicle health”, “SV health” or “signal health”. For this

standard, there are four possible SPS SIS health conditions: “healthy”, “marginal”, “unhealthy”,

and “not applicable”. The mapping of the real-time health-related information broadcast by the

satellite to these four conditions is given in the paragraphs below for each SPS SIS signal.

Note:

1. The condition of “marginal” collectively encompasses several situations identified in the interface

specifications where the SPS SIS cannot be categorized as either “healthy” or “unhealthy”.

2.3.2.1 C/A-Code Signal

"Healthy". The C/A-code signal is healthy when all of the following four conditions are

present:

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(1) There is no C/A-code signal alarm indication present. C/A-code signal alarm

indications are one component of the SPS SIS integrity. The presence of any one

of the nine C/A-code signal alarm indications listed in paragraph 2.3.4 means the

information provided by the C/A-code signal may not be correct.

(2) The C/A-code signal indicates the SPS SIS is healthy. More specifically, the six-

bit health status word given in subframe 1 of the LNAV message is set to all zeros

(i.e., binary 000000

, meaning “all NAV data are OK, all signals are OK”).

(3) The User Range Accuracy (URA) alert flag for the C/A-code signal is not raised

(i.e., bit 18 of the LNAV HOW is set to 0 (meaning the URA is not worse than the

URA index value transmitted in subframe 1).

(4) The transmitted SPS URA index for the C/A-code signal in subframe 1 is less than

8 (“N” <8). The URA index is an integer that equates to a range of URA values. A

URA index of less than 8 equates to a URA of less than or equal to 48 meters.

The above descriptions only relate to how each condition applies to a healthy C/A-code

signal. Definitions for all the different settings of conditions 2, 3, and 4 are given in IS-

GPS-200.

"Marginal". The C/A-code signal is marginal when the C/A-code signal would otherwise

have been defined as healthy except that one or more of the following three warning

conditions is or are present:

(1) The C/A-code signal indicates that any one of the satellite’s SIS components may

not be fully capable. More specifically, the Most Significant Bit (MSB) of the six-bit

health status word given in subframe 1 of the LNAV message is set to 0

(“all NAV

data are OK”) and the 5 Least Significant Bits (LSBs) of the six-bit health status

word in subframe 1 of the LNAV message are set to anything other than 00000

(all signals are OK), 00010

(all signals dead), or 11100

(“SV is temporarily out”).

(2) The URA alert flag is raised (i.e., bit 18 of the LNAV HOW is set to 1) and the SPS

URA does not apply. This means the URA may be worse than the URA index

value transmitted in subframe 1. See IS-GPS-200 for details.

(3) The transmitted SPS URA index in subframe 1 is greater than or equal to 8 ("N"=8).

A URA index of 8 or greater indicates that the URA is greater than 48 meters or

that there is no URA prediction available. See IS-GPS-200 for details.

"Unhealthy". The C/A-code signal is unhealthy when any one or more of the following

four conditions is or are present:

(1) There is a C/A-code signal alarm indication present. C/A-code signal alarm

indications are one component of the SPS SIS integrity. The presence of any one

of the nine C/A-code signal alarm indications listed in paragraph 2.3.4 means the

information provided by the C/A-code signal may not be correct,

(2) The MSB of the six-bit health status word given in subframe 1 of the LNAV

message is set to 1

(“some or all NAV data are bad”).

(3) The 5 LSBs of the six-bit health status word in subframe 1 of the LNAV message

are set to 00010

(“all signals dead”) or 11100

(“SV is temporarily out”).

(4) The transmitted C/A-code signal is untrackable.

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"Not Applicable". The “not applicable” health condition does not apply to the C/A-code

signal since it is not an allowed SPS SIS broadcast configuration (see Table 1.6-2).

Notes:

1. The C/A-code signal is unhealthy when the MSB of the six-bit health status word in subframe 1 is

set to 1

(“some or all NAV data are bad”) and/or the 5 LSBs of the six-bit health status word in

subframe 1 are set to 11111

(“more than one combination would be required to describe

anomalies”). The Control Segment frequently uses this particular combination to indicate a “dead”

satellite.

2. Subframes 4 and 5 of the LNAV message also contain information related to the SIS health of all

satellites in the constellation. This is not real-time information. It is more of a long-term indicator

and may not correspond to the actual health of the C/A-code signal from the transmitting satellite

or from other satellites in the constellation. The preceding definitions in this paragraph take

precedence over the information in subframes 4 and 5.

3. From the above C/A-code signal characteristics, it follows that: (a) a “healthy” C/A-code signal is

necessarily trackable, (b) a “marginal” C/A-code signal is necessarily trackable, and (c) an

“unhealthy” C/A-code signal may either be trackable or untrackable.

4. If broadcast, CNAV messages may also contain information related to the SIS health of the signals

from various satellites in the constellation. This information may not correspond to the actual health

of the C/A-code signal from the broadcasting satellite or from other satellites in the constellation.

The most reliable information about a satellite’s C/A-code signal health is obtained directly from

that satellite’s C/A-code signal. The preceding definitions in this paragraph take precedence over

the information in the CNAV messages.

2.3.2.2 CM-Code Signal

"Healthy". The CM-code signal is healthy when all of the following four conditions are

present:

(1) There is no CM-code signal alarm indication present. CM-code signal alarm

indications are one component of the SPS SIS integrity. The presence of any one

of the nine CM-code signal alarm indications listed in paragraph 2.3.4 means the

information provided by the CM-code signal may not be correct.

(2) The CM-code signal indicates the CM-code signal is healthy. More specifically,

the L2 health bit of the three-bit signal health field given in MT-10 of the CNAV

message is set to 0.

(3) The URA alert flag for the CM-code signal is not raised (i.e., bit 38 of each CNAV

message is set to 0).

(4) The transmitted SPS URA

index in MT-10 and the SPS URA

NED0

index in MT-3x

are both less than 8. The URA

index and URA

NED0

index are integers that equate

to a range of URA

and URA

NED0

values respectively. A URA

and URA

NED0

index of less than 8 equates to a URA

, and URA

NED0

value of less than or equal

to 48 meters.

The above descriptions only relate to how each condition applies to a healthy CM-code

signal. Definitions for all the different settings of conditions 2, 3, and 4 are given in IS-

GPS-200.

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"Marginal". The CM-code signal health is marginal when the CM-code signal would

otherwise have been defined as healthy except that one or more of the following three

warning conditions is or are present:

(1) Default CNAV data (i.e., MT-0) is being transmitted in lieu of MT-10 and/or MT-11

and/or MT-3x on the L2C-code signal (e.g., a current and consistent CEI data set

is not available).

(2) The URA alert flag is raised (i.e., bit 38 of each CNAV message is set to 1) and

the L2C-code signal URA components do not apply. This means the CM-code

signal URA may be worse than indicated by the URA index components

transmitted in MT-10 and MT-3x. See IS-GPS-200 for details.

(3) Either or both the transmitted SPS URA

index in MT-10 and the SPS URA

NED0

index in MT-3x are greater than or equal to 8 ("N"=8). A URA

index or URA

NED0

index of 8 or greater indicates that the L2C-code signal URA is greater than 48

meters or that there is no CM-code signal URA prediction available. See IS-GPS-

200 for details.

"Unhealthy". The CM-code signal health is unhealthy when any one or more of the

following three conditions is or are present:

(1) There is an CM-code signal alarm indication present. CM-code signal alarm

indications are one component of the SPS SIS integrity. The presence of any one

of the nine CM-code signal alarm indications listed in paragraph 2.3.4 means the

information provided by the CM-code signal may not be correct,

(2) The CM-code signal indicates the CM-code signal is bad. More specifically for the

CM-code signal, the L2 health bit of the three-bit signal health field given in MT-10

of the CNAV message is set to 1.

(3) The transmitted CM-code signal is untrackable.

"Not Applicable". The “not applicable” health condition applies to the CM-code signal

when the satellite is not broadcasting an CM-code signal either: (a) because the satellite

was designed without an CM-code signal capability as indicated by the broadcast

configuration code for the satellite, or (b) because the satellite’s CM-code signal capability

has been disabled and the broadcast configuration code for the satellite has been changed

to reflect the residual capabilities of the satellite. See the relevant SPS SIS broadcast

configurations given in Table 1.6-2.

Notes:

1. If broadcast, the almanac data in MT-31 and/or MT-12 and/or MT-37 of the CNAV message also

contains information related to the L1/L2/L5 SIS health of the satellites in the constellation. This is

not real-time information. It is more of a long-term indicator and may not correspond to the actual

health of the L1/L2/L5 SIS from the transmitting satellite or from other satellites in the constellation.

2. From the above SIS characteristics, it follows that: (a) a “healthy” CM-code signal is necessarily

trackable, (b) a “marginal” CM-code signal is necessarily trackable, (c) an “unhealthy” CM-code

signal may either be trackable or untrackable, and (d) a “not applicable” CM-code signal does not

exist.

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3. The most reliable information about a satellite’s CM-code signal health is obtained directly from

that satellite’s CM-code signal. The preceding definitions in this paragraph take precedence over

other information in the CNAV and LNAV messages.

2.3.2.3 CL-Code Signal

"Healthy". The CL-code signal is healthy when all of the following four conditions are

present:

(1) There is no CL-code signal alarm indication present. CL-code signal alarm

indications are one component of the SPS SIS integrity. The presence of any one

of the four CL-code signal alarm indications listed in paragraph 2.3.4 means the

information provided by the CL-code signal may not be correct.

(2) The CM-code signal indicates the CL-code signal is healthy. More specifically, the

L2 health bit of the three-bit signal health field given in MT-10 of the CNAV

message is set to 0.

(3) The URA alert flag for the CM-code signal is not raised (i.e., bit 38 of each CM-

code signal CNAV message is set to 0).

(4) The transmitted SPS URA

index in the CM-code signal MT-10 and the SPS

URA

NED0

index in the CM-code signal MT-3x are both less than 8.

The above descriptions only relate to how each condition applies to a healthy CL-code

signal. Definitions for all the different settings of conditions 2, 3, and 4 are given in IS-

GPS-200.

"Marginal". The CL-code signal health is marginal when the CL-code signal would

otherwise have been defined as healthy except that one or more of the following three

warning conditions is or are present:

(1) Default CNAV data (i.e., MT-0) is being transmitted in lieu of MT-10 and/or MT-11

and/or MT-3x (e.g., a current and consistent CEI data set is not available) on the

CM-code signal.

(2) The URA alert flag for the I5-code signal is raised (i.e., bit 38 of each CNAV

message is set to 1) and the CM-code signal URA components do not apply to the

CL-code signal.

(3) Either or both the transmitted SPS URA

index in the CM-code signal MT-10 and

the SPS URA

NED0

index in the CM-code signal MT-3x are greater than or equal to

8 ("N"=8).

"Unhealthy". The CL-code signal health is unhealthy when any one or more of the

following three conditions is or are present:

(1) There is a CL-code signal alarm indication present. CL-code signal alarm

indications are one component of the SPS SIS integrity. The presence of any one

of the four CL-code signal alarm indications listed in paragraph 2.3.4 means the

information provided by the CL-code signal may not be correct,

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(2) The CM-code signal indicates the CL-code signal is bad. More specifically for the

CL-code signal, the L2 health bit of the three-bit signal health field given in the CM-

code signal MT-10 of the CNAV message is set to 1.

(3) The transmitted CL-code signal is untrackable.

"Not Applicable". The “not applicable” health condition applies to the CL-code signal

when the satellite is not broadcasting a CL-code signal either: (a) because the satellite

was designed without a CL-code signal capability as indicated by the broadcast

configuration code for the satellite, or (b) because the satellite’s CL-code signal capability

has been disabled and the broadcast configuration code for the satellite has been changed

to reflect the residual capabilities of the satellite. See the relevant SPS SIS broadcast

configurations given in Table 1.6-2.

Notes:

1. From the above SIS characteristics, it follows that: (a) a “healthy” CL-code signal is necessarily

trackable, (b) a “marginal” CL-code signal is necessarily trackable, (c) an “unhealthy” CL-code

signal may either be trackable or untrackable, and (d) a “not applicable” CL-code signal does not

exist.

2. The most reliable information about a satellite’s CL-code signal health is obtained directly from that

satellite’s CL-code signal and then indirectly from that satellite’s CM-code signal. The preceding

definitions in this paragraph take precedence over other information in the CNAV and LNAV

messages.

2.3.2.4 I5-Code Signal

"Healthy". The I5-code signal is healthy when all of the following four conditions are

present:

(1) There is no I5-code signal alarm indication present. I5-code signal alarm

indications are one component of the SPS SIS integrity. The presence of any one

of the nine I5-code signal alarm indications listed in paragraph 2.3.4 means the

information provided by the I5-code signal may not be correct.

(2) The I5-code signal indicates the I5-code signal is healthy. More specifically, the

L5 health bit of the three-bit signal health field given in MT-10 of the CNAV

message is set to 0.

(3) The URA alert flag for the I5-code signal is not raised (i.e., bit 38 of each CNAV

message is set to 0).

(4) The transmitted SPS URA

index in MT-10 and the SPS URA

NED0

index in MT-3x

are both less than 8. The URA

index and URA

NED0

index are integers that equate

to a range of URA

and URA

NED0

values respectively. A URA

and URA

NED0

index of less than 8 equates to a URA

, and URA

NED0

value of less than or equal

to 48 meters.

The above descriptions only relate to how each condition applies to a healthy I5-code

signal. Definitions for all the different settings of conditions 2, 3, and 4 are given in IS-

GPS-705.

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"Marginal". The I5-code signal health is marginal when the I5-code signal would

otherwise have been defined as healthy except that one or more of the following three

warning conditions is or are present:

(1) Default CNAV data (i.e., MT-0) is being transmitted in lieu of MT-10 and/or MT-11

and/or MT-3x on the I5-code signal (e.g., a current and consistent CEI data set is

not available).

(2) The URA alert flag is raised (i.e., bit 38 of each CNAV message is set to 1) and

the I5-code signal URA components do not apply. This means the I5-code signal

URA may be worse than indicated by the URA index components transmitted in

MT-10 and MT-3x. See IS-GPS-705 for details.

(3) Either or both the transmitted SPS URA

index in MT-10 and the SPS URA

NED0

index in MT-3x are greater than or equal to 8 ("N"=8). A URA

index or URA

NED0

index of 8 or greater indicates that the I5-code signal URA is greater than 48

meters or that there is no I5-code signal URA prediction available. See IS-GPS-

705 for details.

"Unhealthy". The I5-code signal health is unhealthy when any one or more of the

following three conditions is or are present:

(1) There is an I5-code signal alarm indication present. I5-code signal alarm

indications are one component of the SPS SIS integrity. The presence of any one

of the nine I5-code signal alarm indications listed in paragraph 2.3.4 means the

information provided by the I5-code signal may not be correct,

(2) The I5-code signal indicates the I5-code signal is bad. More specifically for the I5-

code signal, the L5 health bit of the three-bit signal health field given in MT-10 of

the CNAV message is set to 1.

(3) The transmitted I5-code signal is untrackable.

"Not Applicable". The “not applicable” health condition applies to the I5-code signal

when the satellite is not broadcasting an I5-code signal either: (a) because the satellite

was designed without an I5-code signal capability as indicated by the broadcast

configuration code for the satellite, or (b) because the satellite’s I5-code signal capability

has been disabled and the broadcast configuration code for the satellite has been changed

to reflect the residual capabilities of the satellite. See the relevant SPS SIS broadcast

configurations given in Table 1.6-2.

Notes:

1. If broadcast, the almanac data in MT-31 and/or MT-12 and/or MT-37 of the CNAV message also

contains information related to the L1/L2/L5 SIS health of the satellites in the constellation. This is

not real-time information. It is more of a long-term indicator and may not correspond to the actual

health of the L1/L2/L5 SIS from the transmitting satellite or from other satellites in the constellation.

2. From the above SIS characteristics, it follows that: (a) a “healthy” I5-code signal is necessarily

trackable, (b) a “marginal” I5-code signal is necessarily trackable, (c) an “unhealthy” I5-code signal

may either be trackable or untrackable, and (d) a “not applicable” I5-code signal does not exist.

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3. The most reliable information about a satellite’s I5-code signal health is obtained directly from that

satellite’s I5-code signal. The preceding definitions in this paragraph take precedence over other

information in the CNAV and LNAV messages.

2.3.2.5 Q5-Code Signal

"Healthy". The Q5-code signal is healthy when all of the following four conditions are

present:

(1) There is no Q5-code signal alarm indication present. Q5-code signal alarm

indications are one component of the SPS SIS integrity. The presence of any one

of the four Q5-code signal alarm indications listed in paragraph 2.3.4 means the

information provided by the Q5-code signal may not be correct.

(2) The I5-code signal indicates the Q5-code signal is healthy. More specifically, the

L5 health bit of the three-bit signal health field given in MT-10 of the CNAV

message is set to 0.

(3) The URA alert flag for the I5-code signal is not raised (i.e., bit 38 of each I5-code

signal CNAV message is set to 0).

(4) The transmitted SPS URA

index in the I5-code signal MT-10 and the SPS

URA

NED0

index in the I5-code signal MT-3x are both less than 8.

The above descriptions only relate to how each condition applies to a healthy Q5-code

signal. Definitions for all the different settings of conditions 2, 3, and 4 are given in IS-

GPS-705.

"Marginal". The Q5-code signal health is marginal when the Q5-code signal would

otherwise have been defined as healthy except that one or more of the following three

warning conditions is or are present:

(1) Default CNAV data (i.e., MT-0) is being transmitted in lieu of MT-10 and/or MT-11

and/or MT-3x (e.g., a current and consistent CEI data set is not available) on the

I5-code signal.

(2) The URA alert flag for the I5-code signal is raised (i.e., bit 38 of each CNAV

message is set to 1) and the I5-code signal URA components do not apply to the

Q5-code signal.

(3) Either or both the transmitted SPS URA

index in the I5-code signal MT-10 and

the SPS URA

NED0

index in the I5-code signal MT-3x are greater than or equal to 8

("N"=8).

"Unhealthy". The Q5-code signal health is unhealthy when any one or more of the

following three conditions is or are present:

(1) There is a Q5-code signal alarm indication present. Q5-code signal alarm

indications are one component of the SPS SIS integrity. The presence of any one

of the four Q5-code signal alarm indications listed in paragraph 2.3.4 means the

information provided by the Q5-code signal may not be correct,

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(2) The I5-code signal indicates the Q5-code signal is bad. More specifically for the

Q5-code signal, the L5 health bit of the three-bit signal health field given in the I5-

code signal MT-10 of the CNAV message is set to 1.

(3) The transmitted Q5-code signal is untrackable.

"Not Applicable". The “not applicable” health condition applies to the Q5-code signal

when the satellite is not broadcasting a Q5-code signal either: (a) because the satellite

was designed without a Q5-code signal capability as indicated by the broadcast

configuration code for the satellite, or (b) because the satellite’s Q5-code signal capability

has been disabled and the broadcast configuration code for the satellite has been changed

to reflect the residual capabilities of the satellite. See the relevant SPS SIS broadcast

configurations given in Table 1.6-2.

Notes:

1. From the above SIS characteristics, it follows that: (a) a “healthy” Q5-code signal is necessarily

trackable, (b) a “marginal” Q5-code signal is necessarily trackable, (c) an “unhealthy” Q5-code

signal may either be trackable or untrackable, and (d) a “not applicable” Q5-code signal does not

exist.

2. The most reliable information about a satellite’s Q5-code signal health is obtained directly from that

satellite’s Q5-code signal and then indirectly from that satellite’s I5-code signal. The preceding

definitions in this paragraph take precedence over other information in the CNAV and LNAV

messages.

2.3.3 SPS SIS Accuracy

The SPS SIS accuracy is described in two statistical ways; one way is as the 95

percentile (95%)

SPS SIS user range error (URE) at a specified age of data (AOD), the other is as the 95% SPS

SIS URE over all AODs. With either statistical expression, the SPS SIS accuracy is also known

as the SPS SIS pseudorange accuracy. In this context, “pseudorange” means the full

pseudorange data set (i.e., the matched combination of a corrected pseudorange measurement

and a pseudorange origin, or equivalently the matched combination of a raw pseudorange

measurement and the associated NAV data). Other accuracy-related SPS SIS performance

parameters include the SPS SIS pseudorange rate (velocity) accuracy defined as the 95% SPS

SIS pseudorange rate error over all AODs and the SPS SIS pseudorange acceleration (rate rate)

accuracy defined as the 95% SPS SIS pseudorange acceleration error over all AODs.

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2.3.4 SPS SIS Integrity

The SPS SIS integrity is defined to be the trust which can be placed in the correctness of the

information provided by the SPS SIS. SPS SIS integrity includes the ability of the SPS SIS to

provide timely alerts to receivers when the SPS SIS should not be used for positioning or timing.

The SPS SIS should not be used when it is providing misleading signal-in-space information

(MSI), where the threshold for “misleading” is a not-to-exceed (NTE) tolerance on the SIS URE.

For this SPS PS, the four components of integrity are the probability of a major service failure,

the time to alert, the SIS URE NTE tolerance, and the alert (either one or the other of two types

of alerts).

Probability of a Major Service Failure. The probability of a major service failure for the

SPS SIS is defined to be the probability that the SPS SIS's instantaneous URE exceeds

the SIS URE NTE tolerance (i.e., MSI) without a timely alert being issued (i.e., unalerted

MSI [UMSI]). Alerts generically include both alarms and warnings.

Time to Alert. The time to alert (TTA) for the SPS SIS is defined to be the time from the

onset of MSI until an alert (alarm or warning) indication arrives at the receiver's antenna.

Real-time alert information broadcast as part of the NAV message data is defined to arrive

at the receiver’s antenna at the end of the NAV message subframe which contains that

particular piece of real-time alert information.

SIS URE NTE Tolerance. The SPS SIS URE NTE tolerance for a trackable and healthy

SPS SIS is defined to be 4.42 times the integrity assured URA (IAURA) value currently

being broadcast by the satellite. For LNAV, the IAURA is equal to the upper bound on the

URA value corresponding to the URA index "N" currently broadcast by the satellite in

subframe 1. For CNAV, the IAURA is the RSS of an elevation-dependent function of the

upper bound value of the URA

component and a non-elevation-dependent function of

the upper bound value of the URA

NED

component currently broadcast by the satellite in

MT-10 and MT-3x respectively. The SIS URE NTE tolerance for a marginal SPS SIS is

not defined and there is no SIS URE NTE tolerance for an unhealthy SPS SIS.

Alert – Alarm Indications. An otherwise healthy SPS SIS signal or marginal SPS SIS

signal becomes unhealthy when it is the subject of a SPS SIS alarm indication. Each SPS

SIS signal has its own set of alarm indications as detailed below.

C/A-Code Signal

The presence of any of the 9 alarm indications listed below means the information

provided by the C/A-code signal may not be correct. The C/A-code signal alarm

indications are defined to include the following:

(1) The C/A-code signal becomes untrackable (e.g., ≥ 20 dB decrease in transmitted

signal power, ≥ 20 dB increase in correlation loss):

(a) The C/A-code signal ceases transmission.

(b) The elimination of the standard C/A-code (e.g., gibberish code).

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(d) The substitution of PRN C/A-code number 37 for the standard C/A-code.

(2) The failure of parity on 5 successive words of NAV data (3 seconds).

(3) The broadcast IODE does not match the 8 LSBs of the broadcast IODC (excluding

normal data set cutovers, see IS-GPS-200).

(4) The transmitted bits in words 3-10 of subframe 1, 2, or 3 are all set to 0's or all set

to 1's.

(5) Default NAV data is being transmitted in subframes 1, 2, or 3 (see IS-GPS-200).

(6) The 8-bit preamble does not equal 10001011

, decimal 139, or hexadecimal 8B.

CM-Code Signal

The presence of any of the 9 alarm indications listed below means the information

provided by the CM-code signal may not be correct. The CM-code signal alarm

indications are defined to include the following:

(1) The CM-code signal becomes untrackable (e.g., ≥ 20 dB decrease in transmitted

signal power, ≥ 20 dB increase in correlation loss):

(a) The CM-code signal ceases transmission.

(b) The elimination of the standard CM-code (e.g., gibberish code).

(d) The substitution of PRN CM-code number 37 for the standard CM-code.

(2) The failure of the cyclic redundancy check (CRC) on 5 successive CNAV

messages (60 seconds).

(3) The broadcast time of ephemeris (t

) is not current or does not match the

broadcast time of clock (t

) (excluding normal data set cutovers, see IS-GPS-200).

(4) The broadcast t

is not consistent across the MT-10, -11, and -3x messages which

comprise the current CEI data set (excluding normal data set cutovers, see IS-

GPS-200).

(5) The transmitted bits in MT-10, MT-11, and MT-3x are all set to 0's or all set to 1's.

(6) The 8-bit preamble does not equal 10001011

, decimal 139, or hexadecimal 8B.

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CL-Code Signal

The presence of any of the 4 alarm indications listed below means the information

provided by the CL-code signal may not be correct. The CL-code signal alarm

indications are defined to include the following:

(1) The CL-code signal becomes untrackable (e.g., ≥ 20 dB decrease in transmitted

signal power, ≥ 20 dB increase in correlation loss):

(a) The CL-code signal ceases transmission.

(b) The elimination of the standard CL-code (e.g., gibberish code).

(d) The substitution of PRN CL-code number 37 for the standard CL-code.

I5-Code Signal

The presence of any of the 9 alarm indications listed below means the information

provided by the I5-code signal may not be correct. The I5-code signal alarm indications

are defined to include the following:

(1) The I5-code signal becomes untrackable (e.g., ≥ 20 dB decrease in transmitted

signal power, ≥ 20 dB increase in correlation loss):

(a) The I5-code signal ceases transmission.

(b) The elimination of the standard I5-code (e.g., gibberish code).

(d) The substitution of PRN I5-code number 37 for the standard I5-code.

(2) The failure of the CRC on 5 successive CNAV messages (30 seconds).

(3) The broadcast t

is not current or does not match the broadcast t

(excluding

normal data set cutovers, see IS-GPS-705).

(4) The broadcast t

is not consistent across the MT-10, -11, and -3x messages which

comprise the current CEI data set (excluding normal data set cutovers, see IS-

GPS-705).

(5) The transmitted bits in MT-10, MT-11, and MT-3x are all set to 0's or all set to 1's.

(6) The 8-bit preamble does not equal 10001011

, decimal 139, or hexadecimal 8B.

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Q5-Code Signal

The presence of any of the 4 alarm indications listed below means the information

provided by the Q5-code signal may not be correct. The Q5-code signal alarm

indications are defined to include the following:

(1) The Q5-code signal becomes untrackable (e.g., ≥ 20 dB decrease in transmitted

signal power, ≥ 20 dB increase in correlation loss):

(a) The Q5-code signal ceases transmission.

(b) The elimination of the standard Q5-code (e.g., gibberish code).

(d) The substitution of PRN Q5-code number 37 for the standard Q5-code.

Alert – Warning Indications. An otherwise trackable and healthy SPS SIS becomes

marginal or unhealthy when it is the subject of a SPS SIS warning indication. SPS SIS

warnings are typically provided in advance of the onset of potential MSI events (i.e.,

preemptive setting of the six-bit health status word in subframe 1 prior to scheduled

maintenance). SPS SIS warnings are also common after an SPS SIS alarm and at

satellite end of life. The SPS SIS warning indications are defined in paragraph 2.3.2

above, plus:

(1) An appropriately inflated URA index "N" value or appropriately inflated set of

URA

and URA

NED

values (appropriately inflated to cover the expected risk of an

abnormally large SPS SIS URE).

Notes:

1. A SPS SIS alarm indication exists when the satellite is not trackable because it is not transmitting

the standard PRN code modulation on the L-band carrier signal. As indicated above, specific SPS

SIS alarm indications include the following: (a) when the L-band carrier signal has no modulation

(i.e., unmodulated carrier signal), (b) when the L-band carrier signal is modulated by nonstandard

PRN code, and (c) when the L-band carrier signal is modulated by PRN code number 37. These

SPS SIS alarm indications are specifically called out above because of their relatively high

probability of occurrence.

2. The SPS SIS alarm indications related to the LNAV and CNAV message data are considered

“weak” indications since SPS receivers do not necessarily continuously read each satellite’s LNAV

or CNAV message data either by design or by circumstance (e.g., radio-frequency interference

[RFI] can prevent reading NAV message data). These weak SPS SIS alarm indications are

assumed to have a five minute lag time before SPS receivers take notice of them for alerting

purposes.

3. The SPS SIS alarm indications related to the NAV message data are indicative of a problem

onboard the satellite. GPS receivers may perceive similar indications caused by local effects that

are unrelated to the broadcast SPS SIS.

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4. MSI is the SIS range domain analog of hazardously misleading information (HMI) in the user

position domain. SPS SIS MSI may or may not cause some SPS receivers to output HMI.

5. See Appendix A, Section A.5 for additional background information on integrity.

6. In addition to SPS SIS alarm indications, other conditions may also cause GPS signals to become

temporarily untrackable, such as ionospheric signal fades, local signal masking, or local

interference.

2.3.5 SPS SIS Continuity

The SPS SIS continuity for a trackable and healthy SPS SIS is the probability that the SPS SIS

will continue to be trackable and healthy without unscheduled interruption over a specified time

interval. Scheduled interruptions which are announced at least 48 hours in advance do not

contribute to a loss of continuity. Scheduled SPS SIS interruptions are announced by way of the

Control Segment issuing a "Notice Advisory to Navstar Users" (NANU). NANUs are similar to the

"Notices to Airmen" (NOTAMs) issued regarding scheduled interruptions of ground-based air

navigation aids. CS internal procedures are to issue NANUs for scheduled interruptions at least

96 hours in advance.

Note:

1. In this edition of the SPS PS, the continuity performance standards in Section 3.6 only apply to the

C/A-code signals. There are no continuity performance standards yet for the CM-code signals, the

CL-code signals, the I5-code signals, or the Q5-code signals,

2.3.6 SPS SIS UTC(USNO) Accuracy

The SPS UTC(USNO) accuracy for a healthy or marginal SPS SIS is defined to be the 95% error

in the parameters (ref. 20.3.3.5.2.4 of IS-GPS-200) contained in that SPS SIS which relate GPS

time to UTC(USNO).

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2.4 Usage Assumptions for SPS Performance Standards

This SPS PS is conditioned upon certain assumptions regarding use of the SPS SIS. Those

assumptions are as follows.

2.4.1 SPS User

This SPS PS assumes an SPS user with an SPS receiver.

The SPS user is assumed to be a human or an automated (sub)system desiring to use the SPS

positioning and timing service described in this document for a peaceful civil, commercial, or

scientific purpose.

The SPS receiver is assumed to comply with the technical requirements related to the interface

between the Space Segment and SPS receivers as established by IS-GPS-200 and IS-GPS-705.

The SPS receiver is also assumed to properly account for the performance impacts caused by

use of any NAV data outside the curve fit interval for that data and for SIS extended operations.

2.4.2 SPS SIS Configuration

This SPS PS assumes the GPS receiver is tracking, processing, and using one or more of the

SPS SIS component combinations listed in Table 2.2-2 as appropriate to the SPS SIS component

transmitted by each satellite. Pseudorange measurements are assumed to be made by PRN

code tracking with an early-minus-late correlator at 1 chip spacing using an exact replica of the

waveform within an ideal sharp-cutoff filter bandwidth at 24 MHz with linear phase centered at the

carrier frequency. Carrier phase measurement processing is not assumed.

2.4.3 Single-/Dual-/Triple-Frequency Operation

This SPS PS assumes a GPS receiver operates with any of the SF/DF/TF combinations listed in

Table 2.2-2. The performance standards in Section 3 of this SPS PS are independent of whether

the GPS receiver uses the satellite-transmitted ionospheric parameters for SF model-based

ionospheric delay compensation purposes or not.

This SPS PS assumes a SF GPS receiver will apply the group delay time correction (T

) term in

accordance with IS-GPS-200 or IS-GPS-705.

2.4.4 SPS SIS Health

This SPS PS preferentially uses the terms "SPS SIS health" or "SPS signal health" to describe

the status indicated by the real-time health-related information broadcast by each satellite as part

of the SPS SIS (see paragraph 2.3.2). Occasionally, for consistency with prior usage, this SPS

PS may also use the terms "satellite health” or “space vehicle health”.

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2.4.4.1 Limitations on SPS SIS Health

This SPS PS assumes a GPS receiver will only consider using an SPS SIS component whose

health status is indicated as healthy. This SPS PS explicitly assumes a GPS receiver will not use

a SPS SIS component whose health status is indicated as either marginal or unhealthy.

Notes:

1. It is recognized that GPS receivers may gain operational benefit in certain situations by cautiously

using a SPS SIS component whose status is indicated as marginal. Such situations include periods

of reduced satellite visibility due to terrain masking, body masking, abnormal receiving antenna

orientation, default CNAV messages, or instances where an additional pseudorange data set is

needed for fault exclusion. Although potentially beneficial, use of a SPS SIS whose status is

indicated as marginal is not recommended by this SPS PS. If an indicated-as-marginal SPS SIS

is used, the reason for the marginal status should be ascertained and the impact on performance

(accuracy, integrity, continuity) should be accounted for in that use.

2. It is further recognized that many GPS augmentation systems (e.g., maritime differential GPS

(DGPS) services operating in accordance with the recommendations in RTCM Paper 194-

93/SC104-STD, Satellite–Based Augmentation System (SBAS) services operating in accordance

with RTCA/DO-229) can override many parts of the real-time and long-term health-related

information transmitted by each satellite. If a GPS augmentation system does override any of the

health-related information transmitted by the satellites, that GPS augmentation system is explicitly

responsible for any and all consequences of the override.

2.4.4.2 Priority of SPS SIS Health Information

This SPS PS assumes a GPS receiver will prioritize the application of the real-time health-related

information transmitted by each satellite ahead of the long-term health-related information

transmitted by that satellite or any other satellite. The real-time health-related information is as

described earlier in paragraph 2.3.2 of this SPS PS. The long-term health-related information is

contained in words 3 through 10 of the various pages of subframe 4 and subframe 5 of the LNAV

data message as shown in Figure 2.2-1 and as further described in IS-GPS-200, or in MT-31

and/or MT-12 and/or MT-37 of the CNAV message as further described in IS-GPS-200 and IS-

GPS-705. In this context, “prioritize” means the GPS receiver will use current real-time health-

related information for a GPS SIS whenever it is available in lieu of long-term health-related

information for that GPS SIS.

2.4.4.3 Timely Application of SPS SIS Health Information

This SPS PS assumes a GPS receiver will monitor, process, and apply the real-time health-

related information transmitted by each satellite (including SPS SIS alert indications) each time

the information is transmitted. For real-time health-related information broadcast as part of the

NAV message data, the assumed time of application is 2.0 seconds after the end of the NAV

message subframe or message which contains the particular piece of real-time health-related

information.

Notes:

1. Real-time alert information broadcast as part of the NAV message data is assumed to require an

additional 2 seconds for application as opposed to real-time alert information not broadcast as part

of the NAV message data.

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2. The Control Segment will endeavor to operate the SPS SIS in such a manner to allow GPS

receivers at least five minutes to receive, process, and apply the real-time health-related

information broadcast as part of the NAV message data before taking any action that could cause

a large SPS SIS URE under normal operations and maintenance (O&M) conditions.

It is recognized that GPS receivers cannot always monitor the broadcast NAV message data

since interruptions may be caused by temporary signal blockages, abnormal receiving antenna

orientation, RFI (particularly jamming), and intermittent environmental effects. Although the GPS

receiver is responsible for taking appropriate action when it cannot monitor, process, or apply the

current real-time health-related information in the NAV message data, it is possible for the Control

Segment to aid some GPS receivers by giving them some advance warning of impending SPS

SIS health changes. This action will only be beneficial for SPS SIS integrity if the SPS SIS health

changes from healthy to marginal, or from healthy to unhealthy. An example of such an in-

advance warning would be setting the 5 LSBs of the six-bit health status word in subframe 1 of

the LNAV message to 11101

(“SV will be temporarily out”) for a period of time before setting the

MSB of the six-bit health status word given in subframe 1 of the LNAV message to 1

(“some or

all NAV data are bad”) and/or setting the 5 LSBs of the six-bit health status word in subframe 1 to

11100

(“SV is temporarily out”).

Note:

1. The Control Segment currently provides advance warning of large SPS SIS URE by setting the

MSB of the six-bit health status word in subframe 1 to 1

(“some or all NAV data are bad”) and

setting the 5 LSBs of the six-bit health status word in subframe 1 to either 11100

(“SV is temporarily

out”) or 11111

(“more than one combination would be required to describe anomalies”). These

are conservative courses of action. The impact of these courses of action has already been

factored into the SPS SIS availability standards.

2.4.5 Excluded Errors

The performance standards in Section 3 of this SPS PS do not take into consideration any error

source that is not under direct control of the Space Segment or Control Segment. Specifically

excluded errors include those due to the effects of:

 Signal distortions caused by ionospheric and/or tropospheric scintillation

 Residual receiver ionospheric delay compensation errors

 Residual receiver tropospheric delay compensation errors

 Receiver noise (including received signal power and interference power) and resolution

 Receiver hardware/software faults

 Multipath and receiver multipath mitigation

 User antenna effects

 Operator (user) error

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2.4.6 “Dummy” Satellites

This SPS PS assumes a GPS receiver will not intentionally attempt to acquire, acquire, track, or

use the signals broadcast by any satellite indicated as being a “dummy” satellite in the almanac

data broadcast by the non-dummy satellites in the constellation. The performance standards in

Section 3 of this SPS PS do not apply to dummy satellites.

Note:

1. See the definition of a dummy satellite in IS-GPS-200.

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SECTION 3.0 SPS SIS Performance Standards

This section establishes SPS SIS performance standards for GPS operations. The USSF is

committed to operating GPS in accordance with these standards, in a manner consistent with

system capabilities and subject to budgetary constraints. The USG reserves the right to adjust

GPS constellation management practices as necessary to support military and civil end users.

One of the potential adjustments to increase the robustness of constellation availability and

enhance the overall SPS SIS performance is an expandable 24-slot constellation consisting of

the expansion of as many as six of the baseline 24-slot constellation slots. The USG also reserves

the right to optimize performance to support high priority mission needs over an AOO (e.g.,

maximizing accuracy and availability at the site of a major natural disaster). Any such optimization

will not degrade the SPS SIS performance beyond the standards defined in this section for areas

outside the AOO.

3.1 Overview

The SPS SIS performance is specified in terms of minimum performance standards for each

performance parameter. Each standard includes a definition of conditions and constraints

applicable to the provision of the specified service. The phrase “any [healthy] SPS SIS”, when

listed as a condition or constraint for any of the performance standards in this section, refers to

the individual SPS signal in space transmissions from each satellite.

SPS SIS performance standards do not include any element not under the direct control of the

GPS Control/Space Segments. Any performance parameters not specified in this section are not

considered to be part of the SPS SIS performance standards. Performance parameters not

specified in this section, in IS-GPS-200D/E/F/G/H, or in IS-GPS-705D do not represent a part of

the minimum service being provided to the user community.

These SPS SIS performance standards do not directly represent the end performance users will

experience. The standards provide a definition of the components of GPS performance that,

when combined with a signal reception environment and assumptions concerning the GPS

receiver, allow users to define for themselves the end performance they can expect for their

particular application. The USG recognizes that these metrics have little direct meaning to the

average end user (e.g., pilot, navigator, driver), but they are absolutely essential for GPS receiver

designers, system integrators, application engineers, infrastructure and augmentation system

developers, space/control segment operators and maintainers, and usage regulators. In support

of end users, Appendix B provides an expanded description of the position domain performance

implied by the SPS SIS performance standards combined with the typical performance

assumptions, including ionosphere, troposphere, and receiver noise error contributions, for a

range of GPS receivers to give a sense of the operational characteristics that can be expected

under a wide spectrum of operating conditions. Appendix B also gives examples of how to

translate the expected pseudorange domain characteristics into end user PVT performance

terms.

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3.1.1 SPS SIS Performance Standards Organization

The SPS SIS performance standards are organized in a ‘build-up sequence’ order. They start

with the most basic performance standard (coverage) and conclude with the position/time domain

standards which are a culmination of the preceding SIS performance standards interpreted

through a representative SPS receiver.

The build-up sequence of SPS SIS performance standards (and the specific section references)

is as follows:

Constellation (3.2)

Deployment of the SPS SIS transmitters (satellites)

Coverage (3.3)

Broadcast region served by each SPS SIS transmitter

Accuracy (3.4)

Level of errors in the SPS SIS transmissions over the broadcast region

Integrity (3.5)

Trustworthiness of the claimed ‘level of errors’ in the SPS SIS transmissions

Continuity (3.6)

Reliability (continuous availability) of the SPS SIS transmissions with integrity

Availability (3.7)

Restoration/replacement of SPS SIS transmitters when they lose continuity

Position/Time Domain (3.8)

Result of constellation & coverage, plus accuracy/integrity/continuity, considering availability

3.1.2 CM-Code, CL-Code, I5-Code, and Q5-Code SIS Performance Standards

This 5th Edition of the SPS PS only addresses the operation of CM-code, CL-code, I5-code, and

Q5-code signals prior to IOC. Until IOC is declared for each SPS SIS, the CM-code, CL-code,

I5-code, and Q5-code signals will only meet the Constellation (Section 3.2), Coverage (Section

3.3), Accuracy (Section 3.4) and Integrity (Section 3.5) standards; and then only when trackable

and “Healthy” as defined in Sections 2.3.2.2, 2.3.2.3, and 2.3.2.4. Subsequent editions of this

SPS PS will address post-IOC and post-FOC standards for the L2C and L5 signals.

The applicability of SPS SIS performance standards to each of the SPS SIS signals in this edition

of the SPS PS are graphically illustrated in Table 3.1-1.

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Table 3.1-1. SPS SIS Performance Standards vs. SPS SIS Signals

C/A-Code

CM-Code

CL-Code

I5-Code

Q5-Code

Constellation



Coverage



Accuracy



Integrity



Continuity



Availability



Position/Time Domain



Note:

* Performance Standards Marked by an Asterisk will be addressed in a Future Edition of this

SPS PS

3.2 24-Slot Constellation Definitions

The GPS baseline 24-slot constellation consists of 24 slots in six orbital planes with four slots per

plane. The baseline satellites will occupy these slots. Six of the 24 slots are expandable. For

historical reasons, the expandable slots are divided into two sets; one set spanning the B/D/F

planes and the other set spanning the A/C/E planes. Any combination of expandable slots

(including none or all) may be expanded at a given moment. Any auxiliary satellites that exist on

orbit will occupy other locations in the orbital planes. There are no a priori specified slots for

auxiliary satellites.

The baseline 24-slot constellation satellites will be placed in the orbital slots whose nominal slot

centers are defined by Table 3.2-1. Satellites will be maintained relative to those slot centers in

accordance with the reference orbit specifications and tolerances in Table 3.2-3.

Slots for the baseline 24-slot constellation are specified in terms of the Right Ascension of the

Ascending Node (RAAN) and the Argument of Latitude for a defined epoch. The corresponding

Groundtrack Equatorial Crossing (GEC) values (also known as the Geographic Longitude of the

Ascending Node [GLAN] values) are also provided in Table 3.2-1. Tables 3.2-1 and 3.2-3 define

the nominal, properly geometrically spaced, baseline 24-slot constellation for GPS.

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Table 3.2-1. Baseline 24-Slot Constellation Slot Assignments as of the Defined Epoch

Slot

RAAN

Argument of

Latitude

GEC

(GLAN)

288.85

239.54

127.85

A2*

288.85

133.20

74.68

288.85

343.09

179.63

288.85

13.22

14.69

B1*

348.85

52.37

94.27

348.85

144.75

140.46

348.85

281.39

28.78

348.85

175.79

155.98

48.85

83.29

169.73

48.85

343.21

119.69

48.85

311.08

103.62

C4*

48.85

212.97

54.57

108.85

106.64

61.40

D2*

108.85

236.86

126.51

108.85

6.57

11.37

108.85

138.77

77.47

168.85

168.46

152.31

168.85

274.01

25.09

E3*

168.85

37.48

86.82

168.85

305.10

40.63

228.85

210.30

53.23

F2*

228.85

316.64

106.40

228.85

76.62

166.39

228.85

106.76

1.46

Notes:

Epoch: 23:59:43 UTC, 31 December 2016 (Week 1930 / Time of Week 0000 GPS time)

Leap second offset (GPS-UTC) transitioned from 17 to 18 sec at 23:59:60 UTC, 31

December 2016

Greenwich Hour Angle: 100.765°

Referenced to FK5/J2000.00 Coordinates

Orbital Slot IDs are Arbitrarily Numbered

* Orbital Slots Marked by an Asterisk are Expandable

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The expandable 24-slot constellation consists of the baseline 24-slot constellation modified to

include at least one expandable slot occupied by a pair of satellites in its expanded configuration

defined in Table 3.2-2. The fore (F) and aft (A) locations in an expandable slot are defined relative

to the baseline slot center in the direction of satellite motion. Together, Tables 3.2-1 and 3.2-2

define a total of up to 30 orbital locations and 64 variations of the expandable-24 constellation.

There are 6 variations to occupy 25 orbital locations using any 1 expanded slot, 15 variations to

occupy 26 orbital locations using any 2 expanded slots, 20 variations to occupy 27 orbital locations

using any 3 expanded slots, 15 variations to occupy 28 orbital locations using any 4 expanded

slots, 6 variations to occupy 29 orbital locations using any 5 expanded slots, and 1 variation to

occupy 30 orbital locations using all 6 expanded slots.

Table 3.2-2. Expandable 24-Slot Constellation Slot Assignments as of the Defined Epoch

Expandable Slot

RAAN

Argument of

Latitude

GEC

(GLAN)

B1 Expands To:

B1F

348.85

66.33

101.25

B1A

348.85

37.77

86.97

D2 Expands To:

D2F

108.85

254.09

135.13

D2A

108.85

229.39

122.78

F2 Expands To:

F2F

228.85

331.87

114.02

F2A

228.85

305.43

100.80

A2 Expands To:

A2F

288.85

144.40

80.28

A2A

288.85

117.96

67.06

C4 Expands To:

C4F

48.85

220.43

58.30

C4A

48.85

195.73

45.95

E3 Expands To:

E3F

168.85

52.08

94.18

E3A

168.85

23.52

79.84

Note that the actual satellite RAAN values will vary from the nominal values in Table 3.2-1 and

Table 3.2-2 due to initial launch dispersion, perturbation forces acting over each satellite's lifetime

(particularly the inclination-dependent forces due to the Earth’s geopotential oblateness [J2

term]), and variations in other forces affecting each unique satellite orbit nodal regression rate.

Maintenance of the satellite argument of latitude (ArgLat) values and relative spacing of the slots

are the controls employed to compensate for orbit plane drift and sustain constellation geometry

at acceptable levels. It is also possible for the inclination to drift out of the operational range.

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Table 3.2-3. Reference Orbit Parameters

Reference Orbit Parameter

Nominal Value

Operational Range

Required Tolerance

Semi-Major Axis, km

Eccentricity

Inclination, deg

RAAN, deg

Argument of Perigee, deg

Argument of Latitude at

Epoch, deg

Nodal Regression Rate,

deg/day

26,559.8

0.0

55.0

Note 3

0.0

Note 3

-0.0402

Note 1

0.0 to 0.02

 3

 180

N/A

Note 2

0.0 to 0.03

N/A

Note 1

N/A

Note 1: The satellite’s semi-major axis and orbital period will be adjusted to optimize the overlap between the

satellite’s footprint on the surface of the Earth and the slot center’s footprint on the surface of the Earth. For

practical reasons, such as the prudent use of a satellite’s on-orbit fuel supply, the maximization of footprint

overlaps will only be approximate.

Note 2: The nominal value shown provides stationary ground tracks.

Note 3: See Tables 3.2-1 and 3.2-2.

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3.3 SPS SIS Coverage

This section provides the SPS SIS coverage standards.

There are two components of SPS SIS coverage: (1) the per-satellite coverage, and (2) the

baseline/expandable 24-slot constellation coverage. These two components are interrelated.

The per-satellite coverage depends primarily on the satellite antenna subsystem design, the on-

orbit satellite pointing accuracy, and the satellite altitude (where the allowed range of satellite

altitudes is defined by the 24-slot constellation architecture). The baseline/expandable 24-slot

constellation coverage depends primarily on the per-satellite coverage coupled with the

baseline/expandable 24-slot constellation architecture.

Each component of SPS SIS coverage shall be as specified below.

3.3.1 Per-Satellite Coverage

The terrestrial service volume for per-satellite coverage comprises the portion of the near-Earth

region which extends from the surface of the Earth up to an altitude of 3,000 km above the surface

of the Earth which is visible from the satellite's orbital position (i.e., those portions of the resulting

spherical shell surrounding the Earth which are not otherwise physically obscured by the Earth or

by localized obstructions). The per-satellite coverage performance standards apply at the worst-

case satellite antenna pointing angle relative to the Earth.

The space service volume for per-satellite coverage comprises the near-Earth region which

extends from an altitude of 3,000 km above the surface of the Earth up to and including 36,000

km above the Earth’s surface which is visible from the satellite's orbital position. The space

service volume coverage is limited by the transmitting satellite’s antenna gain pattern and free-

space path loss. The limits of coverage are determined by the received power contour surface of

-182 dBW assuming a linear receiving antenna with a minimum gain that varies from +3 dBic at

3,000 km altitude to +7 dBic at 36,000 km altitude. There are no explicit per-satellite coverage

standards for the space service volume.

The per-satellite coverage shall be as specified in Table 3.3-1.

Table 3.3-1. SPS SIS Per-Satellite Coverage Standards

SIS Per-Satellite Coverage Standard

Conditions and Constraints

Terrestrial Service Volume:

 100% Coverage

Space Service Volume:

 No Coverage Performance Specified

 For any healthy or marginal SPS SIS

Notes:

1. The guaranteed minimum user-received SPS SIS power levels for the terrestrial service volume

are specified in IS-GPS-200 and IS-GPS-705. Although IS-GPS-200 and IS-GPS-705 uses a 5

degree elevation angle as the reference value for specifying the guaranteed minimum user-

April 2020 GPS SPS PS

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received SPS SIS power levels, the per-satellite coverage is not restricted to just those locations

where the satellite viewing angle is greater than or equal to 5 degrees above the local horizon. The

user-received SPS SIS power levels may be less than the guaranteed minimum at viewing angles

below 5 degrees above the local horizon.

3.3.2 Baseline/Expandable Constellation Coverage

The terrestrial service volume for the baseline 24-slot constellation and expandable 24-slot

constellation coverage comprises the entire near-Earth region which extends from the surface of

the Earth up to an altitude of 3,000 km above the surface of the Earth which is not physically

obscured by localized obstructions.

The space service volume for the baseline 24-slot constellation and expandable 24-slot

constellation coverage comprises the near-Earth region which extends from an altitude of 3,000

km above the surface of the Earth up to and including 36,000 km above the Earth’s surface. At

certain times and locations in the space service volume, the baseline/expandable 24-slot

constellations do not provide adequate coverage for instantaneous position solutions. Users

operating at those times and locations are therefore limited to time-filtered position solutions

propagated over time. There are no explicit constellation coverage standards for the space

service volume.

The constellation coverage shall be as specified in Table 3.3-2.

Table 3.3-2. SPS SIS Constellation Coverage Standards

SIS Constellation Coverage Standard

Conditions and Constraints

Terrestrial Service Volume:

 100% Coverage

Space Service Volume:

 No Coverage Performance Specified

 From only healthy or marginal SPS SIS

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3.4 SPS SIS Accuracy

This section provides the SPS SIS accuracy standards. The SPS SIS accuracy standards apply

to the SIS portion of the GPS error budgets for the user equivalent range error (UERE).

There are four main aspects of SPS SIS accuracy. The standards for each of these aspects are

given in this section. The four main aspects are:

1. The pseudorange accuracy (i.e., “User Range Error” or URE)

2. The time derivative of the URE (i.e., “User Range Rate Error” or URRE)

3. The second time derivative of URE (i.e., “User Range Acceleration Error” or URAE)

4. The UTC Offset Error (UTCOE)

The standards for each of the four main aspects of SPS SIS accuracy are different depending on

the operational application and/or condition of utilization. Refer to Section A.4 in Appendix A for

descriptions of the operational applications conditions of utilization. Different SPS SIS accuracy

standards are given in this section for:

a. Across all AODs versus at a specified AOD (i.e., either at zero AOD or at maximum AOD)

b. Normal operations versus extended operation (see paragraph A.4.3.2)

c. Any trackable and healthy SPS SIS versus across all trackable and healthy SPS SISs

Regardless of SPS SIS component(s) or operational application/utilization, each of the four main

aspects of SPS SIS accuracy are addressed in terms of a "global statistic" performance standard.

In this case, "global statistic" means the statistic across the portion of the globe in view of the

satellite (or constellation ensemble of satellites) over at least the ergodic period (see Appendix A

for additional details on the meaning of "global statistic"). All of the SPS SIS performance

standards in this section are expressed at the 95% probability level in accordance with

international standards.

The SPS SIS accuracy standards given in the following tables apply to the SPS SIS from all

satellites regardless of whether they are occupying locations in the baseline/expandable 24-slot

constellation or not. These SPS SIS accuracy standards therefore apply equally to the SPS SIS

from baseline/expandable slot satellites and from auxiliary satellites.

Notes:

1. The accuracy performance standards do not apply beyond the defined bounds of SPS SIS

coverage (see Section 3.3).

2. The ergodic period contains the minimum number of samples such that the sample statistic is

representative of the population statistic. Under a one-upload-per-day scenario, for example, the

traditional approximation for the URE ergodic period is 30 days.

3. Normal operations and extended operations refer to two different GPS operating modes with

distinctly different accuracy levels. In the normal operations mode, the satellites are uploaded with

fresh NAV message data by the Control Segment on a regular basis. In the extended operations

April 2020 GPS SPS PS

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mode, one or more satellites are not uploaded with fresh NAV message data by the Control

Segment on a regular basis. See paragraph A.4.3.2 for additional information.

3.4.1 SPS SIS URE Accuracy Standards

The SPS SIS URE accuracy shall be as specified in Table 3.4-1.

Table 3.4-1. SPS SIS URE Accuracy Standards

SIS Accuracy Standard

Conditions and Constraints

Each SPS SIS Component Combination per

Table 2.2-2:

  7.0 m 95% Global Statistic URE during

Normal Operations over all AODs

  3.8 m 95% Global Statistic URE during

Normal Operations at Zero AOD

  9.7 m 95% Global Statistic URE during

Normal Operations at Any AOD

 For any trackable and healthy SPS SIS

 Neglecting SF ionospheric delay model

errors

 Including group delay time correction (T

)

errors at L1

 Including inter-signal bias (P(Y)-code to

C/A-code) errors at L1

 Including ISC errors

Each SPS SIS Component Combination per

Table 2.2-2:

  30 m 99.94% Global Statistic URE during

Normal Operations

  30 m 99.79% Worst Case Single Point

Statistic URE during Normal Operations

 For any trackable and healthy SPS SIS

 Neglecting SF ionospheric delay model

errors

 Including group delay time correction (T

)

errors at L1

 Including inter-signal bias (P(Y)-code to

C/A-code) errors at L1

 Including ISC errors

 Standard based on measurement interval

of one year; statistic of daily values within

the service volume

 Standard based on 3 service failures per

year, lasting no more than 6 hours each

SF C/A-Code:

  388 m 95% Global Statistic URE during

Extended Operations after 14 Days without

Upload

 For any trackable and healthy SPS SIS

Each SPS SIS Component Combination per

Table 2.2-2:

  2.0 m 95% Global Statistic URE during

Normal Operations over all AODs for the

ensemble of constellation slots

 Across all trackable and healthy SPS SISs

from satellites occupying constellation slots

 Neglecting SF ionospheric delay model

errors

 Including group delay time correction (T

)

errors at L1

 Including inter-signal bias (P(Y)-code to

C/A-code) errors at L1

 Including ISC errors

April 2020 GPS SPS PS

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Notes:

1. For SF URE, see Appendix A for information on how to factor in the SF ionospheric delay model

errors for L1. SF ionospheric delay model errors are specifically excluded from the SPS SIS URE

standard as an explicit constraint to emphasize they are neglected despite the fact that the SF

ionospheric delay model parameters are part of the broadcast SPS SIS.

2. The “over all AODs” performance standards are the ones which are the most directly representative

of the URE experienced by SPS receivers. See Appendix A for further information.

3. The ≤ 7.0 m 95% SPS SIS URE performance standard is statistically equivalent to a ≤ 3.6 m RMS

SPS SIS URE performance standard, assuming a normal distribution with zero mean.

4. Numerical values apply to the absolute value of the “95% global statistic URE”.

3.4.2 SPS SIS URRE Accuracy Standards

The SPS SIS URRE accuracy shall be as specified in Table 3.4-2.

Table 3.4-2. SPS SIS URRE Accuracy Standards

SIS Accuracy Standard

Conditions and Constraints

Each SPS SIS Component Combination per

Table 2.2-2:

  0.006 m/sec 95% Global Statistic URRE

over any 3-second interval during Normal

Operations at Any AOD

 For any trackable and healthy SPS SIS

 Neglecting all perceived pseudorange rate

errors attributable to pseudorange step

changes caused by NAV message data

cutovers

 Neglecting SF ionospheric delay model

errors

Notes:

1. The normal operations performance standards are consistent with an early GPS Block II clock

stability of 1x10

-11

at a tau of 3 seconds for either Rubidium clocks or Cesium clocks.

2. Root-sum-squaring the SIS-caused URRE with the receiver-caused pseudorange rate error, and

neglecting any correlated components, the combined pseudorange rate error perceived by the GPS

receiver is known as the User Equivalent Range Rate Error (UERRE).

3. Because all SIS components are coherently derived onboard each satellite, the URRE performance

standard applies equally to the transmitted code rate error and transmitted carrier rate error.

4. Numerical values apply to the absolute value of the “95% global statistic URRE”.

3.4.3 SPS SIS URAE Accuracy Standards

The SPS SIS URAE accuracy shall be as specified in Table 3.4-3.

April 2020 GPS SPS PS

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Table 3.4-3. SPS SIS URAE Accuracy Standards

SIS Accuracy Standard

Conditions and Constraints

Each SPS SIS Component Combination per

Table 2.2-2:

  0.002 m/sec/sec 95% Global Statistic

URAE over any 3-second interval during

Normal Operations at Any AOD

 For any trackable and healthy SPS SIS

 Neglecting all perceived pseudorange

acceleration errors attributable to

pseudorange step changes caused by

NAV message data cutovers

 Neglecting SF ionospheric delay model

errors

Notes:

1. The normal operations performance standards are consistent with an early GPS Block II clock

stability of 1x10

-11

at a tau of 3 seconds for either Rubidium clocks or Cesium clocks.

2. Root-sum squaring the SIS-caused URAE with the receiver-caused pseudorange acceleration

error, and neglecting any correlated components, the combined pseudorange acceleration error

perceived by the GPS receiver is known as the User Equivalent Range Acceleration Error

(UERAE).

3. Because all SIS components are coherently derived onboard each satellite, the URAE performance

standard applies equally to the transmitted code acceleration error and transmitted carrier

acceleration error.

4. Numerical values apply to the absolute value of the “95% global statistic URAE”.

3.4.4 SPS SIS UTCOE Accuracy Standards

The SPS SIS UTCOE accuracy shall be as specified in Table 3.4-4.

Table 3.4-4. SPS SIS UTCOE Accuracy Standards

SIS Accuracy Standard

Conditions and Constraints

Each SPS SIS Component Combination per

Table 2.2-2:

  30 ns 95% Global Statistic UTCOE during

Normal Operations at Any AOD

 For any trackable and healthy SPS SIS

Notes:

1. This is the accuracy of the UTC(USNO) offset data in the broadcast navigation message portion of

the SPS SIS which relates GPS time (as maintained by the Control Segment) to UTC (as

maintained by the U.S. Naval Observatory).

2. Root-sum squaring the UTCOE with a receiver’s solution accuracy for GPS time gives the total

UTC accuracy for that receiver. See Appendix B for further information.

3. Numerical values apply to the absolute value of the “95% global statistic UTCOE”.

April 2020 GPS SPS PS

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3.5 SPS SIS Integrity

This section provides the SPS SIS integrity standards. For a positioning/timing system, integrity

is defined as the trust which can be placed in the correctness of the positioning/timing information

provided by the system. Integrity includes the ability of that system to provide timely alerts when

it should not be used for positioning/timing. See Appendix A, Section A.5 for further definition.

The SPS SIS integrity standards given in the following tables apply to the SPS SIS from all

satellites regardless of whether they are occupying locations in the baseline/expandable 24-slot

constellation or not. These SPS SIS integrity standards therefore apply equally to the SPS SIS

from baseline/expandable slot satellites and from auxiliary satellites.

A timely alert is defined to be an alert provided at the GPS receiver antenna no later than 8

seconds after an instantaneous error exceeds the relevant NTE tolerance for any alert method

except SatZap and non-standard code (NSC). An additional 2 seconds is assumed for the GPS

receiver response time. For SatZap and NSC, a timely alert is defined to be an alert provided at

the GPS receiver antenna no later than 10 seconds after an instantaneous error exceeds the

relevant NTE tolerance.

3.5.1 SPS SIS Instantaneous URE Integrity Standards

The SPS SIS instantaneous URE integrity shall be as specified in Table 3.5-1.

Table 3.5-1. SPS SIS Instantaneous URE Integrity Standards

SIS Integrity Standard

Conditions and Constraints

Each SPS SIS Component Combination per

Table 2.2-2:

  1x10

-5

Probability Over Any Hour of the

SPS SIS Instantaneous URE Exceeding the

NTE Tolerance Without a Timely Alert

 Applies to any trackable and healthy SPS

SIS

 SPS SIS URE NTE tolerance defined to be

4.42 times the relevant IAURA value

currently broadcast by the satellite

 Given that the maximum SPS SIS

instantaneous URE did not exceed the

NTE tolerance at the start of the hour

 UMSI occurs if no timely alert issued after

SPS SIS URE NTE tolerance exceeded

 Worst case for delayed alert is 6 hours

 Neglecting SF ionospheric delay model

errors

For the maximum possible 32 satellites in the current broadcast LNAV almanac

(baseline/expandable slot satellites plus auxiliary satellites), the corresponding average annual

number of SPS SIS instantaneous URE integrity losses is 3. Assuming each of these 3 losses of

SPS SIS integrity lasts for no more than 6 hours, the equivalent worst-case probability of users

experiencing UMSI is 18 hours divided by 8760 hours or 0.002. For an average 1-hour loss of

SPS SIS instantaneous URE integrity, recognizing that each satellite is only visible to a third of

the Earth at any one time, the equivalent average probability of users experiencing UMSI is

approximately 1 hour divided by 8760 hours or 0.0001.

April 2020 GPS SPS PS

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Notes:

1. In 1 year, 32 satellites each continuously transmitting a trackable and healthy SPS SIS will

accumulate a total of approximately 2.8x10

hours of operation. For a probability of 1x10

-5

/hour of

maintaining integrity, the expected number of losses of SPS SIS integrity in 1 year across the entire

constellation is approximately 3.

2. The worst-case probability of a user experiencing UMSI assumes that each of the 3 losses of SPS

SIS integrity results in UMSI for 6 hours and that the worst-case user is unlucky enough to be using

each of the 3 satellites during the full duration of each loss of SPS SIS integrity.

3. The duration statistics for losses of SPS SIS integrity are: (a) worst case ≤ 6 hours, (b) 99.9

percentile ≤ 3 hours, (c) 50

percentile ≤ 1 hour.

3.5.2 SPS SIS Instantaneous URRE Integrity Standards

The SPS SIS instantaneous URRE (i.e., pseudorange rate error) integrity shall be as specified in

Table 3.5-2.

Table 3.5-2. SPS SIS Instantaneous URRE Integrity Standards

SIS Integrity Standard

Conditions and Constraints

Each SPS SIS Component Combination per

Table 2.2-2:

 No Integrity Performance Specified

 A future edition of this SPS PS may

establish a standard

Notes:

1. Although there is no SPS SIS URRE NTE tolerance defined, a high-probability (6-sigma) upper

bound on the SPS SIS Instantaneous URRE which is typically used for design purposes is 0.02

m/sec over any 3-second interval during normal operations at any AOD. This is consistent with an

early Block II clock stability of 1x10

-11

at a tau of 3 seconds for either Rubidium clocks or Cesium

clocks.

2. Short-term fluctuations in the ionosphere can produce very large SPS SIS instantaneous URREs

for SF C/A-Code operations.

3. Instantaneous URREs due to upload data cutovers and data set pageovers last for less than 3

seconds.

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3.5.3 SPS SIS Instantaneous URAE Integrity Standards

The SPS SIS instantaneous URAE (i.e., pseudorange acceleration error) integrity shall be as

specified in Table 3.5-3.

Table 3.5-3. SPS SIS Instantaneous URAE Integrity Standards

SIS Integrity Standard

Conditions and Constraints

Each SPS SIS Component Combination per

Table 2.2-2:

 No Integrity Performance Specified

 A future edition of this SPS PS may

establish a standard

Notes:

1. Although there is no SPS SIS URAE NTE tolerance defined for SF operation, a high-probability

upper bound on the SPS SIS Instantaneous URAE which is typically used for design purposes is

0.007 m/sec/sec over any 3-second interval during normal operations at any AOD. This is

consistent with an early Block II clock stability of 1x10

-11

at a tau of 3 seconds for either Rubidium

clocks or Cesium clocks.

2. Short-term fluctuations in the ionosphere can produce extremely large SPS SIS instantaneous

URAEs for SF operations.

3. Instantaneous URAEs due to upload data cutovers and data set pageovers last for less than 3

seconds.

3.5.4 SPS SIS Instantaneous UTCOE Integrity Standards

The SPS SIS instantaneous UTCOE (i.e., UTC(USNO) offset error) integrity shall be as specified

in Table 3.5-4.

Table 3.5-4. SPS SIS Instantaneous UTCOE Integrity Standards

SIS Integrity Standard

Conditions and Constraints

Each SPS SIS Component Combination per

Table 2.2-2:

  1x10

-5

Probability Over Any Hour of the

SPS SIS Instantaneous UTCOE Exceeding

the NTE Tolerance Without a Timely Alert

during Normal Operations

 Applies to any trackable and healthy SPS

SIS

 SPS SIS UTCOE NTE tolerance defined to

be 120 ns

 Given that the maximum SPS SIS

instantaneous URE did not exceed the

NTE tolerance at the start of the hour

 Worst case for delayed alert is 6 hours

Notes:

1. This is the integrity of the UTC(USNO) offset data in the broadcast navigation message portion of

the SPS SIS which relates GPS time (as maintained by the Control Segment) to UTC (as

maintained by the U.S. Naval Observatory).

April 2020 GPS SPS PS

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2. Adding the UTCOE to a receiver’s solution error for GPS time gives the total UTC error for that

receiver. See Appendix B for further information.

3.5.5 P

sat

and P

const

Standards

The SPS SIS instantaneous P

sat

and P

const

shall be as specified in Table 3.5-5.

Table 3.5-5. SPS SIS Instantaneous P

sat

and P

const

Standards

SIS Integrity Standard

Conditions and Constraints

Each SPS SIS Component Combination per

Table 2.2-2:

  1x10

-5

Fraction of Time When the SPS SIS

Instantaneous URE Exceeds the NTE

Tolerance Without a Timely Alert (P

sat

)

 Applies to any trackable and healthy SPS

SIS

 SPS SIS URE NTE tolerance defined to be

4.42 times the relevant IAURA value

currently broadcast by the satellite

 Average case for delayed alert is 1 hour

 Neglecting SF ionospheric delay model

errors

Each SPS SIS Component Combination per

Table 2.2-2:

  1x10

-8

Fraction of Time When the SPS SIS

Instantaneous URE from Two or More

Satellites Exceeds the NTE Tolerance Due to

a Common Cause Without a Timely Alert

const

)

 Applies across all trackable and healthy

SPS SIS

 SPS SIS URE NTE tolerance defined to be

4.42 times the relevant IAURA value

currently broadcast by the satellite

 Average case for delayed alert is 1 hour

 Neglecting SF ionospheric delay model

errors

Notes:

1. The difference between the P

sat

in Table 3.5-5 and the probability of UMSI (P(UMSI)) in Table 3.5-

1 is the duration.

2. Common cause faults are assumed to have common mode effects.

3. Per satellite fault modes are assumed to be independent with high probability.

April 2020 GPS SPS PS

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3.6 SPS SIS Continuity

This section provides the SPS SIS continuity standards. The SPS SIS continuity for a trackable

and healthy SPS SIS is the probability that the SPS SIS will continue to be healthy and trackable

without unscheduled interruption over a specified time interval.

Planned interruptions of the SPS are subject to a minimum of 48-hour advance notice provided

by the Control Segment to the Coast Guard Navigation Center and the FAA Notice to Airmen

(NOTAM) system (e.g., scheduled satellite maintenance). An interruption is defined as a period

in which the SIS from a satellite does not comply with the standards defined in this SPS PS. A

scheduled interruption is defined as a period announced at least 48 hours in advance in which

the SIS from a satellite is not planned to comply with the standards defined in this SPS PS.

Unscheduled interruptions resulting from system malfunctions or maintenance occurring outside

the scheduled period will be announced to the Coast Guard and the FAA as soon as possible.

Scheduled interruptions which are announced at least 48 hours in advance do not constitute a

loss of continuity.

3.6.1 SPS SIS Continuity Standards – Unscheduled Failure Interruptions

The SPS SIS continuity shall be as specified in Table 3.6-1 for the composite of all unscheduled

interruptions in service (long-term [LT] hard failures, short term [ST] hard failures, soft failures, as

well as end-of-life [EOL] failures and satellite O&M activity interruptions without a timely NANU;

see Section A.6).

Table 3.6-1. SPS SIS Unscheduled Failure Interruption Continuity Standards

SIS Continuity Standard

Conditions and Constraints

Unscheduled Failure Interruptions of SF C/A-

Code:

  0.9998 Probability Over Any Hour of Not

Losing the SPS SIS Availability from a Slot

Due to Unscheduled Interruption

 Calculated as an average over all slots in

the 24-slot constellation, normalized

annually

 Given that SF C/A-Code is available from

the slot at the start of the hour

 NANU timeliness standard per Table 3.6-3

Note:

1. At this point in time, there are no unscheduled failure interruption continuity standards for any SPS

SIS component combination per Table 2.2-2 other than SF C/A-code.

3.6.2 SPS SIS Continuity Standards – Unscheduled Maintenance Interruptions

The SPS SIS continuity shall be as specified in Table 3.6-2 for the composite of all unscheduled

maintenance interruptions in service (e.g. planned EOL failure events and satellite O&M activity

interruptions; see Section A.6).

April 2020 GPS SPS PS

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Table 3.6-2. SPS SIS Unscheduled Maintenance Interruption Continuity Standards

SIS Continuity Standard

Conditions and Constraints

Unscheduled Maintenance Interruptions:

 No Performance Specified

 A future edition of this SPS PS may

establish a standard

Note:

1. Table 3.6-2 is effectively a placeholder for a future standard that defines the level of rigor the Control

Segment must maintain in issuing the NANUs.

3.6.3 Status and Problem Reporting Standards

The commitments for maintaining continuity result in support for status and problem

reporting standards as presented in Table 3.6-3.

Table 3.6-3. SPS Status and Problem Reporting Standards

Status and Problem Reporting Standard

Conditions and Constraints

Scheduled Event Affecting Service

 Appropriate NANU issued to the Coast Guard

and the FAA at least 48 hours prior to the event

for 95% of the events

 For any SPS SIS Component Combination

per Table 2.2-2

Unscheduled Outage or Problem Affecting

Service

 Appropriate NANU issued to the Coast Guard

and the FAA as soon as possible after the event

 For any SPS SIS Component Combination

per Table 2.2-2

Notes:

1. A CS internal goal (not a performance standard) is to issue the NANU 96 hours prior to the event.

2. NANUs not issued at least 48 hours prior to a scheduled event affecting service means the

scheduled event counts against the “Unscheduled Failure Interruption Continuity Standards” per

paragraph 3.6.1.

April 2020 GPS SPS PS

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3.7 SPS SIS Availability

This section provides the SPS SIS availability standards.

There are two components of SPS SIS availability: (1) the per-slot availability, and (2) the

constellation availability. These two components are related. The per-slot availability depends

primarily on the satellite design and the Control Segment procedures for on-orbit maintenance

and failure response. The constellation availability depends primarily on the per-slot availability

coupled with the satellite launch policies and satellite disposal criteria.

Each component of SPS SIS availability shall be as specified below. These availability standards

apply to satellites broadcasting PRN codes numbered in the range of 1 to 32. Satellites

broadcasting higher numbered PRN codes (“expanded PRN codes”) are not eligible for satisfying

these availability standards due to backward compatibility limitations.

This section also provides the operational satellite count standard.

Notes:

1. The term "operational satellite" applies to any satellite which appears in the current almanac data

(e.g., subframe 4 or 5 of the LNAV message). This designation is used even when a satellite’s SIS

is untrackable or is set unhealthy. It applies to nearly every satellite in the constellation.

2. At this point in time, there are no SPS SIS availability standards for any SPS SIS component

combination per Table 2.2-2 other than SF C/A-code.

3. A slot is occupied by a satellite when the satellite’s footprint on the surface of the Earth overlaps

95% of the slot center’s footprint on the surface of the Earth averaged over an orbit revolution.

4. Satellites operating satisfactorily beyond their design life may be administratively declared as

occupying their assigned slot even if the satellite’s footprint on the Earth’s surface fails to overlap

95% of the slot center’s footprint on the Earth’s surface averaged over an orbit revolution.

3.7.1 SPS SIS Per-Slot Availability Standards

The SPS SIS per-slot availability shall be as specified in Table 3.7-1 for slots in either the baseline

24-slot constellation configuration or the expandable 24-slot constellation configuration.

Table 3.7-1. SPS SIS Per-Slot Availability Standards

SIS Availability Standard

Conditions and Constraints

  0.957 Probability that a Slot in the Baseline

24-Slot Configuration will be Occupied by a

Satellite Broadcasting a Healthy SF C/A-

Code SPS SIS

  0.957 Probability that a Slot in the

Expanded Configuration will be Occupied by

a Pair of Satellites Each Broadcasting a

Healthy SF C/A-Code SPS SIS

 Calculated as an average over all slots in

the 24-slot constellation, normalized

annually

 Applies to satellites broadcasting a healthy

SF C/A-Code SPS SIS which also satisfy

the other performance standards in this

SPS PS

April 2020 GPS SPS PS

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Notes:

1. The SPS SIS availability standards given in Tables 3.7-1 apply to the SPS SIS from all slots in the

24-slot constellation.

2. Expanded slot availability includes occupancy by a pair of satellites in an equivalent-or-better non-

standard configuration. See Section A.7 in Appendix A for example equivalent-or-better non-

standard configurations.

3. The loss of availability caused by an expanded slot which has lost one of its pair of satellites can

be remedied by either replacing the lost satellite or by returning the slot back to its baseline

configuration. See Section A.7 in Appendix A for further information.

4. These SPS SIS availability standards do not apply to auxiliary satellites not occupying a slot in the

24-slot constellation.

5. The SPS SIS per-slot availability measurement interval must be substantially longer than the mean

time between per-slot interruptions and the mean time to restore per-slot availability.

3.7.2 SPS SIS Constellation Availability Standards

The SPS SIS constellation availability shall be as specified in Table 3.7-2 for the ensemble of all

24 slots in either the baseline 24-slot configuration or the expandable 24-slot configuration.

Table 3.7-2. SPS SIS Constellation Availability Standards

SIS Availability Standard

Conditions and Constraints

  0.98 Probability that at least 21 Slots out of

the 24 Slots will be Occupied Either by a

Satellite Broadcasting a Healthy SF C/A-

Code SPS SIS in the Baseline 24-Slot

Configuration or by a Pair of Satellites Each

Broadcasting a Healthy SF C/A-Code SPS

SIS in the Expanded Slot Configuration

  0.99999 Probability that at least 20 Slots

out of the 24 Slots will be Occupied Either by

a Satellite Broadcasting a Healthy SF C/A-

Code SPS SIS in the Baseline 24-Slot

Configuration or by a Pair of Satellites Each

Broadcasting a Healthy SF C/A-Code SPS

SIS in the Expanded Slot Configuration

 Calculated as an average over all slots in

the 24-slot constellation, normalized

annually

 Applies to satellites broadcasting a healthy

SF C/A-Code SPS SIS which also satisfies

the other performance standards in this

SPS PS

Notes:

1. So long as at least 21 slots out of the 24 slots are occupied either by a satellite broadcasting a

trackable and healthy SPS SIS in the baseline 24-slot configuration or by a pair of satellites each

broadcasting a trackable and healthy SPS SIS in the expanded slot configuration, the all-in-view

position dilution of precision (PDOP) with a mask angle of 5 degrees will be 6 or less for: a) 98%

of the global average over any sidereal day, and b) 88% at the worst site over any sidereal day.

See Appendix A for further information.

April 2020 GPS SPS PS

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2. Expandable slots occupied by a pair of satellites in the expanded configuration can provide more

robust constellation availability to enhance the overall SPS SIS performance. However, since there

are no standards given in this SPS PS for the probabilities of the expandable slots being in their

expanded configurations and occupied by pairs of satellites, no credit can be taken for them relative

to the baseline 24-slot constellation availability with all slots in their baseline configuration.

3. The SPS SIS constellation availability measurement interval must be substantially longer than the

mean time between constellation interruptions and the mean time to restore constellation

availability.

3.7.3 Operational Satellite Count Standards

The total number of operational satellites in the constellation shall be as specified in Table 3.7-3.

Table 3.7-3. Operational Satellite Count Standards

Operational Satellite Count Standard

Conditions and Constraints

  0.95 Probability that the Constellation will

Have at least 24 Operational Satellites

Regardless of Whether Those Operational

Satellites are Located in Slots or Not

 Applies to the total number of operational

satellites in the constellation (averaged

over any day); where any satellite which

appears in the transmitted navigation

message almanac is defined to be an

operational satellite regardless of whether

that satellite is currently broadcasting a

trackable and healthy SPS SIS or not and

regardless of whether the broadcast SPS

SIS also satisfies the other performance

standards in this SPS PS or not

April 2020 GPS SPS PS

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3.8 SPS Position/Time Domain Standards

This section provides the SPS position/time domain performance standards. SPS position/time

domain performance standards are a key element of backward compatibility and are based on a

specific set of receiver assumptions. The user assumptions include the error exclusions identified

in paragraph 2.4.5 as well as the following SPS receiver assumptions.

The use of a representative SPS receiver that:

• is designed in accordance with IS-GPS-200.

• is tracking the SF C/A-code SPS SIS from all satellites in view above a 5° mask angle with

respect to the local horizon (no local obscura are considered). It is assumed the receiver

is operating in a nominal noise environment that does not interrupt receiver acquisition

and tracking capabilities.

• accomplishes satellite position and geometric range computations in the most current

realization of the World Geodetic System 1984 (WGS 84) Earth-Centered, Earth-Fixed

(ECEF) coordinate system.

• generates a position/time domain solution from data broadcast by all satellites in view

transmitting PRNs 1-32.

• compensates for dynamic Doppler shift effects on nominal SPS ranging signal carrier

phase and C/A-code measurements.

• processes the health-related information in the SIS and excludes marginal and unhealthy

SISs from the position solution.

• ensures up-to-date and internally consistent CEI and UTC offset (UTCO) data within the

respective curve fit intervals for all satellites used in the position solution.

• generates range rate measurements using carrier phase tracking (for velocity solution).

• loses track in the event a GPS satellite stops transmitting a trackable SIS.

• is operating at a surveyed location (for a time transfer receiver).

The SPS position/time domain performance standards are defined for a position/time solution

meeting the representative receiver conditions and operating within the service volume over any

sidereal day (approximately 24-hour interval). For information regarding different receiver

assumptions, see Appendix B.

April 2020 GPS SPS PS

5th Edition Page 57

3.8.1 PDOP Availability Standards

The commitments for maintaining the constellation (Sections 3.2 and 3.7) and coverage (Section

3.3) result in support for position dilution of precision (PDOP) standards presented in Table 3.8-

Table 3.8-1. PDOP Availability Standards

PDOP Availability Standard

Conditions and Constraints

≥ 98% global PDOP of 6 or less

≥ 88% worst site PDOP of 6 or less

3.8.2 SPS Position Availability Standards

The commitments for maintaining PDOP (Table 3.8-1) and SPS SIS URE accuracy (Table 3.4-1)

result in support for position availability standards as presented in Table 3.8-2.

Table 3.8-2. SPS Position Availability Standards

Position Availability Standard

Conditions and Constraints

≥ 99% Horizontal Service Availability, average

location

≥ 99% Vertical Service Availability, average

location

 15 m horizontal (SIS only) 95% threshold

 33 m vertical (SIS only) 95% threshold

≥ 90% Horizontal Service Availability, worst-

case location

≥ 90% Vertical Service Availability, worst-case

location

 15 m horizontal (SIS only) 95% threshold

 33 m vertical (SIS only) 95% threshold

April 2020 GPS SPS PS

5th Edition Page 58

3.8.3 SPS Position/Velocity/Time Accuracy Standards

The commitments for maintaining PDOP (Table 3.8-1), SPS SIS URE accuracy (Table 3.4-1),

and SPS SIS URRE accuracy (Table 3.4-2) result in support for position/velocity/time accuracy

standards as presented in Table 3.8-3.

Table 3.8-3. SPS Position/Velocity/Time Accuracy Standards

Position/Time Accuracy Standard

Conditions and Constraints

Global Average Position Accuracy

 ≤ 8 m 95% Horizontal Error

 ≤ 13 m 95% Vertical Error

 Defined for a position/time solution meeting

the representative user conditions

 Position/time solution is available

Worst Site Position Accuracy

 ≤ 15 m 95% Horizontal Error

 ≤ 33 m 95% Vertical Error

 Defined for a position/time solution meeting

the representative user conditions

 Position/time solution is available

Global Average Velocity Accuracy

 ≤ 0.2 m/sec 95% velocity error, any axis

 Defined for a position/velocity/time solution

meeting the representative user conditions

 Position/velocity/time solution is available

Time Transfer Accuracy

 ≤ 30 ns Time Transfer error 95% of time (SIS

only)

 Defined for a time transfer solution meeting

the representative user conditions

 Time transfer solution is available

April 2020 GPS SPS PS

5th Edition Page 59

SECTION 4.0 References

This section identifies the Government documents and non-Government documents explicitly

referenced in or related to the content of this SPS PS.

4.1 Government Documents

SPECIFICATIONS:

Federal

8 December 1993 Global Positioning System Standard Positioning Service

Signal Specification, (1

Edition)

2 June 1995 Global Positioning System Standard Positioning Service

Signal Specification, 2

Edition)

4 October 2001 Global Positioning System Standard Positioning Service

Performance Standard, 3

Edition

23 February 2007 Global Positioning System Precise Positioning Service

Performance Standard, 1

Edition

September 2008 Global Positioning System Standard Positioning Service

Performance Standard, 4

Edition

Military

None

Program

SS-GPS-300G System Specification for the Navstar Global Positioning

5 August 2005 System

Other Government Activity

10 USC 2281 Title 10 United States Code, Section 2281

NIMA TR8350.2 Department of Defense

3 January 2000 World Geodetic System 1984

Its Definition and Relationship with Local Geodetic Systems

STANDARDS:

Federal

TSO-C145e Technical Standard Order (TSO), Airborne Navigation

5 May 2017 Sensors Using the Global Positioning System Augmented

By The Satellite Based Augmentation System (SBAS)

April 2020 GPS SPS PS

5th Edition Page 60

TSO-C146e Technical Standard Order (TSO), Stand-Alone Airborne

5 May 2017 Navigation Equipment Using the Global Positioning System

Augmented by The Satellite Based Augmentation System

(SBAS)

TSO-C196b Technical Standard Order (TSO), Airborne Supplemental

20 December 2013 Navigation Sensors for Global Positioning System

Equipment Using Aircraft-Based Augmentation

Global Positioning System Wide Area Augmentation

31 October 2008 System (WAAS) Performance Standard, 1

Edition

Military

MSO-C129b Military Standard Order (MSO), Airborne Supplemental

13 October 2005 Navigation Equipment Using the Global Positioning System

(GPS) / Precise Positioning Service (PPS)

MSO-C145b Military Standard Order (MSO), Airborne Navigation

12 August 2016 Sensors Using the Global Positioning System (GPS) /

Precise Positioning Service (PPS) for Area Navigation (RNAV)

in Required Navigation Performance (RNP) Airspace and

for Automatic Dependent Surveillance- Broadcast (ADS-B)

Program

IS-GPS-200 Navstar GPS Space Segment / Navigation User

Current Revision Interfaces

IS-GPS-200H Navstar GPS Space Segment / Navigation User

24 September 2013 Interfaces

IS-GPS-200G Navstar GPS Space Segment / Navigation User

5 September 2012 Interfaces

IS-GPS-200F Navstar GPS Space Segment / Navigation User

21 September 2011 Interfaces

IS-GPS-200E Navstar GPS Space Segment / Navigation User

8 June 2010 Interfaces

IS-GPS-200D Navstar GPS Space Segment / Navigation User

7 December 2004 Interfaces

ICD-GPS-200C Navstar GPS Space Segment / Navigation User

10 October 1993 Interfaces

IS-GPS-705 Navstar GPS Space Segment / User Segment L5

Current Revision Interfaces

Other Government Activity

April 2020 GPS SPS PS

5th Edition Page 61

OTHER PUBLICATIONS:

Manuals

None

Regulations

None

Plans

DOT-VNTSC-OST-R-15-01 2017 Federal Radionavigation Plan (FRP)

20 September 2017

Miscellaneous

28 December 2001 Headquarters Air Force Space Command

Concept of Operations for the Global Positioning System

(Also known as the “GPS CONOPS”)

Tech Report No. 96 U.S. Air Force Aeronautical Chart and Information Center,

February 1962 Principles of Error Theory and Cartographic

Applications

Publication No. 9 N. Bowditch; The American Practical Navigator, an Epitome

2002 of Navigation; 2002 Bicentennial Edition; prepared and

published by the National Imagery and Mapping Agency;

Bethesda, MD

AC 20-138D Federal Aviation Administration Advisory Circular (AC)

Change 2 Airworthiness Approval of Positioning and

4 April 2016 Navigation Systems

AC 90-96A Federal Aviation Administration Advisory Circular (AC)

Change 1 Approval of U.S. Operators and Aircraft to Operate Under

12 November 2010 Instrument Flight Rules (IFR) in European Airspace d

Designate for Basic Area Navigation (BRNAV) and

Precision Area Navigation (PRNAV)

TOR S3-G-89-01 Technical Operating Report (TOR)

23 March 1989 Navstar Global Positioning System Standardized Accuracy

Definitions and Relationships

April 2020 GPS SPS PS

5th Edition Page 62

4.2 Non-Government Documents

SPECIFICATIONS:

None

STANDARDS:

SARPs Annex 10 International Civil Aviation Organization (ICAO)

Amendment 89 Annex 10 to the Convention on International Civil Aviation,

13 November 2014 International Standards and Recommended Practices,

Aeronautical Telecommunications, Volume 1,

Radio Navigation Aids

RTCA/DO-229E RTCA Document, Special Committee 159

15 December 2016 Minimum Operational Performance Standards for Global

Positioning System/Wide Area Augmentation System

Airborne Equipment

RTCA/DO-236C RTCA Document, Special Committee 181

23 September 2014 Minimum Aviation System Performance Standards:

Required Navigation Performance for Area Navigation

RTCA/DO-316 RTCA Document, Special Committee 159

14 April 2009 Minimum Operational Performance Standards for Global

Positioning System/Aircraft Based Augmentation System

Airborne Equipment

RTCM Paper 136-2001/SC104-STD RTCM Document, Special Committee 104

August 2001 RTCM Recommended Standards for Differential

Navstar GS Service, Version 2.3

OTHER PUBLICATIONS:

[No Number] M. Ananda, J. Leung, P. Munjal, and B. Siegel;

September 1994 RAIM Detection and Isolation Integrity Availability With

and Without CAG; Proceedings of ION GPS-94, the 7

International Technical Meeting of the Satellite Division of

the Institute of Navigation, Salt Lake City, UT

[No Number] P. Massatt and M. Zeitzew; The GPS Constellation

January 1998 Design – Current and Projected; Proceedings of the 1998

National Technical Meeting of the Institute of Navigation,

Long Beach, CA

A.953(23) International Maritime Organization (IMO)

26 February 2004 World-wide Radionavigation System

April 2020 GPS SPS PS

5th Edition

GLOBAL POSITIONING SYSTEM

STANDARD POSITIONING SERVICE

PERFORMANCE STANDARD

APPENDIX A

SPS SIGNAL-IN-SPACE (SIS)

BACKGROUND INFORMATION

April 2020

Integrity - Service - Excellence

April 2020 GPS SPS PS

5th Edition

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April 2020 GPS SPS PS

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TABLE OF CONTENTS

SECTION A.1 Introduction ....................................................................................... A-1

A.1.1 Scope ...................................................................................................................................... A-1

A.1.2 Limitations .............................................................................................................................. A-1

SECTION A.2 Constellation ..................................................................................... A-2

A.2.1 Relationship with Section 3.2 ................................................................................................. A-2

A.2.2 Baseline 24-Slot Constellation Configuration .......................................................................... A-2

A.2.3 Expandable 24-Slot Constellation Configuration ..................................................................... A-3

A.2.4 Propagating the Constellation over Time ................................................................................ A-4

SECTION A.3 Coverage ........................................................................................... A-7

A.3.1 Relationship with Section 3.3 ................................................................................................. A-7

A.3.2 Per-Satellite Coverage ............................................................................................................ A-7

A.3.2.1 Satellite Footprint ............................................................................................................ A-7

A.3.2.2 Mask Angles .................................................................................................................... A-7

A.3.3 Constellation Coverage ........................................................................................................... A-8

A.3.3.1 Seamless Coverage ........................................................................................................ A-8

A.3.3.2 Terrestrial Service Volume .............................................................................................. A-8

A.3.3.3 Space Service Volume .................................................................................................... A-8

SECTION A.4 Accuracy ............................................................................................ A-9

A.4.1 Relationship with Section 3.4 ................................................................................................. A-9

A.4.2 Reserved ................................................................................................................................. A-9

A.4.3 Time Dependency ................................................................................................................... A-9

A.4.3.1 Graceful Degradation ...................................................................................................... A-9

A.4.3.2 Normal Operations vs. Extended Operations ................................................................. A-9

A.4.3.3 Variations During Normal Operations ............................................................................. A-10

A.4.3.4 Accuracy at Time of Upload ............................................................................................ A-13

A.4.4 Contingency Uploads .............................................................................................................. A-14

A.4.5 UERE Budgets ........................................................................................................................ A-14

A.4.6 URE Over All AODs................................................................................................................. A-16

A.4.7 URE Time Derivative Accuracies ............................................................................................ A-17

A.4.8 UTC(USNO) Offset Accuracy .................................................................................................. A-19

A.4.9 Single-Frequency Ionospheric Delay Model Errors ................................................................. A-19

A.4.10 Group Delay Time Correction (T

) Errors ............................................................................ A-20

A.4.11 Spatial Dependency .............................................................................................................. A-20

A.4.12 Inter-Signal Bias (ISB) & Inter-Signal Correction (ISC) Errors .............................................. A-23

April 2020 GPS SPS PS

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SECTION A.5 Integrity ............................................................................................ A-25

A.5.1 Relationship with Section 3.5 ............................................................................................... A-25

A.5.2 URA Relationship to Integrity ................................................................................................ A-25

A.5.3 IAURA-Derived Integrity Tolerance ....................................................................................... A-26

A.5.4 Nature of SPS SIS URE ........................................................................................................ A-27

A.5.4.1 Integrity Failure Modes and Effects............................................................................... A-27

A.5.4.2 Control Segment Monitoring and Intervention .............................................................. A-29

A.5.4.3 Control Segment Preemptive Actions ........................................................................... A-30

A.5.4.4 Satellite On-Board Monitoring and Intervention ............................................................ A-31

A.5.5 Timely Alert Considerations .................................................................................................. A-32

A.5.6 Nature of SPS SIS URE Time Derivatives ............................................................................ A-33

A.5.6.1 URE Time Derivative Illustrations for Integrity .............................................................. A-33

A.5.6.2 URE Second Time Derivative Illustrations .................................................................... A-34

A.5.6.3 Combinations of URE Time Derivatives ........................................................................ A-36

A.5.7 SPS SIS Component Combination Integrity Considerations ................................................ A-36

A.5.7.1 Per ‘SPS SIS Component Combination’ Integrity Accounting ...................................... A-36

A.5.7.2 Independence of SPS SIS Component Combination Integrity ..................................... A-38

SECTION A.6 Continuity ........................................................................................ A-39

A.6.1 Relationship with Section 3.6 ............................................................................................... A-39

A.6.2 Various Types of Failures and the Impacts on Continuity ..................................................... A-39

A.6.2.1 Hard Failures ................................................................................................................. A-39

A.6.2.2 Wear-Out Failures ......................................................................................................... A-40

A.6.2.3 Soft Failures .................................................................................................................. A-41

A.6.2.4 Satellite O&M Activities ................................................................................................. A-41

A.6.3 Losses of Continuity .............................................................................................................. A-42

A.6.4 Expected Frequencies for Losses of Continuity .................................................................... A-42

A.6.5 Expandable Slot Continuity ................................................................................................... A-43

SECTION A.7 Availability ....................................................................................... A-44

A.7.1 Relationship with Section 3.7 ............................................................................................... A-44

A.7.2 Per-Slot Availability................................................................................................................ A-44

A.7.2.1 Satellite Outage Categories .......................................................................................... A-44

A.7.2.2 Conservative Satellite/Slot Availability Model ............................................................... A-44

A.7.2.3 Satellite/Slot Availability Computation ........................................................................... A-45

A.7.2.4 Expandable Slot Availability .......................................................................................... A-46

A.7.2.5 Equivalent-or-Better Non-Standard Expanded Slot Configurations .............................. A-46

A.7.3 Constellation Availability ........................................................................................................ A-49

A.7.4 Slot Occupancy ..................................................................................................................... A-49

SECTION A.8 Position/Time Domain .................................................................... A-52

A.8.1 Relationship with Section 3.8 ............................................................................................... A-52

A.8.2 Availability of Geometry (and PVT Determination) ................................................................ A-52

A.8.3 Availability of Geometry Impacts Due to Expandable24 Slots .............................................. A-53

A.8.4 Availability of Geometry Impacts Due to Auxiliary Satellites ................................................. A-53

A.8.5 Position/Time Domain Time Derivatives ............................................................................... A-53

April 2020 GPS SPS PS

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List of Figures

Figure A.3-1 Illustration of Satellite Footprint ......................................................................................... A-7

Figure A.3-2 Illustration of Terrestrial Service Volume .......................................................................... A-8

Figure A.4-1 UERE Graceful Degradation ............................................................................................. A-10

Figure A.4-2a UERE as a Function of AOD, One-Upload-Per-Day Scenario ....................................... A-11

Figure A.4-2b UERE as a Function of AOD, Three-Uploads-Per-Day Scenario ................................... A-11

Figure A.4-3 Statistical PPS SIS URE as a Function of AOD, One-Upload-Per-Day Scenario ............ A-12

Figure A.4-4 Instantaneous SPS SIS URE as a Function of Time ........................................................ A-12

Figure A.4-5a Instantaneous SPS SIS URE as a Function of Time ...................................................... A-18

Figure A.4-5b Instantaneous SPS SIS URRE as a Function of Time ................................................... A-18

Figure A.4-5c Instantaneous SPS SIS URAE as a Function of Time .................................................... A-18

Figure A.4-6 Illustration of Spatial Dependency - Horizontal Orbit Error ............................................... A-21

Figure A.4-7 Illustration of Spatial Dependency - Radial Orbit Error ..................................................... A-22

Figure A.5-1 Types of URE Effects ........................................................................................................ A-28

Figure A.5-2 Types of URE/URRE Effects ............................................................................................. A-34

Figure A.5-3 Types of URE/URAE Effects ............................................................................................. A-35

Figure A.5-4 Combined URE Time Derivative Examples ...................................................................... A-37

Figure A.7-1 Illustration of Nominal and Expanded Slot Configurations ................................................ A-46

Figure A.7-2 Illustration of Equivalent-or-Better Non-Standard Configurations (1 of 2) ........................ A-47

Figure A.7-2 Illustration of Equivalent-or-Better Non-Standard Configurations (2 of 2) ........................ A-48

Figure A.7-3 Nominal Slot Ellipse .......................................................................................................... A-50

List of Tables

Table A.2-1 Baseline 24-Slot Constellation Almanac, at GPS Epoch of 0000 on 1 Jan 17 .................. A-2

Table A.2-2 Expandable 24-Slot Constellation Almanac, at GPS Epoch of 0000 on 1 Jan 17 ............. A-3

Table A.2-3 Baseline 24-Slot Constellation at 00:00:00 UTC 1 Jan 17 ................................................. A-4

Table A.2-4 Reference Orbit Parameters .............................................................................................. A-5

Table A.2-5 Baseline 24-Slot Constellation at 00:00:00 UTC 1 Jan 18 ................................................. A-6

Table A.4-1 DF L1 C/A – L2C UERE Budget ......................................................................................... A-15

Table A.4-2 SF L1 C/A-Code UERE Budget .......................................................................................... A-16

Table A.4-3 Reserved ............................................................................................................................ A-16

Table A.4-4 Reserved ............................................................................................................................ A-16

Table A.5-1 LNAV URA Index to Expected URE Relationship .............................................................. A-25

Table A.5-2 Normal Distribution Bounds vs. Probability of Exceeding Those Bounds .......................... A-26

Table A.5-3 Potential Integrity Failure Modes ........................................................................................ A-28

Table A.7-1 Per-Satellite/Slot Availability Model Parameters for Baseline Slots ................................... A-45

Table A.7-2 Constellation Availability ..................................................................................................... A-49

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SECTION A.1 Introduction

A.1.1 Scope

This appendix provides further background information on the SPS SIS and its performance

standards. The performance standards given in Section 3 of this SPS PS, along with the

referenced ISs, comprise a full and complete description of the SPS SIS interface provided to the

User Segment's SPS receivers. The background information in this appendix serves to place

those standards into context and explain the rationale behind them.

A.1.2 Limitations

Nothing in this appendix shall be deemed to change, modify, or alter the performance standards

in Section 3 of this SPS PS. The background information in this appendix is for reference

purposes only.

April 2020 GPS SPS PS

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SECTION A.2 Constellation

A.2.1 Relationship with Section 3.2

Section 3.2 contains the SPS SIS performance standards for the satellite constellation. The

orbital slots defined in Section 3.2 are the nominal originating locations for the SPS SIS. For

convenience, this section further describes those nominal SPS SIS originating locations in a

manner equivalent to the LNAV almanac data sets defined by IS-GPS-200 which may be more

familiar to some readers.

A.2.2 Baseline 24-Slot Constellation Configuration

The baseline 24-slot constellation defined by Tables 3.2-1 and 3.2-3 can be expressed in an

"almanac type" representation using parameters and units equivalent to those in IS-GPS-200 –

see Table A.2-1. (Recognize that IS-GPS-200 uses semicircles rather than degrees, meters

rather than meters, and includes clock offset parameters which are not shown in Table A.2-1).

The almanac reference week (WN

) is 1930 and the almanac reference time (t

) is 0000.

Table A.2-1. Baseline 24-Slot Constellation Almanac, at GPS Epoch of 0000 on 1 Jan 17

Slot

(unit less)



(degrees)

OMEGADOT

(deg/sec)

(meters)

OMEGA

(degrees)



(degrees)

0.000

+1.00

-4.6528E-7

26,559,800

188.085

0.00

239.54

0.000

+1.00

-4.6528E-7

26,559,800

188.085

0.00

133.20

0.000

+1.00

-4.6528E-7

26,559,800

188.085

0.00

343.09

0.000

+1.00

-4.6528E-7

26,559,800

188.085

0.00

13.22

0.000

+1.00

-4.6528E-7

26,559,800

248.085

0.00

52.37

0.000

+1.00

-4.6528E-7

26,559,800

248.085

0.00

144.75

0.000

+1.00

-4.6528E-7

26,559,800

248.085

0.00

281.39

0.000

+1.00

-4.6528E-7

26,559,800

248.085

0.00

175.79

0.000

+1.00

-4.6528E-7

26,559,800

308.085

0.00

83.29

0.000

+1.00

-4.6528E-7

26,559,800

308.085

0.00

343.21

0.000

+1.00

-4.6528E-7

26,559,800

308.085

0.00

311.08

0.000

+1.00

-4.6528E-7

26,559,800

308.085

0.00

212.97

0.000

+1.00

-4.6528E-7

26,559,800

8.085

0.00

106.64

0.000

+1.00

-4.6528E-7

26,559,800

8.085

0.00

236.86

0.000

+1.00

-4.6528E-7

26,559,800

8.085

0.00

6.57

0.000

+1.00

-4.6528E-7

26,559,800

8.085

0.00

138.77

0.000

+1.00

-4.6528E-7

26,559,800

68.085

0.00

168.46

0.000

+1.00

-4.6528E-7

26,559,800

68.085

0.00

274.01

0.000

+1.00

-4.6528E-7

26,559,800

68.085

0.00

37.48

0.000

+1.00

-4.6528E-7

26,559,800

68.085

0.00

305.10

0.000

+1.00

-4.6528E-7

26,559,800

128.085

0.00

210.30

0.000

+1.00

-4.6528E-7

26,559,800

128.085

0.00

316.64

0.000

+1.00

-4.6528E-7

26,559,800

128.085

0.00

76.62

0.000

+1.00

-4.6528E-7

26,559,800

128.085

0.00

106.76

April 2020 GPS SPS PS

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Notes:

e = Eccentricity



= Delta-inclination relative to a nominal value of 0.30 semi-circles (54 degrees)

OMEGADOT = Rate of Right Ascension

A = Semi-major axis

OMEGA

= Geographic Longitude of the Ascending Node at the Weekly Epoch

 = Argument of perigee

= Mean anomaly at the reference time

A.2.3 Expandable 24-Slot Constellation Configuration

Table A.2-2 provides the equivalent "almanac type" representation for when all of the expandable

slots are in their expanded configuration in accordance with Tables 3.2-1, 3.2-2, and 3.2-3. Note

that each expandable slot may individually be in its non-expanded configuration or in its expanded

configuration. There is no linkage between the expandable slots.

Table A.2-2. Expandable 24-Slot Constellation Almanac, at GPS Epoch of 0000 on 1 Jan 17

SlotID

(unit less)



(degrees)

OMEGADOT

(deg/sec)

(meters)

OMEGA

(degrees)



(degrees)

0.000

+1.00

-4.6404E-7

26,559,800

188.085

0.00

239.54

A2F

0.000

+1.00

-4.6404E-7

26,559,800

188.085

0.00

144.40

A2A

0.000

+1.00

-4.6404E-7

26,559,800

188.085

0.00

176.96

0.000

+1.00

-4.6404E-7

26,559,800

188.085

0.00

343.09

0.000

+1.00

-4.6404E-7

26,559,800

188.085

0.00

13.22

B1F

0.000

+1.00

-4.6404E-7

26,559,800

248.085

0.00

66.33

B1A

0.000

+1.00

-4.6404E-7

26,559,800

248.085

0.00

37.77

0.000

+1.00

-4.6404E-7

26,559,800

248.085

0.00

144.75

0.000

+1.00

-4.6404E-7

26,559,800

248.085

0.00

281.39

0.000

+1.00

-4.6404E-7

26,559,800

248.085

0.00

175.79

0.000

+1.00

-4.6404E-7

26,559,800

308.085

0.00

83.29

0.000

+1.00

-4.6404E-7

26,559,800

308.085

0.00

343.21

0.000

+1.00

-4.6404E-7

26,559,800

308.085

0.00

311.08

C4F

0.000

+1.00

-4.6404E-7

26,559,800

308.085

0.00

220.43

C4A

0.000

+1.00

-4.6404E-7

26,559,800

308.085

0.00

195.73

0.000

+1.00

-4.6404E-7

26,559,800

8.085

0.00

106.64

D2F

0.000

+1.00

-4.6404E-7

26,559,800

8.085

0.00

254.09

D2A

0.000

+1.00

-4.6404E-7

26,559,800

8.085

0.00

229.39

0.000

+1.00

-4.6404E-7

26,559,800

8.085

0.00

6.57

0.000

+1.00

-4.6404E-7

26,559,800

8.085

0.00

138.77

0.000

+1.00

-4.6404E-7

26,559,800

68.085

0.00

168.46

0.000

+1.00

-4.6404E-7

26,559,800

68.085

0.00

274.01

E3F

0.000

+1.00

-4.6404E-7

26,559,800

68.085

0.00

52.08

E3A

0.000

+1.00

-4.6404E-7

26,559,800

68.085

0.00

23.52

0.000

+1.00

-4.6404E-7

26,559,800

68.085

0.00

305.10

0.000

+1.00

-4.6404E-7

26,559,800

128.085

0.00

210.30

F2F

0.000

+1.00

-4.6404E-7

26,559,800

128.085

0.00

331.87

F2A

0.000

+1.00

-4.6404E-7

26,559,800

128.085

0.00

305.43

0.000

+1.00

-4.6404E-7

26,559,800

128.085

0.00

76.62

0.000

+1.00

-4.6404E-7

26,559,800

128.085

0.00

106.76

April 2020 GPS SPS PS

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A.2.4 Propagating the Constellation over Time

The GPS baseline constellation defined in Section 3.2 is designed to be a ground track repeating

orbit with a period of a half of a sidereal day. The constellation RAAN values change over time

with the mean nodal regression rate of -0.0402 deg/day. The repeating ground track for the

nominal constellation require that any propagation of the constellation is relative to time in UTC.

The baseline constellation at 1 January 2017 00:00:00 UTC (JD

= 2457754.5) is:

Table A.2-3. Baseline 24-Slot Constellation at 00:00:00 UTC 1 Jan 17

Slot

RAAN (Ω)

Argument of Latitude

288.85°

239.68°

288.85°

133.34°

288.85°

343.24°

288.85°

13.36°

348.85°

52.52°

348.85°

144.90°

348.85°

281.54°

348.85°

175.94°

48.85°

83.44°

48.85°

343.36°

48.85°

311.22°

48.85°

213.12°

108.85°

106.78°

108.85°

237.00°

108.85°

6.72°

108.85°

138.92°

168.85°

168.60°

168.85°

274.16°

168.85°

37.62°

168.85°

305.24°

228.85°

210.44°

228.85°

316.78°

228.85°

76.76°

228.85°

106.90°

Note:

1. The epoch time of the above table is slightly different from the corresponding table in Section 3.2.

For propagation purposes, the following reference orbit parameters from Table 3.2-3 are held

fixed with their nominal values as shown below. The indicated symbols are used in the equations

which follow.

April 2020 GPS SPS PS

5th Edition Page A-5

Table A.2-4. Reference Orbit Parameters

Reference Orbit Parameter

Nominal Value

Semi-Major Axis, km (a)

Eccentricity (e)

Inclination (i)

Argument of Perigee ()

26,599.8

0.0

55.0°

0.0°

Only the RAAN (Ω) and the argument of latitude are time dependent. Because eccentricity (e)

and argument of perigee () are both zero, the argument of latitude equals the mean anomaly

(M). For simplicity M will be used for propagation.

To propagate the RAAN forward to a new epoch whose Julian Date is JD

Δt = JD

- JD

= mod

360

(-0.0402 x Δt + Ω

)

where Ω

is the satellite RAAN at 1 January 2017 00:00:00 UTC.

The repeating ground tracks mean that the Geographic Longitude of the Ascending Node (GLAN)

values (also known as the Groundtrack Equatorial Crossing [GEC] values and denoted as λ), for

the nominal constellation are constant over time. When a satellite is at a nodal crossing, λ may

be expressed as a function of M, Ω, and θ

, where θ

is the Greenwich hour angle. Taking into

account that the ground tracks repeat twice a sidereal day, at any time t:

= Ω

+ M

/2 - θ

Furthermore, because the ground tracks for the nominal constellation repeat, λ

= λ

for all future

times t. Thus:

= mod

360

+ 2(Ω

- Ω

) - 2(θ

- θ

))

As an example, the 1 January 2017 00:00:00 UTC constellation configuration shown in Table A.2-

3 and A.2-4 propagated forward to a later epoch of 1 January 2018 00:00:00 UTC results in the

equivalent constellation configuration shown in Table A.2-6 and A.2-7.

April 2020 GPS SPS PS

5th Edition Page A-6

Table A.2-5. Baseline 24-Slot Constellation at 00:00:00 UTC 1 Jan 18

Slot

RAAN (Ω)

Argument of Latitude

274.18°

268.55°

274.18°

162.21°

274.18°

12.11°

274.18°

42.23°

334.18°

81.39°

334.18°

173.77°

334.18°

310.41°

334.18°

204.81°

34.18°

112.31°

34.18°

12.23°

34.18°

340.09°

34.18°

241.99°

94.18°

135.65°

94.18°

265.87°

94.18°

35.59°

94.18°

167.79°

154.18°

197.47°

154.18°

303.03°

154.18°

66.49°

154.18°

334.11°

214.18°

239.31°

214.18°

345.65°

214.18°

105.63°

214.18°

135.77°

This propagation defines the baseline constellation at any epoch after 1 January 2017 00:00:00

UTC. The GLANs (λ) for operational satellites are adjusted by the right ascension dispersion of

the satellite:

λ - λ

= Ω - Ω

Where λ is the GLAN for the operational satellite, λ

is the GLAN for the nominal satellite orbit as

defined in the baseline satellite constellation, Ω is the right ascension of ascending node for the

operational satellite, and Ω

is the right ascension for the nominal satellite orbit as defined in the

baseline satellite constellation.

April 2020 GPS SPS PS

5th Edition Page A-7

SECTION A.3 Coverage

A.3.1 Relationship with Section 3.3

Section 3.3 contains the SPS SIS performance standards for coverage from each satellite and

from the satellite constellation. This section provides further information relative to both aspects

of the SPS SIS coverage.

A.3.2 Per-Satellite Coverage

A.3.2.1 Satellite Footprint

The portion of the surface of the Earth which is visible from a satellite's orbital location is known

as the satellite’s “footprint”. See Figure A.3-1.

The footprint of each satellite occupying a slot in the baseline/expandable 24-slot constellation

covers approximately 38% of the Earth’s surface. The use of artificial mask angles will reduce

the satellite’s effective footprint. With a 5-degree mask angle, a satellite’s effective footprint is

reduced to slightly more than one-third of the Earth's surface (33.9%).

Figure A.3-1. Illustration of Satellite Footprint

A.3.2.2 Mask Angles

The 5 degree elevation angle used in IS-GPS-200 and IS-GPS-705 for specifying the guaranteed

minimum user-received power level should not be interpreted as specifying, recommending, or

suggesting a particular GPS receiver mask angle. The GPS receiver mask angle (if any) should

27.8°

Satellite Footprint

Not to scale

April 2020 GPS SPS PS

5th Edition Page A-8

be determined as a function of the IS-GPS-200 and/or IS-GPS-705 guaranteed minimum user-

received power levels, the GPS SIS frequencies used, the GPS receiver antenna gain patterns

at each frequency, the GPS receiver front-end sensitivity at each frequency, the particular types

of mission(s) to be accomplished, and related factors. Some common GPS receiver mask angles

are 15 degrees, 10 degrees, 7.5 degrees, 5 degrees, 2 degrees, and 0 degrees. Some GPS

receivers have no mask angle. Some older GPS receivers have a variable mask angle. See AC

20-138 for some aviation mask angle examples.

A.3.3 Constellation Coverage

A.3.3.1 Seamless Coverage

The coverage of a "global positioning system" should obviously be global and seamless. The

same is not necessarily true for a "global navigation satellite system" (GNSS) which incorporates

wide area or local area augmentations. The SPS SIS constellation coverage is seamless.

A.3.3.2 Terrestrial Service Volume

The near-Earth region extending from the surface of the Earth up to an altitude of 3,000 km above

the surface of the Earth is also known as the "terrestrial service volume". See Figure A.3-2.

Figure A.3-2. Illustration of Terrestrial Service Volume

A.3.3.3 Space Service Volume

The spherical shell extending from the outer surface of the terrestrial service volume up to an

altitude of 36,000 km above the surface of the Earth (approximately the geosynchronous orbit

altitude) is known as the "space service volume". There are no explicit constellation coverage

standards for the space service volume.

Alt = 3000 km

41.3°

Terrestrial Service Volume

Not to scale

April 2020 GPS SPS PS

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SECTION A.4 Accuracy

A.4.1 Relationship with Section 3.4

Section 3.4 contains the SPS SIS performance standards for accuracy. This section provides

background information relative to the SPS SIS accuracy performance standards.

A.4.2 Reserved

A.4.3 Time Dependency

A.4.3.1 Graceful Degradation

GPS UERE budgets and corresponding SPS SIS accuracy standards vary as a function of time.

The accuracy variation over time will be significant if the Control Segment is unable to upload

fresh NAV message data to the satellites in the constellation. Such a condition could occur as

the result of total loss of the Control Segment due to a natural or man-made disaster, or it could

occur on a per-satellite basis if a satellite were to become unable to accept and process the

uploaded data. In either case, the UERE will degrade gracefully over time as illustrated in Figure

A.4-1.

A.4.3.2 Normal Operations vs. Extended Operations

When the satellites are being uploaded on a routine basis, the SPS SIS accuracy standards which

apply are for the normal operations mode. During normal operations, each satellite in the

constellation is uploaded at least once per day. Additional (contingency) uploads may be

necessary for certain satellites as described in the following section. The normal operations mode

is shown at the far left-hand side of Figure A.4-1. The SPS SIS indicates when the satellite is in

the normal operations mode by way of the C/A-code signal LNAV data stream fit interval flag

Figure A.4-1. UERE Graceful Degradation

UERE (95%), meters

Days Since Last Upload

0 2 4 6 8 10 12 14

400

300

200

100

Not to scale

Normal Operations

Extended Operations

April 2020 GPS SPS PS

5th Edition Page A-10

being set to “0” (zero) in accordance with IS-GPS-200. When the fit interval flag is set to “1” (one),

the satellite is operating in the extended operations mode. Special SPS SIS accuracy standards

apply for the extended operations mode. See IS-GPS-200 for details on the fit interval flag.

Notes:

1. Three uploads per day is a worst-case assumption for the normal operations period. One upload

per day is a best-case assumption. Many satellites typically only require one upload per day.

2. There is no equivalent ‘normal operations mode’ flag (fit interval flag) in the CNAV data stream on

the CM-code signal or the I5-code signal.

The extended operations period is shown at the center and right-hand side of Figure A.4-1. A

different set of SPS SIS accuracy standards applies when a satellite is operating in the extended

operations mode. All satellites will provide a usable (i.e., healthy or marginal) C/A-code signal

for at least 14 days in the extended operations mode (i.e., at least 14 days after the last upload

of fresh LNAV message data from the Control Segment). Most satellites will continue to provide

a usable C/A-code signal for even longer than 14 days. The extended operations mode for CNAV

message data from the CM-code signal and the I5-code signal is only 2 days for the Block IIR-M

and Block IIF satellites. See IS-GPS-200 for further details on the capability of different types of

satellites to continue in the extended operations mode beyond 14 days.

A.4.3.3 Variations During Normal Operations

During the normal operations period, the GPS UERE and SPS SIS accuracy vary as a function

of the time since upload in the same general manner as shown in Figure A.4-1, but with the

maximum time since last upload for each satellite limited to no more than about a day. The

smallest UERE and best SIS accuracy will generally occur immediately after an upload of fresh

NAV message data to a satellite, while the largest UERE and worst SIS accuracy will usually be

with the stalest NAV message data just prior to the next upload to that satellite.

The metric used to characterize whether the NAV message data being transmitted by a satellite

is fresh or stale is the age of data (AOD), where the AOD is the elapsed time since the Control

Segment generated the satellite clock/ephemeris prediction used to create the NAV message

data upload. The AOD is approximately equal to the time since last upload plus the time it took

the Control Segment to create the NAV message data and upload it to the satellite.

For normal operations, the GPS UERE budget and the traditional SPS SIS accuracy

specifications apply at each AOD. Because the largest UERE and worst SIS accuracy usually

occur with the stalest NAV message data, the UERE budget and traditional SPS SIS accuracy

specifications are taken as applying at the maximum AOD.

Figure A.4-2 shows close-up views of the normal operations period. The horizontal axes are

given in terms of the AOD. In a best-case one-upload-per-day scenario for a satellite with a very

stable clock (Figure A.4-2a), the maximum AOD is assumed to be 26 hours based on: (a) 1 hour

to create the NAV message data and upload it to satellite, (b) 24 hours mean time between

uploads, and (c) 1 hour schedule variation for the subsequent upload. In a worst-case three-

upload-per-day scenario for a satellite with a less stable clock (Figure A.4-2b), the maximum AOD

is assumed to be 10 hours based on: (a) 1 hour to create the NAV message data and upload it

to satellite, (b) 8 hours mean time between uploads, and (c) 1 hour schedule variation for the

subsequent upload.

April 2020 GPS SPS PS

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Figure A.4-2a. UERE as a Function of AOD, One-Upload-Per-Day Scenario

Figure A.4-2b. UERE as a Function of AOD, Three-Uploads-Per-Day Scenario

Recognize that a large portion of the UERE in Figure A.4-2a (and in Figure A.4-2b) does not vary

as a function of the AOD. The dominant component which does not vary is the SPS receiver's

contribution to the UERE (known as the user equipment error or UEE). Factoring out the SPS

receiver's UEE leaves just the SPS SIS contribution to the UERE (i.e., the SPS SIS URE). As

shown in Figure A.4-3, the SPS SIS URE exhibits a stronger dependence on the AOD than the

SPS UERE shown in Figure A.4-2a.

Figure A.4-3, like the preceding figures, is a statistical plot. It shows the cumulative SPS SIS

URE-as-a-function-of-AOD results over many uploads (e.g., all uploads to a satellite over the

course of a year in a one-upload-per-day scenario). Figure A.4-4 shows a representative example

of four uploads performed over a day to a single satellite in a three-upload-per-day scenario.

Unlike the preceding figures, Figure A.4-4 shows the instantaneous URE as a function of time

rather than the cumulative statistical URE as a function of the AOD.

UERE (95%), meters

Age of Data (AOD), hours

0 2 4 6 8 10

Not to scale

UERE (95%), meters

Age of Data (AOD), hours

0 2 4 6 8 10 12 14 16 18 20 22 24 26

Not to scale

April 2020 GPS SPS PS

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Figure A.4-3. Statistical SPS SIS URE as a Function of AOD, One-Upload-Per-Day

Scenario

Figure A.4-4. Instantaneous SPS SIS URE as a Function of Time

Note:

1. The instantaneous URE can be positive or negative as shown in Figure A.4-4. The statistical URE

is always unsigned (illustrated as positive) as shown in Figures A.4-1 through A.4-3.

In Figure A.4-4, the uploads are shown as occurring at approximately 00:42, 08:28, 16:29, and

23:57. Each upload is characterized by the instantaneous SPS SIS URE resetting to near zero

as a result of the satellite starting to broadcast the fresh NAV message which has just been

uploaded. The transition from the stale old NAV message data to the fresh new NAV message

data is known as an "upload cutover". In addition to the large discontinuities at the upload

cutovers, Figure A.4-4 also shows much smaller discontinuities occurring at the 2-hour

URE (Instantaneous), meters

Time,

hours

0 4 8 12 16 20 24

Not to scale

Upload

URE (95%), meters

Age of Data (AOD), hours

0 2 4 6 8 10 12 14 16 18 20 22 24 26

Not to scale

-4

-8

April 2020 GPS SPS PS

5th Edition Page A-13

boundaries (slightly exaggerated for clarity). These smaller discontinuities are the result of the

satellite switching from broadcasting one 2-hour data set to the next 2-hour data set from the

same upload. Even though sequential 2-hour data sets come from the same upload, minor

differences in clock/ephemeris curve fitting introduce small discontinuities between data sets (i.e.,

between curve fits). The transition from one 2-hour data set to the next 2-hour data set is known

as a "data set cutover". See IS-GPS-200 and IS-GPS-705 for further details on upload cutovers

and data set cutovers.

A.4.3.4 Accuracy at Time of Upload

As shown in the preceding figures, uploads of fresh NAV message data reset the instantaneous

SPS SIS URE to near zero, but they do not always reset the instantaneous SPS SIS URE to

exactly zero. There are three main types of errors which prevent fresh NAV message data from

being 100% accurate at the time of the upload. In descending order of impact on upload accuracy,

the three types of limiting errors are:

a. The first error type is the result of the inability of the Control Segment to perfectly

determine a satellite’s clock offset from GPS time and its location in orbit at every instant

in time. This is often called the zero age of data (ZAOD) error. The MCS uses a Kalman

filter to process the GPS SIS tracking data supplied by the MSs and generates estimates

for the satellite clock/ephemeris parameters in near real time. Because Kalman filters do

not react instantaneously to unpredictable changes, and because there are only a limited

number of MSs providing GPS SIS tracking measurements, the near real time estimated

clock/ephemeris parameters in the MCS are always slightly inaccurate. The ZAOD errors

tend to be lower when more MSs are tracking a satellite, and they tend to be larger when

a satellite’s clock or orbit is changing in an unpredictable manner. Inaccurately estimated

clock/ephemeris parameters from the Control Segment’s Kalman filter map directly into

the upload based on those parameters.

b. The second error type is the result of there only being a limited number of data bits in the

NAV message data to represent a satellite’s predicted clock and ephemeris. This is known

as the curve fit error. The two “curves” which have the fitting error are: (1) the quadratic

curve specified in IS-GPS-200/IS-GPS-705 for representing a satellite’s predicted clock

offset from GPS time, and (2) the quasi-Keplerian curve specified in IS-GPS-200/IS-GPS-

705 for representing a satellite’s predicted ephemeris. This is the same error which is

responsible for the small discontinuities occurring at the 2-hour boundaries shown in

Figure A.4-4. See IS-GPS-200/IS-GPS-705 for further details on curve fit errors. LNAV

curve fit errors make up a significant portion of the C/A-code signal URE at the time of

upload. CNAV curve fit errors are insignificant contributors to the CM-code signal URE,

the I5-code signal URE, and the Q5-code signal URE regardless of the time of upload.

c. The third error type is the result of the delay between the time that the Control Segment’s

Kalman filter generates its estimates of the satellite clock/ephemeris parameters and the

time that the satellite starts transmitting new NAV message data from an upload based on

those Kalman filter estimates. The AOD starts counting at the time the Kalman filter

generates the satellite clock/ephemeris parameters, not at the time of the upload.

Because of the delay, new NAV message data already has a significant non-zero AOD

when a satellite starts transmitting it.

April 2020 GPS SPS PS

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A.4.4 Contingency Uploads

The Control Segment has some ability to manage the SPS SIS contribution to the overall UERE.

The Control Segment can do this by monitoring the current instantaneous GPS SIS URE from

each satellite and performing a "contingency upload" if the URE starts to become large relative to

the allocated Space/Control portions of the UERE budget. Done consistently, this puts an

effective bound on the maximum SPS SIS URE. This SPS PS contains no requirement for the

Control Segment to perform contingency uploads nor does it give any SPS SIS URE threshold

for prompting a contingency upload. The contingency upload threshold (CUT), if any, is under

the purview of the Control Segment. The only SPS SIS accuracy-related requirements on the

Control Segment for uploading are: (1) uploading each satellite a minimum of approximately once

per day, and (2) satisfying the requirements of Tables 3.4-1 through 3.4-4.

A.4.5 UERE Budgets

For reference, the GPS UERE budgets for typical DF L1 C/A – L2C receivers and for SF L1 C/A-

code receivers at zero AOD, at maximum AOD in normal operations, and at 14.5 day AOD in

extended operations, are shown in Tables A.4-1 and A.4-2. The breakouts of the individual

segment components of the UERE budgets shown in these tables are given for illustration

purposes only. The actual SPS SIS accuracy standards are given in Table 3.4-1.

Recognize that those portions of the GPS UERE budgets related to the SPS receivers are shown

strictly for illustration purposes only. The actual SPS receiver UEE contributions to the overall

GPS UERE budgets will vary significantly as a function of SPS receiver design and performance

under different environmental conditions.

Notes:

1. Deleted.

2. The normal operations UERE budgets are consistent with an early Block II clock stability of 5x10

at a tau of 10

seconds for either Rubidium clocks or Cesium clocks and an average of three

uploads per day per satellite assuming an average maximum AOD of 8.5 hours. Under a three-

uploads-per-day scenario, the actual average maximum AOD is on the order of 10 hours.

3. The normal operations UERE budgets are conservative with a later Block II clock stability of 6x10

at a tau of 10

seconds for a Rubidium clock and an average of one upload per day per satellite

assuming an average maximum AOD of 26 hours.

4. The extended operations UERE budgets are consistent with an "average" early Block II clock

stability of 3.5x10

-13

at a tau of 10

seconds (i.e., average between a Rubidium clock and a Cesium

clock) and an AOD of 14.5 days.

5. 14 days after the Control Segment ceases uploading satellites with fresh NAV message data under

the system-wide graceful degradation scenario, the average AOD across all satellites in the

constellation will be 14.5 days assuming an average of one upload per day per satellite before the

Control Segment ceased uploading satellites. The extended operations URE standards apply

across the entire constellation 14 days after the Control Segment ceased uploading satellites.

April 2020 GPS SPS PS

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6. Reserved

7. Reserved

8. Actual SF ionospheric delay model errors depend on the point in the 11-year sunspot cycle, the

geomagnetic location, the local solar time of day, and the local satellite elevation angle. Due to

this variability, the SF URE, URRE, and URAE standards do not include the SF ionospheric delay

model errors. Tables A.4-2 and A.4-4 illustrate the typical method for including the SF ionospheric

delay model errors at L1. See paragraph A.4.9 for additional information.

9. The user contributions to the UERE budget illustrate mid-1980s vintage receiving equipment. See

Appendix B for additional information on different SPS receivers and environments.

10. All statistical values are expressed at the 95% probability level in accordance with international

standards.

Table A.4-1. DF L1 C/A – L2C UERE Budget

Segment

Error Source

UERE Contribution (95%)

(meters)

Zero AOD

Max. AOD

in Normal

Operation

14.5 Day

AOD

Space

Clock Stability

Group Delay Stability

Diff'l Group Delay Stability

Satellite Acceleration Uncertainty

Other Space Segment Errors

0.0

1.6

2.4

0.0

1.0

7.5

1.6

2.4

2.0

1.0

257

1.6

2.4

204

1.0

Control

Clock/Ephemeris Estimation

Clock/Ephemeris Prediction

Clock/Ephemeris Curve Fit

Iono Delay Model Terms

Group Delay Time Correction

Other Control Segment Errors

2.0

0.0

0.1

N/A

1.0

2.0

4.4

0.1

N/A

1.0

2.0

206

1.2

N/A

1.0

User*

Ionospheric Delay Compensation

Tropospheric Delay Compensation

Receiver Noise and Resolution

Multipath

Other User Segment Errors

4.5

3.9

2.9

2.4

1.0

4.5

3.9

2.9

2.4

1.0

4.5

3.9

2.9

2.4

1.0

95% System UERE (SPS)

8.0

12.0

388

* For illustration only, actual SPS receiver performance varies significantly -- see Table B.2-1

April 2020 GPS SPS PS

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Table A.4-2. SF L1 C/A-Code UERE Budget

Segment

Error Source

UERE Contribution (95%)

(meters)

Zero AOD

Max. AOD

in Normal

Operation

14.5 Day

AOD

Space

Clock Stability

Group Delay Stability

Diff'l Group Delay Stability

Satellite Acceleration Uncertainty

Other Space Segment Errors

0.0

1.6

0.0

1.0

7.5

1.6

0.0

2.0

1.0

257

1.6

0.0

204

1.0

Control

Clock/Ephemeris Estimation

Clock/Ephemeris Prediction

Clock/Ephemeris Curve Fit

Iono Delay Model Terms

Group Delay Time Correction

Other Control Segment Errors

2.0

0.0

0.6

9.8-19.6

2.3

1.0

2.0

4.4

0.6

9.8-19.6

2.3.0

1.0

2.0

206

1.2

9.8-19.6

2.3.0

1.0

User*

Ionospheric Delay Compensation

Tropospheric Delay Compensation

Receiver Noise and Resolution

Multipath

Other User Segment Errors

N/A

3.9

2.9

2.4

1.0

N/A

3.9

2.9

2.4

1.0

N/A

3.9

2.9

2.4

1.0

95% System UERE (SPS)

11.9-20.7

14.8

-22.6

388

* For illustration only, actual SPS receiver performance varies significantly -- see Table B.2-1

Table A.4-3. Reserved

Table A.4-4. Reserved

A.4.6 URE Over All AODs

The SPS SIS portions of the preceding UERE budgets describe the SPS SIS accuracy at various

specified AODs. During the normal operations period, the statistical SPS SIS URE at the

maximum AOD (i.e., any AOD) has traditionally been taken as being the URE seen by SPS

receivers for system accuracy computations. Although valid under worst-case conditions, SPS

receivers are very unlikely to encounter a condition where all satellites being used are

simultaneously at their maximum AOD. Instead, the general-case condition for a SPS receiver is

for the satellites being used to have a range of AODs. Some satellites will have large AODs,

some satellites will have small AODs, and some satellites will have AODs in the middle. Under

this general-case condition, using the SPS SIS URE at the maximum AOD will result in overly

conservative system accuracy computations.

To avoid over-conservatism for the normal operations period, a different SPS SIS URE statistic

is used. This is the SPS SIS URE over all AODs which occur during the normal operations period.

The SPS SIS URE over all AODs applies to individual satellites over time as well as to the

ensemble of satellites used by a SPS receiver over time. The SPS SIS URE over all AODs should

be used in system accuracy computations.

April 2020 GPS SPS PS

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Notes:

1. Due to the shape of the SPS SIS URE curve as a function of AOD (e.g., see Figure A.4-3), the SPS

SIS URE over all AODs is not equal to the SPS SIS URE at the average AOD.

2. The SPS SIS URE over all AODs is the expected SPS SIS URE at a random point in time over a

long series of uploads (i.e., at a random AOD).

A.4.7 URE Time Derivative Accuracies

There are two time derivatives of the SPS SIS instantaneous URE (instantaneous pseudorange

error) addressed in this SPS PS. The first time derivative of the SPS SIS instantaneous

pseudorange error is the SPS SIS instantaneous pseudorange rate error; also known as the

instantaneous pseudorange velocity error or instantaneous user range rate error (URRE). The

second time derivative of the SPS SIS instantaneous pseudorange error is the SPS SIS

instantaneous pseudorange rate rate error; more commonly known as the instantaneous

pseudorange acceleration error or instantaneous user range acceleration error (URAE).

The inter-relationship of the time derivatives with the SPS SIS instantaneous URE is shown in the

three panels of Figure A.4-5. Because the instantaneous pseudorange rate error is the rate of

change of the instantaneous URE, the instantaneous URRE is simply the slope of the

instantaneous URE. For example; just before the first upload, the slope of the instantaneous URE

is steep and positive -- hence the instantaneous URRE is large and positive. This large and

positive instantaneous URRE is shown in Figure A.4-5(b). One derivative further, the

instantaneous pseudorange acceleration error is the rate of change of the rate of change of the

instantaneous URE, or equivalently the rate of change (the slope) of the instantaneous URRE.

For the example in Figure A.4-5(a); right after the first upload, the slope of the instantaneous URE

starts out at zero and gradually becomes negative over time -- these changing slopes are the

instantaneous URRE over time as shown in Figure A.4-5(b) -- and the changing slopes of the

instantaneous URRE over time are the instantaneous URAE over time as shown in Figure A.4-

5(c).

Just as the behavior of the instantaneous URE over time is specified in terms of a statistical URE,

the behavior of the instantaneous pseudorange rate error over time is specified in terms of a

statistical URRE and the behavior of the instantaneous pseudorange acceleration error over time

is specified in terms of a statistical URAE. Also, like the URE values, the URRE and URAE values

are expressed as 95

percentile accuracies over time, where measurement point is at any AOD

for the normal operations scenario and at 14 days after the Control Segment ceases uploading in

the extended operations scenario.

The infinite spikes in the instantaneous URRE and instantaneous URAE values shown in Figures

A.4-5b and A.4-5c deserves special mention. Whenever a step change occurs in the

instantaneous URE in Figure A.4-5a -- due to a discontinuity caused by either an upload cutover

or a data set cutover -- there is a corresponding spike in the instantaneous URRE. These spikes

occur because the step change in the instantaneous URE at the cutover happens over an

infinitesimally short time and the resulting "slope" of the instantaneous URE at the step change is

infinite. The same principle also causes spikes in the instantaneous URAE whenever a step

change in the instantaneous URRE occurs at a cutover. These spikes in the instantaneous URRE

and instantaneous URAE are infinitely large in size, but only last for an infinitesimally short

duration. These spikes are not included in the statistical URRE 95% and URAE 95% values.

April 2020 GPS SPS PS

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Figure A.4-5a. Instantaneous SPS SIS URE as a Function of Time

Figure A.4-5b. Instantaneous SPS SIS URRE as a Function of Time

Figure A.4-5c. Instantaneous SPS SIS URAE as a Function of Time

Instantaneous URE, meters

Time,

hours

0 4 8 12 16 20 24

Not to scale

Upload

Instantaneous URRE, m/sec

Time,

hours

0 4 8 12 16 20 24

+0.0004

+0.0002

-0.0002

-0.0004

Not to scale

Instantaneous URAE, m/sec/sec

Time,

hours

0 4 8 12 16 20 24

+210

-8

+110

-8

-210

-8

-210

-8

Not to scale

-4

-8

April 2020 GPS SPS PS

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A.4.8 UTC(USNO) Offset Accuracy

The SPS SIS NAV message contains offset data for relating GPS time to UTC(USNO). During

normal operations, the accuracy of this offset data during the transmission interval is such that

the UTC offset error (UTCOE) in relating GPS time (as maintained by the Control Segment) to

UTC (as maintained by the U.S. Naval Observatory) is within 30 nanoseconds 95% (15

nanoseconds 1-sigma). See IS-GPS-200 and/or IS-GPS-705 for additional details regarding the

UTC(USNO) offset data.

Notes:

1. The accuracy of the UTC(USNO) offset data will degrade if the Control Segment is unable to upload

fresh data to the satellites. During extended operations, it is expected that alternate sources of

UTC are no longer available, and that the relative accuracy of the UTC(USNO) offset data will be

sufficient for most users.

2. The UTC(USNO) offset data is intended to be applied by the GPS receiver, or by the user, after

the GPS receiver has solved for its own offset from GPS time. The GPS receiver is not required to

compute a position solution for the UTC(USNO) offset data to be useful, only a time solution is

needed.

3. The Control Segment is not required to update its estimate of the UTC (USNO) offset data prior to

each upload. As such, the same UTC(USNO) offset data is commonly broadcast by several

satellites simultaneously. Depending on the Control Segment's UTC(USNO) offset estimate

update schedule, it is possible for all satellites to be broadcasting the same UTC(USNO) offset

data.

A.4.9 Single-Frequency Ionospheric Delay Model Errors

The accuracy of the single-frequency (SF) ionospheric delay model is better than 50%. Typical

global statistic SF ionospheric delay model errors for L1 vary from 9.8 m to 19.6 m 95% for benign

and disturbed ionospheric conditions respectively. SF ionospheric delay model errors for L2 and

L5 are larger because of the lower carrier frequencies. Ionospheric delay model errors can be as

severe as ±100 m or more in some solar conditions, at some latitudes, at some elevation angles,

and at some times of day. The largest errors are usually seen when solar storms occur during or

shortly after a maximum in the 11-year sunspot cycle, within ±15 degrees of the geomagnetic

equator, near the horizon, during the local afternoon. The smallest errors are usually seen when

the sun is quiet during a minimum in the 11-year sunspot cycle, at the geomagnetic mid-latitudes,

at zenith, during the local night. The influence of the local time of day on the SF ionospheric delay

model accuracy is particularly strong. Regardless of the 11-year sunspot cycle phase or

geomagnetic latitude, the ionospheric delay model errors at zenith between local midnight and

local dawn are commonly less than ±1 m.

Table A.4-2 illustrates the typical method for including the SF ionospheric delay model errors in

the Control Segment contribution to the L1 C/A-code SPS SIS URE. The same method is also

used for including the SF ionospheric delay model errors in the Control Segment contribution to

the L2 CM-code and L5 I5-code/Q5-code SPS SIS UREs.

April 2020 GPS SPS PS

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A.4.10 Group Delay Time Correction (T

) Errors

As described in IS-GPS-200 and IS-GPS-705, the group delay time correction (T

) is broadcast

in the LNAV and CNAV messages for the benefit of SF GPS receivers. Errors in the broadcast

value affect the URE experienced by SF GPS receivers which apply that T

value.

Table A.4-2 illustrates the typical method for including T

errors in the Control Segment

contribution to the L1 C/A-code SPS SIS URE. The same method is also used for including T

errors in the Control Segment contribution to the L2 CM-code and L5 I5-code/Q5-code SPS SIS

UREs. T

error effects are included in the performance standards in Section 3 of this SPS PS.

A.4.11 Spatial Dependency

As described earlier in this Section, the GPS UERE budgets and the SPS SIS accuracy standards

vary as a function of the elapsed time since upload. The UERE budgets and SIS accuracy

standards do not vary as a function of the spatial “look angles” relative to the satellites. The UERE

budgets and accuracy standards apply equally at every point within the satellite’s coverage

footprint.

In reality, however, the SPS SIS URE does vary significantly across each satellite’s coverage.

The sources of this spatial dependency are errors in the satellite orbit. Satellite orbit errors are

primarily due to either: (a) unpredictable satellite accelerations, or (b) inaccurate ephemeris data

uploads. The distinction between these sources of satellite orbit errors is manifested in the UERE

budgets of Tables A.4-1 through A.4-4. Unpredictable satellite accelerations are satellite specific,

and the Space Segment has a UERE budget allocation for them in Tables A.4-1 through A.4-4.

Inaccurate ephemeris data uploads are due to a mix of estimation/prediction errors plus curve fit

limitations, and the Control Segment has UERE budget allocations for each of them in Tables

A.4-1 through A.4-4. There are also secondary causes of satellite orbit errors, such as mis-

orientation of the lever arm from the satellite center of mass to the broadcast antenna phase

center. These secondary causes of satellite orbit errors are also in the UERE budgets of Tables

A.4-1 through A.4-4 under the “other segment errors” lines for the Space Segment and for the

Control Segment.

The SIS URE’s spatial dependency on satellite orbit errors is partially explained in Figure A.4-6.

This figure shows a horizontal orbit error “H” (also known as a tangential orbit error) resulting from

some combination of Space/Control Segment errors. In Figure A.4-6, the horizontal orbit error

vector is oriented due north, with the satellite actually being located further south than the location

indicated by the broadcast ephemeris data (the sense of the error vector is “indicated minus

truth”).

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Figure A.4-6. Illustration of Spatial Dependency - Horizontal Orbit Error

An observer located at the edge of the satellite’s coverage footprint due north of the sub-satellite

point will perceive a positive instantaneous URE because the satellite’s true location is further

away from the observer than the location indicated by the broadcast ephemeris data. An observer

located at the edge of the satellite’s coverage footprint due south of the sub-satellite point will

perceive a negative instantaneous URE because the satellite’s true location is closer than the

location indicated by the broadcast ephemeris data. And an observer located at center of the

satellite’s coverage footprint exactly at the sub-satellite point will perceive zero instantaneous

URE because the satellite’s true location is just as far away as the location indicated by the

broadcast ephemeris data.

This sinusoidal variation of the instantaneous URE across the coverage footprint depending on

the look angle projection is characteristic of horizontal orbit errors. In the customary radial-

alongtrack-crosstrack (RAC) orbital coordinate system, the alongtrack (A) and crosstrack (C)

orbital errors are the two orthogonal horizontal error components and each has the same

sinusoidal characteristic in its impact on the instantaneous URE. Over a satellite’s coverage

footprint on the Earth’s surface, their maximum impact on the instantaneous URE is ±0.240 x A

and ±0.240 x C. Across the coverage footprint on the Earth’s surface at a 2° mask, their root-

mean-square (RMS) effect on the URE is 0.141 x A and 0.141 x C. At the edge of the terrestrial

service volume (0° mask, 3,000 km above a mean Earth radius of 6,371 km), their maximum

impact on the instantaneous URE is ±0.353 x A and ±0.353 x C.

The second part of URE’s spatial dependency on satellite orbit errors is explained in Figure A.4-

7. This figure shows a radial orbit error (“R”) resulting from a combination of Space/Control

Segment errors. In Figure A.4-7, the radial orbit error vector is oriented towards the Earth, with

the satellite actually being located further away from the Earth than the location indicated by the

broadcast ephemeris data.

13.88°

Projection = +H x sin(13.88°)

Not to scale

13.88°

Projection = -H x sin(13.88°)

Projection = 0

April 2020 GPS SPS PS

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Figure A.4-7. Illustration of Spatial Dependency - Radial Orbit Error

An observer located anywhere within the satellite’s coverage footprint (north, south, east, west,

or centered) will perceive a positive instantaneous URE because the satellite’s true location is

always further away from the observer than the location indicated by the broadcast ephemeris

data. The URE spatial dependency for radial (R) orbital errors is cosinusoidal rather than

sinusoidal as for the horizontal orbital errors. The impact of an R orbital error on the instantaneous

URE does not change algebraic signs at the sub-satellite point as does the impact of A or C orbital

errors.

Over the coverage footprint on the Earth’s surface, the maximum impact on the instantaneous

URE of a radial error is 1.000 x R and the minimum impact is 0.971 x R. Across the coverage

footprint on the Earth’s surface at a 2° mask, the RMS effect on the URE is 0.980 x R. At the

edge of the terrestrial service volume (0° mask, 3,000 km above a mean Earth radius of 6,371

km), the minimum impact on the instantaneous URE is 0.936 x R.

SPS SIS timing errors (e.g., satellite clock, inter-signal delays, group delays) do not cause the

URE to vary across a satellite’s coverage. SPS SIS timing errors are omni-directional. They

affect the URE equally at every point within the satellite’s coverage.

As described in Section 3.4, the SPS SIS accuracy performance standards are addressed in

terms of a "global statistic URE" where "global statistic URE" means the statistical URE across

the portion of the globe in view of the satellite. There are two generally accepted methods for

computing the global statistic URE for a satellite at a particular instant in time as an (RMS) value.

Those two methods are:

1. Brute Force RMS. The instantaneous URE values can be evaluated at a large number of

spatial points spread evenly across the satellite’s coverage, and the global statistic URE

value can then be computed as the RMS of the instantaneous URE value at each of those

spatial points.

13.88°

Projection = +R x cos(13.88°)

Not to scale

13.88°

Projection = +R x cos(13.88°)

Projection = +R

April 2020 GPS SPS PS

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2. Piecewise RMS. The satellite’s instantaneous alongtrack, crosstrack, and radial orbit error

components, plus its total SPS SIS timing error can be used piecewise in the following

equation:

Global Statistic URE

RMS

= ((cxT)

+(0.980xR)

+(0.141xA)

+(0.141xC)

-1.960xcxTxR)

(A-1)

where

c = speed of light

T = total Timing error

R = Radial orbit error

A = Alongtrack orbit error

C = Crosstrack orbit error

and where the final term in the equation accounts for the correlation (possibly significant)

between the total timing error and the radial orbit error.

Rather than computing an RMS statistic, there are two similar methods for computing the global

statistic URE for a satellite at a particular instant in time as a 95% value. Those two methods are:

1. Brute Force 95%. The instantaneous URE values can be evaluated at a large number of

spatial points spread evenly across the satellite’s coverage, and the global statistic URE

value can then be computed as the 95

percentile of the instantaneous URE value at each

of those spatial points.

2. Piecewise 95%. The satellite’s instantaneous alongtrack, crosstrack, and radial orbit error

components, plus its total SPS SIS timing error can be used piecewise in the following

equation:

|Global Statistic URE

95%

| ≤ (|(cxT)+R|+0.240x(A

)

) (A-2)

Where c, T, R, A, and C are as above.

Note:

1. The two piecewise methods above are only applicable at a particular instant in time. The two brute

force methods are easily extended to cover multiple instants in time.

A.4.12 Inter-Signal Bias (ISB) & Inter-Signal Correction (ISC) Errors

The inter-signal bias (ISB) in transmit time between the reference L1 P(Y)-code signal and the L1

C/A-code signal contributes directly to the L1 C/A-code signal SPS URE. It is accounted for in

Table A.4-2 as part of the group delay stability, and its effects are included in the performance

standards in Section 3 of this SPS PS.

The LNAV data stream does not contain any information relative to the ISB for the C/A-code

signal. The CNAV data stream does, however, contain ‘inter-signal correction’ (ISC) parameters

for the C/A-code signal, the L2C signals (the same ISC applies to both the CM and CL signals),

the I5 signal, and the Q5 signal. Each of these ISC parameters is an estimate of the ISB in

transmit time between the reference L1 P(Y)-code signal and the relevant SPS SIS signal. An

April 2020 GPS SPS PS

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SPS receiver which applies the broadcast ISC values to the measured pseudoranges in

accordance with IS-GPS-200/IS-GPS-705 is able to cancel out the ISB effects on the measured

pseudoranges from the relevant SPS SIS signal.

Cancelling out the ISB effects on the measured pseudoranges for any SPS SIS signal with the

ISC value provides a direct one-to-one benefit on the SF SPS SIS URE for that signal. The SF

L1 C/A-code URE after applying the broadcast ISC value for that C/A-code signal is better (lower)

than the SF L1 C/A-code URE obtained without applying the broadcast ISC value. The accuracy

improvement effects of properly applying the broadcast ISC values are included in the SF SPS

SIS URE performance standards in Section 3 of this SPS PS for the L2C signals, the I5 signal,

and the Q5 signal.

The DF ionospheric delay correction equations in IS-GPS-200 and IS-GPS-705 utilize the

differences between pseudoranges measured across frequencies to determine the total

ionospheric delay affecting the SIS at each frequency. Due to scaling factors which effectively

act as multipliers of the differential ISB between the two DF signals, properly applying the pair of

ISC values to those signals is critical to obtaining an accurate DF ionospheric delay correction.

The accuracy improvement effects of properly applying the broadcast ISC values are included in

the performance standards in Section 3 of this SPS PS for the identified DF SPS SIS component

combinations.

April 2020 GPS SPS PS

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SECTION A.5 Integrity

A.5.1 Relationship with Section 3.5

Section 3.5 contains the SPS SIS performance standards for integrity. This section provides

background information relative to the SPS SIS integrity performance standards.

A.5.2 URA Relationship to Integrity

One of the axioms of information theory is that all data is useful provided one knows how much

weight to give to the data. This axiom applies well to data from the SPS SIS. Some satellites

inherently provide more accurate data on average than other satellites do. One should logically

place more weight on the data from the inherently more accurate satellites than on data from

inherently less accurate satellites.

The LNAV URA index, "N", included in each satellite's broadcast LNAV data stream describes

the satellite's expected accuracy (i.e., 1-sigma bounds on the expected URE) as illustrated below.

Note that this table is equivalent to the look-up table in IS-GPS-200. For most accuracy-related

or data weighting purposes, the ‘typical expected URE’ values in the second column are

adequate. For integrity assurance purposes however, the upper bound values in the furthest right

column are the ‘integrity assured URA’ (IAURA) values for each LNAV URA index, "N".

Table A.5-1. LNAV URA Index to Expected URE Relationship

LNAV URA

Index "N"

Typical Expected URE,

1-sigma

Numerical URA Value, Representing the

Bounds on the Expected URE, 1-sigma

2.0 m

0.00 m

 URA 

2.40 m

2.8 m

2.40 m

 URA 

3.40 m

4.0 m

3.40 m

 URA 

4.85 m

5.7 m

4.85 m

 URA 

6.85 m

8.0 m

6.85 m

 URA 

9.65 m

11.3 m

9.65 m

 URA 

13.65 m

16.0 m

13.65 m

 URA 

24.00 m

32.0 m

24.00 m

 URA 

48.00 m

64.0 m

48.00 m

 URA 

96.00 m

128.0 m

96.00 m

 URA 

192.00 m

256.0 m

192.00 m

 URA 

384.00 m

512.0 m

384.00 m

 URA 

768.00 m

1024.0 m

768.00 m

 URA 

1536.00 m

2048.0 m

1536.00 m

 URA 

3072.00 m

4096.0 m

3072.00 m

 URA 

6144.00 m

No Expectation Provided

6144.00 m

 URA

Use at own risk

Notes:

1. The LNAV URA Index, and its corresponding ‘Typical Expected URE’ and ‘Numerical URA Value’,

includes all SPS SIS error components except for those specific to SF C/A-code operation (i.e.,

inaccuracy, L1 P-to-L1 C/A-code biases).

April 2020 GPS SPS PS

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2. If the LNAV URA were completely reliable, then the SPS SIS would have full integrity with regards

to all SPS SIS error components except for those specific to SF operation. For instance, say that:

(1) all of the satellites except one always broadcast a SPS URA index of 3 and the actual SPS URE

for those satellites always follows a normal distribution with a 1-sigma dispersion of 5.7 meters,

and (2) one satellite always broadcasts a SPS URA index of 7 and the actual SPS URE for that

satellite always follows a normal distribution with a 1-sigma dispersion of 32.0 meters. Each SPS

SIS has full integrity in this case because the user (i.e., the SPS receiver) can decide whether the

satellite which always broadcasts a SPS URA index of 7 should be used for navigation in the

context of the particular mission to be accomplished. As a general rule, most modern SPS

receivers would still use the satellite broadcasting a SPS URA index of 7, but they would deweight

it by a factor of about 6 relative to the satellites broadcasting a SPS URA index of 3.

In the CNAV data stream, there are four separate URA indexes: URA

, URA

NED0

, URA

NED1

, and

URA

NED2

. IS-GPS-200 and IS-GPS-705 provide the equations necessary to use these four

separate URA indexes to compute the ‘typical expected URE’ value for accuracy-related or data

weighting purposes and to compute the IAURA value for integrity purposes.

Note:

1. The CNAV URA

NED0

Index, and its corresponding ‘Typical Expected URE’ and ‘Numerical URA

Value’, include all zero

order SPS SIS error components including those specific to SF C/A-code

operation (i.e., T

inaccuracy, L1 P-to-L1 C/A-code biases).

A.5.3 IAURA-Derived Integrity Tolerance

The broadcast URA index in the LNAV data stream or the broadcast URA indexes in the CNAV

data stream are used to determine the user's (the SPS receiver's) expectation for the SPS URE

and to set the IAURA value for each satellite. For accuracy-related or data weighting purposes,

the ‘typical expected URE’ value can be used directly as a conservative RMS estimate assuming

a normal distribution. For integrity purposes, the IAURA value must be converted into a pass/fail

integrity tolerance.

Treating the IAURA as a normal distribution does not directly lend itself to developing a pass/fail

integrity tolerance because a normal distribution has no outer bounds per se. As can be seen in

Table A.5-2, the probability of exceeding a given outer bound drops off as the outer bound

increases to ever larger values; but it never becomes absolutely zero no matter how far out the

bounds are placed.

Table A.5-2. Normal Distribution Bounds vs. Probability of Exceeding Those Bounds

Normal Distribution

Bounds

Probability of Exceeding

Those Bounds

 1-sigma

0.317310508

 2-sigma

0.045500264

 3-sigma

0.002699796

 4-sigma

0.000063342

 5-sigma

0.000000573

 6-sigma

0.000000002

April 2020 GPS SPS PS

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For integrity assurance purposes, outer bounds have been established at 4.42-sigma relative to

the IAURA value. The corresponding probability of exceeding these bounds for a normal

distribution is 0.00001 (i.e., 1x10

-5

). These outer bounds constitute the not-to-exceed (NTE) SPS

SIS URE tolerance for integrity. A SPS SIS URE exceeding the NTE tolerance is defined to be

misleading signal-in-space information (MSI). MSI may or may not be a loss of SPS SIS integrity

depending on whether a timely alert is issued.

Notes:

1. These outer bounds are consistent with the outer bounds used by legacy civil users who were

subject to SA before the use of SA was discontinued in May 2000. SA was the dominant SPS SIS

error with an a priori assumed 1-sigma dispersion of 33 m. For these civil users,



150 m outer

bounds were established for SPS SIS integrity purposes by rounding the product of 33 m multiplied

by 4.42.

2. SPS SIS MSI may cause some SPS receivers to output hazardously misleading information (HMI).

The factors which determine if SPS SIS MSI will cause HMI or not include: whether the affected

SPS SIS is being used in the position solution, the relative geometry of the set of satellites being

used in the position solution, whether the SPS receiver performs any autonomous integrity

monitoring to detect the occurrence of MSI and/or exclude an affected SPS SIS in a timely manner

(see Appendix B for further information), and the user’s particular tolerance for error in the current

application.

A.5.4 Nature of SPS SIS URE

Neglecting failures and ignoring Control Segment intervention, the SPS SIS URE from satellites

can reasonably be assumed to follow a normal distribution over the long term with zero mean.

Under this assumption, the SPS URA would be a fully satisfactory means of providing SPS SIS

integrity. Unfortunately, GPS failures do occur and many of them can impact the SPS SIS URE

enough to cause the SPS URE to exceed the SPS SIS URE NTE tolerance. Fortunately, the

Control Segment monitors the SPS SIS URE and is able to intervene when such a "soft" GPS

failure has occurred. (As used in this SPS PS, a soft GPS failure is a failure where the SPS SIS

is still healthy but the URE is impacted enough to pose a potential risk to integrity. A hard GPS

failure is a failure where the SPS SIS is no longer trackable and therefore poses no risk to

integrity.)

A.5.4.1 Integrity Failure Modes and Effects

GPS failures which impact the SPS SIS, can occur in the satellites, the Control Segment, or in

the information supplied to the Control Segment by an external source. The soft failure modes

which pose a potential integrity risk are listed in Table A.5-3 along with the representative type of

effect on the URE. The various types of URE effects are illustrated in Figure A.5-1. Table A.5-3

also identifies whether these potential integrity failure modes have a related symptom which is

detectable by a SPS receiver.

April 2020 GPS SPS PS

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Table A.5-3. Potential Integrity Failure Modes

System/

Segment

Failure Mode

Representative

Effect on URE

Spatially

Dependent

Receiver

Detectable

Satellite

Momentum Dump (Thruster Firing)

Step/Ramp/Sinusoid

Yes

Loss of L1 or L2 or L5

Sinusoid

Yes

L1 or L2 or L5 Power Reduction

Noise

Yes

Incorrect PRN

Varies

Some

Yes

Clock Frequency Shift or Instability

Ramp

NAV Message Data Garbled

Varies

Some

Yes

PRN Code Generation Errors

Step

Some

Frequency Synthesizer Upsets

Step

Out-Gassing

Step/Ramp/Sinusoid

Yes

Control

Delayed/Missed Upload

Ramp

Some

Bad Upload: Bad Clock/Ephemeris

Step, Ramp, or Sinusoid

Some

Bad Upload: Wrong/Irrelevant Data

Varies

Some

Most

Operational Error: Health Settings

Step, Ramp, or Sinusoid

Some

Operational Error: Data Content

Step or Sinusoid

Some

GA Induced Errors

Varies

Yes

MS Induced Errors

Step, Ramp, or Sinusoid

Some

Input

Data

Reserved

Bad Earth Orientation Predictions

Ramp or Sinusoid

Yes

Bad Solar Flux Observations

Sinusoid

Yes

Bad UTC(USNO) Offset Data

Other

Figure A.5-1. Types of URE Effects

Instantaneous URE

Time

a. Ramp Error

Failure

Onset

+ SIS URE NTE

Tolerance

- SIS URE NTE

Tolerance

MSI

Start

Onset

Instantaneous

URE

Time

b. Step Error

Failure

Onset

+ SIS URE NTE

Tolerance

- SIS URE NTE

Tolerance

MSI

Start

Onset

Instantaneous URE

Time

c. Sinusoid Error

Failure

Onset

+ SIS URE NTE

Tolerance

- SIS URE NTE

Tolerance

MSI

Start

Onset

Instantaneous URE

Time

d. Noise Error

Failure

Onset

- SIS URE NTE

Tolerance

+ SIS URE NTE

Tolerance

MSI

Start

Onset

April 2020 GPS SPS PS

5th Edition Page A-29

For the potential integrity failure modes which are detectable by a SPS receiver, most of them are

accompanied by a SPS SIS alarm or warning indication which is what is actually detected by the

SPS receiver. The C/A-code SIS alarm indications include, but are not limited to, the following

(CM-code, CL-code, I5-code, and Q5-code are similar):

(1) The apparent cessation of L1 SIS transmission (can also be symptomatic of a hard failure

or Control Segment intervention actions).

(2) The elimination of the standard C/A-code (can also be symptomatic of a hard failure or

Control Segment intervention actions).

(3) The substitution of non-standard C/A-code for the standard C/A-code (an action taken by

the satellite when it autonomously detects certain failures that could compromise the

URA).

(4) The substitution of pseudorandom noise (PRN) C/A-code number 37 for the standard C/A-

code (indicative of Control Segment intervention).

(5) The failure of parity on 5 successive words of LNAV data (can be symptomatic of a "bad

upload: wrong/irrelevant data" failure).

(6) The broadcast Index of Data Ephemeris (IODE) does not match the 8 LSBs of the

broadcast IODC data (can also be symptomatic of a "bad upload: wrong/irrelevant data"

failure; excluding normal data set cutovers, see IS-GPS-200).

(7) All transmitted bits in subframes 1, 2, or 3 are set to 0's or to 1's (can be symptomatic of

a "bad upload: wrong/irrelevant data" failure).

(8) Default LNAV data is being transmitted in subframes 1, 2, or 3 (an action taken by the

satellite when it autonomously detects certain failures that could compromise the URA).

(9) The preamble does not equal 10001011

, or 139

, or 8B

(can be indicative of a failure

in the satellite navigation data unit or in the navigation baseband unit).

A.5.4.2 Control Segment Monitoring and Intervention

One of the Control Segment's major functions is monitoring and assessing the GPS SIS

performance. If the Control Segment determines that a failure has occurred which will adversely

affect the SPS SIS performance, the Control Segment will intervene to prevent, or at least

minimize, the impact of the failure on the SPS SIS performance.

When a soft failure effect on the SPS SIS URE is small, or when rare normal fault-free

performance excursions occur, the typical Control Segment intervention is to perform a

contingency upload (see paragraph A.4.4) to the affected satellite. Contingency uploads serve

both to prevent an integrity fault from occurring and to maintain SPS SIS accuracy.

April 2020 GPS SPS PS

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Note:

1. During fault-free operation, the SPS SIS URE is assumed to follow a normal distribution with zero

mean. Large SPS SIS URE values are expected to occur during fault-free operations but they

should be rare (see Table A.5.2).

When the soft failure effect on the SPS SIS URE is large, the typical Control Segment intervention

is to send a SatZap command to the affected satellite. The SatZap command results in the

satellite immediately switching its assigned PRN code identity to PRN code 37 (i.e., the satellite

starts transmitting P(Y)-code number 37 and C/A-code number 37). This terminates the trackable

SPS SIS from the SatZapped satellite. An equivalent intervention, also sometimes referred to as

SatZap, is commanding the satellite to switch to transmitting non-standard codes in lieu of the

normal PRN codes. Compared to performing a contingency upload or even just performing an

upload to set the satellite unhealthy, SatZap is a much quicker method of intervening -- but it

necessarily renders the SPS SIS untrackable by all SPS receivers.

The Control Segment is able to monitor each satellite's GPS SIS 100% of the time but is not able

to upload or command each satellite 100% of the time due to GA limitations. Although very

unlikely, this SPS PS assumes it is possible for a SPS SIS integrity fault to persist for up to 6

hours before the Control Segment is able to intervene. This intervention delay is comparable to

an average satellite in-view time of 6

hours. Most SPS SIS integrity faults (99.9%) persist for

less than or equal to 3 hours before the Control Segment intervenes. On average (50%), SPS

SIS integrity faults persist for approximately 1 hour.

Note:

1. The 6

hour average in-view time (along with the 6 hour maximum intervention delay) also applies

to SPS SIS UREs that are large, but not so large as to result in an integrity fault or MSI. For receiver

autonomous integrity monitoring (RAIM) purposes, the average in-view time can be conservatively

assumed as the effective correlation time for "large, but not too large" SPS SIS UREs.

A.5.4.3 Control Segment Preemptive Actions

One of the Control Segment's other major functions is conducting on-orbit O&M of the satellites.

Most satellite O&M are scheduled in advance. Certain types of O&M activities are quite likely to

cause a large SPS SIS URE (e.g., station keeping maneuvers and atomic clock maintenance).

In order to prevent a large SPS SIS URE from compromising the URA-derived NTE tolerance and

thereby causing a SPS SIS MSI, the Control Segment will take preemptive action to warn SPS

receivers to not use the SPS SIS from the affected satellite. The preemptive SPS SIS warning

indications include, but are not limited to, causing the satellite to broadcast the following:

(1) An appropriately inflated LNAV URA index "N" value and/or appropriately inflated CNAV

set of URA

and URA

NED

values (appropriately inflated to cover the expected risk of an

abnormally large SPS SIS URE).

(2) The LNAV 6-bit health status word in subframe 1 with the MSB equal to 1

and/or the 5

LSBs equal to anything other than 00000

(a typical "do not use" indication is with the MSB

equal to 1

(“some or all NAV data are bad”) and the 5 LSBs equal to 11100

(“SV is

temporarily out”)) and/or all three bits of the three-bit signal health field in MT-10 of the

CNAV message set to ones.

April 2020 GPS SPS PS

5th Edition Page A-31

(3) An LNAV URA index "N" = 15 and/or CNAV URA

and URA

NED0

= 15 (default action which

may be taken by the Control Segment when a reliable URA cannot be computed).

(4) The URA alert flag is set to 1

To be preemptive, a SPS SIS warning indication must be broadcast to GPS receivers in advance

of the potential integrity fault. So long as the last bit of the LNAV subframe and/or CNAV message

which contains the particular SPS SIS warning indication is received before the NTE tolerance is

breached, no integrity fault will occur because the SPS SIS has provided a timely warning that it

should not be used. The Control Segment may take one or more of the above preemptive actions,

or other preemptive actions (e.g., SatZap) in advance of conducting the O&M that can cause the

large SPS SIS URE. The fact that a preemptive action or actions occurs early with respect to the

NTE tolerance being breached has no adverse impact on SPS SIS integrity. There is no "false

alarm" requirement to constrain how early the Control Segment can take preemptive actions. The

impact of "too early" preemptive actions, or preemptive actions which occur when the subsequent

O&M is cancelled or not completed, is the resulting adverse effect on SPS SIS availability.

Notes:

1. As described in paragraph 2.4.4.3, the Control Segment normally operates the SPS SIS in such a

manner to allow GPS receivers at least five minutes to receive, process, and apply the real-time

health-related information in the NAV message before taking any O&M actions that could cause a

large SPS SIS URE under normal conditions.

2. NANUs are another form of preemptive warning. However, NANUs are not considered as a

warning for integrity purposes. The SPS SIS alarm and warning indications received in real time

by an operating GPS receiver always take precedence over the NANU information received off-line

by an end user. However, NANUs are considered as an adequate warning for continuity purposes

(see Section 3.6).

A.5.4.4 Satellite On-Board Monitoring and Intervention

The satellites are able to autonomously perform a substantial amount of on-board monitoring for

those subsystems which affect the SPS SIS performance. If a satellite determines that a

malfunction has occurred which may adversely affect the SPS SIS performance, an internal alarm

will be generated, and the satellite will intervene to minimize the impact of that failure on the SPS

SIS performance. If the detected malfunction affects the satellite's reference frequency standard

or other critical subsystem, the satellite will provide an integrity alert by switching its broadcast

standard PRN code signals to non-standard code (NSC) signals. If the detected malfunction

affects the satellite's NAV data generation subsystem, the satellite will provide an integrity alert

by switching its broadcast LNAV subframe content to default data (alternating 1s and 0s with

invalid parity) and/or some or all CNAV messages to MT-0. In general, the Block II series of

satellites will switch to broadcasting NSC or default NAV data within 6 seconds of detecting a fault

which can impact SPS SIS integrity.

Many of malfunctions detected by the satellite on-board monitoring are transient, either because

the conditions which cause them only exist for a short while or because the satellite will

autonomously correct the malfunction. If the on-board monitoring determines the detected

April 2020 GPS SPS PS

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malfunction is no longer present, the satellite will return to broadcasting normal PRN-codes and/or

normal NAV data as appropriate. Typical recovery times range from 6 to 24 seconds.

Notes:

1. When a satellite switches over to broadcasting NSC signals in lieu of standard PRN-code signals,

GPS receivers which are currently tracking the satellite signals lose track of the satellite signals at

the time of the switchover. GPS receivers which are currently attempting to acquire the satellite

signals are unable to acquire the satellite signals.

2. When a satellite switches over to broadcasting default NAV data in lieu of normal NAV data, GPS

receivers are assumed to continue tracking the satellite signals through the switchover even though

the GPS receiver is assumed to intentionally not use any of the default NAV data. GPS receivers

which are currently attempting to acquire the satellite signals are assumed to be able to acquire

the satellite signals but will not intentionally use any of the default NAV data.

3. Satellite malfunctions which are not detected by the satellite may require Control Segment

intervention to protect the user (see paragraph A.5.4.2). Control Segment intervention is usually

required to return a satellite to broadcasting a trackable and healthy SPS SIS.

A.5.5 Timely Alert Considerations

The definition of integrity used in this document requires a "timely alert" to be provided when the

SPS SIS should not be used for positioning. Based on operational needs, a threshold of 10

seconds after a breach of the SPS SIS URE NTE tolerance has been established for an alarm or

warning to be issued in order to be considered timely. Alarms and warnings are collectively called

"alerts". This 10 second threshold applies to alerts issued to the end user of the SPS receiver,

and so includes both the time allocated to the SPS SIS and the time allocated to the SPS receiver.

For the SPS SIS, the allocated time (known as the "time to alert" or TTA) is 8 seconds for all alert

indications except for SatZap and NSC. SatZap and NSC are allocated the full 10 seconds. If a

SPS alert indication is transmitted within 8 seconds of an integrity fault occurring, then -- by

definition -- the SPS SIS has converted the MSI into "alerted misleading signal-in-space

information" (AMSI) because a timely alert has been provided. On the other hand, if a SPS alert

indication is not transmitted within 8 seconds of an integrity fault occurring, then the SPS SIS is

defined to have provided "unalerted misleading signal-in-space information" (UMSI). UMSI

constitutes a loss of SPS SIS integrity while AMSI is not a loss of integrity since the alert is timely.

Breaches of the relevant NTE tolerance for less than the overall TTA of 10 seconds do not require

a timely alert.

Preemptive actions taken by the Control Segment (as described in paragraph A.5.4.3) are fully

satisfactory as alerts for integrity purposes. To be timely, the last bit of the NAV message

subframe which contains the particular SPS SIS warning indication must be present at the

receiving antenna within 8 seconds of the NTE tolerance being breached. Preemptive Control

Segment actions are taken well in advance of scheduled O&M activities that are likely to cause a

large SPS SIS URE (e.g., station keeping maneuvers and atomic clock maintenance). The fact

that Control Segment preemptive actions occur early (e.g., 5 minutes or more) with respect to the

NTE tolerance being breached has no adverse impact on SPS SIS integrity.

April 2020 GPS SPS PS

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A.5.6 Nature of SPS SIS URE Time Derivatives

GPS receivers provide end users with velocity information in addition to position and time

information. This is evident in the fact that the outputs of a GPS receiver are commonly referred

to as PVT (i.e., position, velocity, and time). The accuracy and integrity of the GPS receiver's

velocity output depends in large part on the accuracy and integrity of the SIS velocity. The SIS

velocity errors are called the "first time derivatives of the URE", "pseudorange rate errors", or

URRE. GPS receivers generally do not output acceleration information. SIS acceleration errors

-- more properly the "second time derivatives of the URE", "pseudorange acceleration errors", or

URAE -- are important primarily for their effect on the integrity of the GPS receiver's output PVT

information and for the limitations they impose on augmentations like differential GPS (DGPS)

and inertial aiding.

A.5.6.1 URE Time Derivative Illustrations for Integrity

A typical large pseudorange rate error was illustrated by Figure A.5-1(a) given earlier for a URE

ramp error effect. Figure A.5-1(a) showed the instantaneous URE being well behaved before the

start of the failure, then ramping off rapidly after the failure onset at a relatively constant rate.

Since the instantaneous URRE is the slope of the instantaneous URE, the ramp error for the

instantaneous URE thus represents a step error for the instantaneous URRE as shown in Figure

A.5-2(a). This type of instantaneous URE ramp error -- or instantaneous URRE step error -- is

the integrity failure paradigm for testing RAIM algorithms in GPS receivers (see Appendix B for

information on RAIM). The URRE step used for RAIM testing is 5 m/sec. Although fine for test

purposes, it is not a representative failure magnitude for the actual SPS SIS.

Figure A.5-2(b) shows an instantaneous URE step error with a constant (near zero) instantaneous

URRE on both sides of instantaneous URE step error. This sort of instantaneous URE step error

is a much larger version of the URE step changes which are seen at NAV message data

transitions after an upload cutover and the smaller URE step changes which are seen at the NAV

message data set cutovers every two hours (see Figure A.4-4). While none of these

instantaneous URE step changes necessarily involves a finite-duration instantaneous URRE to

get from one instantaneous URE to the other, an instantaneous URRE will be perceived if the

simple difference between two instantaneous UREs which straddle the step change is computed

and divided by the difference in time separating the two instantaneous UREs. In the limit, as the

time difference between two instantaneous UREs becomes smaller and smaller, this perception

will converge to an infinitely large instantaneous URRE occurring over an infinitely short duration.

Another typical URRE error arises from an instantaneous URE sinusoid error effect as illustrated

by Figure A.5-2(c). After the failure occurs in this figure, the instantaneous URE is at first positive,

then it gradually becomes zero at the maximum positive URE, then it becomes negative, and then

April 2020 GPS SPS PS

5th Edition Page A-34

Figure A.5-2. Types of URE/URRE Effects

it starts to become zero again. This is a fairly common type of instantaneous URE. It is seen

when ephemeris errors are mapped along the line-of-sight to a GPS receiver over the full in-view

period. The peak URE magnitudes of these pseudorange rate errors are usually only about half

of the URE budget. Because the instantaneous URRE is the time derivative of the instantaneous

URE, the instantaneous URRE associated with a sinusoidal instantaneous URE is also a sinusoid,

but offset in phase by a quarter period (i.e., the time derivative of a sine wave is a cosine wave).

Depending on the particular time scale involved, there may or may not be noise-like instantaneous

URREs associated with the instantaneous URE noise error like those shown in Figure A.5-2(d).

If the instantaneous URE noise time scale is very long, then there will be instantaneous URREs

whose characteristics are equivalent to many successive ramp errors with random magnitudes

and durations. If the time scale is very short, then there will only be instantaneous URREs with

infinitely large magnitudes occurring over infinitely short durations.

A.5.6.2 URE Second Time Derivative Illustrations

Figure A.5-2(a) showed a step change in the instantaneous URRE where the instantaneous

URRE is near zero before the instantaneous URE ramp starts and jumps up to a near-constant

positive instantaneous URRE value immediately afterwards. Just as there does not need to be a

finite-duration instantaneous URRE when an instantaneous URE step change occurs to get from

one instantaneous URE to the next, there does not need to be a finite-duration instantaneous

a. URE Ramp Error - URRE Step Error

Time

b. URE Step Error - URRE Spike

Failure

Onset

+ SIS URE NTE

Tolerance

- SIS URE NTE

Tolerance

MSI

Start

Onset

Instantaneous URE

/URRE

Instantaneous URE

/URRE

Time

Failure

Onset

+ SIS URE NTE

Tolerance

- SIS URE NTE

Tolerance

MSI

Start

Onset

Time

c. URE Sinusoid Error -

URRE Sinusoid Error

Failure

Onset

+ SIS URE NTE

Tolerance

- SIS URE NTE

Tolerance

MSI

Start

Onset

Instantaneous URE

/URRE

Time

d. URE Noise Error - URRE Noise Error

Failure

Onset

- SIS URE NTE

Tolerance

+ SIS URE NTE

Tolerance

MSI

Start

Onset

Instantaneous URE

/URRE

April 2020 GPS SPS PS

5th Edition Page A-35

URAE when a URRE step change occurs to get from one instantaneous URRE to the next. The

rationale is analogous to that given in the preceding section regarding the URE step change in

Figure A.5-2(b). Some GPS receivers may perceive an instantaneous URAE when an

instantaneous URE ramp error occurs if they simply compute the difference between two

instantaneous URREs which straddle the start of the ramp error and divide by the difference in

time separating the two instantaneous URREs; but there really is no finite-duration instantaneous

URAE for the "sharp" ramp error, only an infinitely narrow instantaneous URAE spike as shown

in Figure A.5-3(a). A "dull" instantaneous URE ramp error, however, may have a large finite-

duration instantaneous URAE -- particularly if the magnitude of the ramp error starts out small but

grows over time.

Figure A.5-3(b) shows a double-headed infinitely narrow instantaneous URAE spike occurring at

the time of the instantaneous step change in the URE. The reason for the double-headed URAE

spike is intuitive if one imagines the instantaneous URE step change being caused by the URRE

spike shown in A.5-2(b). The positive portion of the URAE spike occurs when the URRE spike

starts and the negative portion occurs when the URRE spike ends. Since the URRE spike is

infinitely narrow, the two portion of the URAE spike overlap and result in what appears to be a

double-headed spike.

Figure A.5-3. Types of URE/URAE Effects

Instantaneous URE

/URAE

Time

a. URE Ramp Error - URAE Spike Error

Failure

Onset

+ SIS URE NTE

Tolerance

- SIS URE NTE

Tolerance

MSI

Start

Onset

Time

b. URE Step Error -

URAE Double Spike Error

+ SIS URE NTE

Tolerance

- SIS URE NTE

Tolerance

MSI

Start

Onset

Failure

Onset

Instantaneous URE

/URAE

Time

d. URE Noise Error - URAE Noise Error

Failure

Onset

- SIS URE NTE

Tolerance

+ SIS URE NTE

Tolerance

MSI

Start

Onset

Instantaneous URE

/URAE

Time

c. URE Sinusoid Error -

URAE Sinusoid Error

Failure

Onset

+ SIS URE NTE

Tolerance

- SIS URE NTE

Tolerance

MSI

Start

Onset

Instantaneous URE

/URAE

April 2020 GPS SPS PS

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Figure A.5-3(c) illustrates the most common form of instantaneous URAE. Just as the most

common form of instantaneous URRE has a sinusoidal effect because it is the first time derivative

of an instantaneous URE with a sinusoidal effect, the most common form of instantaneous URAE

has a sinusoidal effect because it is the second time derivative of a sinusoidal URE.

Note:

1. The first time derivative of a sine wave is a cosine wave, and the second time derivative of a sine

wave is an inverted sine wave.

Like Figure A.5-2(d), Figure A.5-3(d) shows instantaneous URAEs with a noise effect. This need

not be the case, however, depending on the particular instantaneous URE noise time scale.

A.5.6.3 Combinations of URE Time Derivatives

Many potential integrity faults manifest themselves via a combination of instantaneous URE time

derivatives. A typical example is shown in Figure A.5-4. This is an example where the satellite

first causes the failure and then autonomously repairs it. Figure A.5-4(a) shows a pair of offsetting

instantaneous URE ramp failures (equal magnitudes but opposite signs) separated by a short

period of constant large instantaneous URE. Recognize the intervening period does not

constitute an instantaneous URE step error although it appears similar. Figure A.5-4(b) shows

this same example in the first time derivative domain. This figure clearly shows the pair of

instantaneous URE ramp failures as equal but opposite sign instantaneous URRE step changes.

Because the second time derivative of an instantaneous URE ramp failure is zero (i.e., no

instantaneous URAE), Figure A.5-2(c) shows a constant instantaneous URAE of zero except for

the four double-headed instantaneous URAE spikes which occur at the start and finish of each

instantaneous URE ramp failure.

A.5.7 SPS SIS Component Combination Integrity Considerations

The preceding information applies specifically to the C/A-code signal and also applies in general

to the other SPS SIS components as indicated. There are some additional considerations,

however, which only apply in the context of the SPS SIS component combinations.

A.5.7.1 ‘Per SPS SIS Component Combination’ Integrity Accounting

As indicated by Tables 3.5-1 through 3.5-5, SPS SIS integrity accounting applies to each SPS

SIS component combination separately. Strictly speaking, a common-cause failure which affects

all SPS SIS components concurrently (e.g., a huge satellite clock frequency shift) can be said to

result in up to twenty integrity failures because there are up to twenty SPS SIS component

combinations identified in Table 2.2-2 that can each be affected. Per se, this also represents an

up to twenty-fold increase in the total number of integrity failures compared to previous editions

of this SPS PS where there was only one SPS SIS component that could suffer an integrity failure.

Although the total number of integrity failures increases, the SIS integrity performance is

unchanged because of the ‘per SPS SIS component combination’ definition specified in Tables

3.5-1 through 3.5-5. The SIS integrity performance standards for ‘L1 C/A-code+LNAV data’ are

the same as they ever were.

April 2020 GPS SPS PS

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Note:

1. If a satellite operating in the minimum SIS broadcast configuration per Table 1.6-2 (i.e., C/A-code

+ LNAV data) suffers an integrity-related failure in its LNAV data, there is obviously only one

integrity failure to impact users. Conversely, if a satellite operating in the objective SIS broadcast

configuration per Table 1.6-2 (i.e., C/A-code + LNAV data, CM-code + CNAV data, CL-code, I5-

code + CNAV data, Q5-code) suffers an integrity-related failure in just its CNAV data, there are

nineteen integrity failures to impact users; the twentieth SPS SIS component combination in Table

2.2-2 (C/A-code + LNAV data) is unaffected by the integrity-related CNAV failure.

Figure A.5-4. Combined URE Time Derivative Examples

a. Pseudorange (URE) Domain

URE Effect

Time

Failure

Onset

+ SIS URE NTE

Tolerance

- SIS URE NTE

Tolerance

HMI

Start

Onset

HMI

End

Onset

Failure

Repaired

Rate Effect

Time

b. Pseudorange Rate (URRE) Domain

Failure

Onset

Failure

Repaired

Acceleration Effect

Time

c. Pseudorange Acceleration (URAE) Domain

Failure

Onset

Failure

Repaired

April 2020 GPS SPS PS

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A.5.7.2 Independence of SPS SIS Component Combination Integrity

Although typically correlated, an integrity failure affecting one SPS SIS component combination

does not necessarily mean there is a corresponding integrity failure affecting other SPS SIS

component combinations. Examples where this independence is observed include:

 LNAV data faults versus CNAV data faults

 LNAV IAURA values versus CNAV IAURA values

 LNAV health changes asynchronous with CNAV health changes (temporary)

 Signal-specific ISC faults

Note:

1. In some cases (such as a failure in the signal generation function of one signal), not all broadcast

signals will be affected. However, this SPS PS does not grant partial credit for this situation

because a large group of users will be impacted even though many may be spared. Similarly,

different integrity standards are not provided for each individual signal or combination, because

they are equivalent for all practical purposes and because the standard is conservative enough that

it masks any small differences between them.

April 2020 GPS SPS PS

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SECTION A.6 Continuity

A.6.1 Relationship with Section 3.6

Section 3.6 contains the SPS SIS performance standards for continuity. This section provides

background information relative to the SPS SIS continuity performance standards. The SPS SIS

performance standards for continuity only apply to the C/A-code signal and the LNAV data stream

in this edition of the SPS PS.

A.6.2 Various Types of Failures and the Impacts on Continuity

A.6.2.1 Hard Failures

Satellites can suffer failures that result in the cessation of SPS SIS transmissions. Such failures

are known as "hard failures". The cessation of SPS SIS transmissions need not be sudden as a

result of the hard failure, it can be gradual (e.g., a steady drop in transmitted SPS SIS power

would be a gradual cessation). Some hard failures result in an immediate cessation of SPS SIS

transmissions, while others result in a delayed cessation (e.g., if a satellite fails such that it can

no longer accept new uploads of NAV message data, it will gracefully degrade in the extended

operations mode for at least 14 days until the SPS SIS becomes unavailable). Many different

types of hard failures are possible.

Hard failures are subdivided into two main categories: (1) long-term failures, and (2) short-term

failures. Long-term (LT) hard failures are basically those failures which result in an irrecoverable

loss of the SPS SIS from the satellite. The normal remedy for LT hard failures is the lengthy

process of launching a replacement satellite. In contrast, short-term (ST) hard failures result in

only a temporary loss of SPS SIS from the satellite. The usual remedy for ST hard failures is the

relatively rapid process of switching the satellite configuration over to using a redundant

subsystem instead of the failed subsystem. All critical satellite subsystems have on-board

redundancy.

Whether the hard failure of an otherwise functional satellite occupying a baseline/expandable slot

results in a loss of continuity or not depends on the Control Segment issuing a NANU in advance

of the SPS SIS interruption. If the nature of the hard failure is such that the Control Segment

issues the NANU at least 48 hours in advance of the interruption, then there is no loss of

continuity. If the hard failure results in a sudden or rapid loss of the SPS SIS from the satellite

such that the Control Segment cannot issue a NANU at least 48 hours in advance, then there is

a loss of continuity.

An alternate means of avoiding a loss of continuity exists in those situations where a second

satellite occupies the same slot in the baseline 24-slot constellation (or occupies the same

position in the case of an expandable 24-slot constellation configuration). So long as the second

satellite does not cease its SPS SIS transmissions, the sudden or rapid loss of the SPS SIS in

either of the two satellites occupying that slot does not cause a loss of continuity.

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Notes:

1. The "48 hour in advance" threshold exists for operational reasons related to air traffic control and

flight planning (i.e., NOTAMs -- see DOT-VNTSC-OST-R-15-01). It is not based on any technical

characteristics of the SPS SIS. Internally, the CS uses a "96 hour in advance" threshold.

2. There are no SPS SIS continuity standards applicable to auxiliary satellites. This is true no matter

where the auxiliary satellite is located. It is expected that most auxiliary satellites will exhibit lower

continuity than satellites occupying slots since most auxiliary satellites will have been made

auxiliary because the satellite is near the end of its useful life and is thus more prone to failures.

A.6.2.2 Wear-Out Failures

Satellites are subject to wear-out failures. Wear-out failures differ from hard failures in that wear-

out failures are generally predictable (i.e., “schedulable”). Hard failures are generally not

predictable. Wear-out failures are a characteristic of the satellite "end-of-life" (EOL) operating

phase. They do not occur on recently launched satellites nor do they occur on satellites in the

"middle age" operating phase. Wear-out failures are all ultimately LT failures, but it is frequently

possible to prolong the usefulness of satellites in the EOL phase by the Control Segment

expending substantial effort.

Historically, one of the most likely wear-out failures that can affect satellites is a gradual reduction

in on-board electrical power capacity which continues over time until the satellite can no longer

sustain full mission capability. When a satellite reaches this point in its life cycle, the Control

Segment can reconfigure the satellite to eliminate some of the load(s) on its electrical subsystem

to maintain at least partial mission capability. Because RF transmitters are one of the major power

loads, disabling one or more of the RF transmission chains is reasonable way of enabling

continued use of the other RF transmitters. In the future, the Control Segment may ultimately

choose to follow this course of action with older satellites and intentionally disable transmission

of the L5 signals to leave enough power to maintain transmission of the L1 and L2 signals.

It is possible for a wear-out failure during the EOL phase to cause a loss of continuity, but this

requires one of two unlikely errors on the part of the Control Segment. It is extremely improbable

that the Control Segment would fail to predict the wear-out failure in advance. Much more

probable, but still unlikely, the Control Segment could underestimate the effort needed to prolong

the life of a satellite in the EOL phase. If the Control Segment chooses to prolong the life of an

EOL satellite but later comes up short and cannot expend the necessary effort due to unforeseen

circumstances, then -- unless a NANU was issued at least 48 hours before the shortfall -- a loss

of continuity will occur.

Notes:

1. The preferred means of avoiding the loss-of-continuity risk posed by a wear-out failure is to simply

replace a satellite occupying a slot when it reaches the EOL operating phase. This is not the

required means of avoiding the loss-of-continuity risk, however. The decision to replace a worn

out satellite in a slot or to instead attempt to prolong its life and accept the loss-of-continuity risk is

made based on many factors including cost and Control Segment operator workload.

2. An alternate means of minimizing the loss-of-continuity risk posed by a wear-out failure is for the

Control Segment to: (a) predict the future point in time where the wear-out failure will become

severe enough to pose a non-trivial loss-of-continuity risk, (b) at least 48 hours (nominally 96 hours)

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in advance of that point in time issue a NANU warning that the SIS/satellite will become unavailable

at that time, and (c) preemptively set the 6-bit health status word in subframe 1 to indicate the SIS

is unhealthy at the planned time.

A.6.2.3 Soft Failures

Section 3.5 addresses integrity failures. Integrity failures are known as "soft failures" in that while

a failure has occurred, the SPS SIS continues to be available without an alert indication (alarm or

warning) that the failure has occurred. Because the SPS SIS continues to be available to users,

soft failures do not -- in and of themselves -- constitute a loss of continuity.

Although soft failures do not constitute a loss of continuity themselves, they can certainly trigger

a loss of continuity. Certain soft failures are autonomously detectable on-board a satellite. If the

satellite detects and reacts to that soft failure by transmitting an alert, it is actually the alert that

makes the SPS SIS unavailable to users and thereby causes the loss of continuity. (Soft failures

are not predictable, so there is no way to issue a NANU regarding them at least 48 hours in

advance). The same principle applies when the Control Segment detects and reacts to a soft

failure. The loss of continuity occurs when the Control Segment reaction causes the SPS SIS to

become unavailable to users without a 48-hour advance warning. This principle is similar to the

one which applies to fault detection alerts issued by the RAIM algorithm in a GPS receiver. A

loss of continuity occurs if the fault cannot be excluded and a "do not use" alert is displayed to

the user.

In the case of a loss of continuity triggered by a soft failure, the Control Segment will provide

notification via a NANU as soon as possible after the event.

A.6.2.4 Satellite O&M Activities

Certain types of routine satellite O&M that are almost certain to cause a large SPS SIS URE (e.g.,

station keeping maneuvers and atomic clock maintenance) are not commonly referred to as

failures. However, from a strict integrity perspective, most of them do result in MSI and could thus

be correctly classified as a form of failure. These types of "O&M-induced failures" are unique

compared to all other failures in that they are planned in advance by the Control Segment. Being

planned, the Control Segment can prepare for them and can take preemptive actions to ensure

that any MSI the O&M activity causes is AMSI (e.g., by performing an upload prior to the start of

the O&M to set the SPS SIS unhealthy in the broadcast NAV data stream, or by SatZapping the

satellite to make the SPS SIS unavailable to users). Taking this preemptive action precludes

UMSI and thus prevents any impact to SPS SIS integrity. Although the preemptive action of

taking the satellite off-line before the O&M activity is good for SPS SIS integrity, it is not

necessarily good for SPS SIS continuity since it is an interruption in service that could potentially

lead to a loss of continuity.

Since these "O&M-induced interruptions in service" are normally planned well in advance, the

Control Segment can also take further preemptive actions to prevent any impacts to SPS SIS

continuity. The typical required preemptive action is issuing a NANU regarding the planned

interruption at least 48 hours in advance of the start of the SPS SIS scheduled outage period.

From a strict continuity perspective, a NANU only needs to be issued for a scheduled interruption

affecting a satellite occupying a slot that is not backed up by a second satellite in the orbital slot

(or location in the case of an expanded slot configuration). However, due to prior convention and

operational reliance on the NANUs, all scheduled interruptions currently require a NANU to be

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issued at least 48 hours in advance to avoid a loss of continuity. The Control Segment normally

issues NANUs at least 96 hours in advance of scheduled outages.

The Control Segment can easily cause a loss of continuity of the SPS SIS by failing to issue the

required NANU at least 48 hours in advance of the scheduled interruption in service. Such a loss

of continuity is considered to be reasonably probable.

Note:

1. Even though O&M-induced interruptions in service are normally short term, the duration of the loss

of SPS SIS availability is not a determining factor for losing continuity. So long as the outages last

longer than the TTA of 8 seconds for integrity (which O&M-induced interruptions do), the outages

will all pose the same risk of a loss of continuity. Each loss of continuity counts as a single loss of

continuity no matter if the loss lasts for one minute or it lasts for a thousand minutes. Although the

duration of the interruption does not affect the SPS SIS continuity, it very much does affect the SPS

SIS availability.

A.6.3 Losses of Continuity

For the SPS SIS from a satellite occupying a slot, continuity is lost any time there is an

unscheduled loss of SPS SIS availability from that slot (unscheduled defined from the user

perspective relative to the 48-hour advance warning threshold). SPS SIS alert indications per

paragraph 2.3.4 and SPS SIS "do not use" health indications per paragraph 2.3.2, which are

defined as ways to maintain integrity, are also further defined as ways to lose continuity if they

occur without at least 48 hours of advance warning.

A.6.4 Expected Frequencies for Losses of Continuity

The expected mean time between failure (MTBF), per-satellite, for the various types of failures

defined in Section A.6.2 are as follows:

LT Hard Failures. The expected MTBF for LT hard failures is about twice the Block IIR

satellite design life of 7.5 years (roughly 15 years MTBF).

ST Hard Failures. The expected MTBF for ST hard failures is about one-fifteenth the

Block IIR satellite design life of 7.5 years (roughly 0.5 years MTBF).

EOL (Hard) Failures. The expected MTBF for EOL (hard) failures is about the same as

the Block IIR satellite design life of 7.5 years (roughly 7.5 years MTBF).

Soft Failures. The expected MTBF for soft failures is no greater than the MTBF for LT

hard failures (roughly 15 years MTBF or less).

Satellite O&M Activities. The expected "MTBF" for satellite O&M activities is no greater

than the MTBF for ST hard failures (roughly 0.5 years "MTBF" or less).

Ideally, there should be no losses of continuity associated with either EOL (hard) failures or

satellite O&M activities since the interruptions are schedulable and the Control Segment can be

arbitrarily rigorous about issuing the required NANUs 48 hours in advance. There is, however,

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no standard which defines the level of rigor the Control Segment must maintain in issuing the

NANUs other than the internal "at least 96 hours in advance" threshold with unspecified probability

of success. The worst case would obviously be no rigor whatsoever. Even though unrealistic,

this does put an upper bound on the expected frequency of the losses of continuity. Comparing

the 7.5 year MTBF for EOL (hard) failures against the 0.5 year "MTBF" for satellite O&M activities,

it is obvious that the satellite O&M activities completely dominate the EOL (hard) failures. The

resulting mean time between loss of continuity (MTBLOC) is thus 0.5 years increased by whatever

level of rigor the Control Segment can maintain.

In contrast to the schedulable interruptions, there should be losses of continuity associated with

all of the unschedulable interruptions due to LT hard failures, all ST hard failures, and most soft

failures. All LT hard failures and all ST hard failures should have an associated loss of continuity

since there is no way the Control Segment can issue NANUs at least 48 hours in advance for

these failures. Most soft failures will also have an associated loss of continuity since they rapidly

lead to an interruption in service and the Control Segment will not be able to issue a NANU at

least 48 hours in advance. Some soft failures will not lead to an interruption in service, however.

Soft failures which the Control Segment resolves using contingency uploads do not involve an

interruption in service. Like with the schedulable interruptions, the MTBF for one of the failure

modes (ST hard failures) completely dominates the MTBFs for the other two failure modes in

determining the composite MTBLOC.

A.6.5 Expandable Slot Continuity

The expandable slots defined in Tables 3.2-1, 3.2-2 and 3.2-3 are considered to lose continuity

when either:

(1) The expandable slot is in the baseline configuration, and the satellite occupying the orbital

location defined in Table 3.2-1 for the slot loses continuity.

(2) The expandable slot is in the expanded configuration, and either one of the pair of

satellites occupying the orbital locations defined in Table 3.2-2 for the slot loses continuity.

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SECTION A.7 Availability

A.7.1 Relationship with Section 3.7

Section 3.7 contains the SPS SIS performance standards for availability. This section provides

background information relative to the SPS SIS availability performance standards. The SPS SIS

performance standards for availability only apply to the C/A-code signal and the LNAV data

stream in this edition of the SPS PS.

The two components of SPS SIS availability (the per-slot availability and the constellation

availability) are interrelated. The per-slot availability depends primarily on the satellite design and

the Control Segment procedures for on-orbit maintenance and failure response. The constellation

availability depends primarily on the per-slot availability coupled with the satellite launch policies

and satellite disposal criteria.

A.7.2 Per-Slot Availability

A.7.2.1 Satellite Outage Categories

The various types of failures and interruptions defined in Section A.6 for continuity reasons can

be categorized as a function of the typical outage duration and whether the Control Segment has

any ability to schedule them in advance. The resulting four categories and the straightforward

mapping of the outage reasons are:

1. Long Term Unscheduled (LTU) Outages

 LT Hard Failures

2. Short Term Unscheduled (STU) Outages

 ST Hard Failures

 Soft Failures

3. Long Term Scheduled (LTS) Outages

 EOL (Hard) Failures

4. Short Term Scheduled (STS) Outages

 Satellite O&M Activities

A.7.2.2 Conservative Satellite/Slot Availability Model

The satellite/slot availability model for the baseline 24-slot constellation slots is both simple and

conservative. The assumed maximum life for the satellites addressed in this SPS PS is 7.5 years,

which corresponds to the design life of the Block IIR satellites. Any satellite reaching this age is

assumed to be at EOL and it is assumed it will be replaced. The replacement timeline is assumed

to be schedulable. Each satellite is assumed to have an a priori probability of 0.6 for reaching

EOL and a 0.4 probability of dying early due to an LT hard failure. For those satellites that die

early, the assumed mean age at the LT hard failure is approximately 3.75 years.

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Before the satellite dies or is replaced, the conservative model assumes ST hard failures and soft

failures happen at random. The same is also true for satellite O&M activities.

When it is time to replace a failed satellite, the conservative model assumes the replacement

process does not start until after either the LT hard failure occurs or after the satellite is disposed

of during EOL operations.

Using the MTBF (MTBLOC) expectations given in Section A.6.4 in this appendix and average

outage durations, the numerical satellite/slot availability model parameters are as summarized in

Table A.7-1.

Table A.7-1. Per-Satellite/Slot Availability Model Parameters for Baseline Slots

Model Parameter

Model Value

Before the LT Hard Failure or EOL Disposal:

Average Number of STU Outages

2.0 per year

Mean STU Outage Duration

36.0 hours

Average Number of STS Outages

2.0 per year

Mean STS Outage Duration

12.0 hours

For the LT Hard Failure or EOL Disposal:

Mean time to LT Hard Failure or EOL Disposal

6.0 years

Mean time to replace after LT Hard Failure or EOL Disposal

0.2 years

Notes:

1. The conservative satellite/slot availability model and the parameters in Table A.7-1 represent the

simple "launch on need" (LON) strategy for constellation sustainment.

2. It can be assumed that the satellite/slot availability model and the parameters in Table A.7-1 will

be conservative with respect to whatever Control Segment on-orbit maintenance and failure

response procedures may be adopted by the USSF in the future. The satellite/slot availability

model and parameters in Table A.7-1 are very conservative compared to the current “launch to

sustain” (LTS) strategy established in the GPS CONOPS. (The LTS strategy is a combination of

the LON and “launch on anticipated need” [LOAN] strategies.)

3. Table A.7-1 also applies to expandable slots in their baseline 24-slot constellation configuration.

A.7.2.3 Satellite/Slot Availability Computation

Using the model parameters in Table A.7-1, the fraction of time that a baseline 24-slot

constellation slot will be occupied by a satellite which is transmitting a trackable and healthy SPS

SIS is no less than 0.957 (95.7%) on a long-term average basis. The resulting fraction of time

that the baseline slot will be occupied by a satellite which is transmitting an untrackable, marginal,

or unhealthy SPS SIS (0.043) is the sum of the fraction of time that the satellite will be transmitting

an untrackable, marginal, or unhealthy SPS SIS with an advance warning having been given via

NANU (e.g., "scheduled downtime" due to on-orbit O&M or disposal activities) plus the fraction of

time that a baseline satellite will be transmitting an untrackable, marginal, or unhealthy SPS SIS

with no advance warning having been given (i.e., "unscheduled downtime" due to an on-orbit

failure).

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A.7.2.4 Expandable Slot Availability

The expandable slots defined in Tables 3.2-1, 3.2-2 and 3.2-3 are considered to be available

when either:

(1) The expandable slot is in the baseline configuration, and the orbital location defined in

Table 3.2-1 for the slot is occupied by a satellite which is transmitting a trackable and

healthy SPS SIS.

(2) The expandable slot is in the expanded configuration, and the pair of orbital locations

defined in Table 3.2-2 for the slot are both occupied by a satellite which is transmitting a

trackable and healthy SPS SIS.

(3) The expandable slot is in an equivalent-or-better non-standard configuration (see A.7.2.5),

and the pair of non-standard orbital locations for the slot are both occupied by a satellite

which is transmitting a trackable and healthy SPS SIS.

There are no performance standards in this document related to how often an expandable slot

can be or must be in its expanded configuration. Each expandable slot may therefore be in its

baseline configuration anywhere from 100% of the time to 0% of the time.

A.7.2.5 Equivalent-or-Better Non-Standard Expanded Slot Configurations

The expandable slots defined in Tables 3.2-1, 3.2-2 and 3.2-3 can be filled by satellites in a non-

standard configuration. Any non-standard configuration which provides equivalent-or-better

performance than the nominal (non-expanded) slot is defined to be an acceptable alternative for

either the nominal slot or the expanded slot.

The nominal and expanded configurations of an expandable slot can be graphically illustrated as

shown in Figure A.7-1.

Figure A.7-1. Illustration of Nominal and Expandable-24 Slot Configurations

a. Expandable Slot Occupied by a

Satellite in the Nominal Orbital Location

Direction of

Satellite Motion

Nominal

b. Expandable Slot Occupied by a Pair of Satellites in

the Expanded Orbital Locations

Direction of

Satellite Motion

Fore

Aft

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Figure A.7-2. Illustration of Equivalent-or-Better Non-Standard Configurations (1 of 2)

a. Condensed Slot - 1

Direction of

Orbital Motion

Nominal

Direction of

Orbital Motion

Fore

Aft

b. Condensed Slot - 2

Direction of

Orbital Motion

Nominal

Fore

Nominal

c. Condensed Slot - 3

d. Expanding a Slot -1

Direction of

Orbital Motion

Nominal

Direction of

Orbital Motion

Fore

Aft

e. Expanding a Slot - 2

Direction of

Orbital Motion

Nominal

Fore

Nominal

f. Expanding a Slot - 3

Aft

g. Expanding a Slot - 4

Direction of

Orbital Motion

Nominal

Fore

Aft

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Figure A.7-2. Illustration of Equivalent-or-Better Non-Standard Configurations (2 of 2)

h. Contracting a Slot – 1

Direction of

Orbital Motion

Nominal

Direction of

Orbital Motion

Fore

Aft

i. Contracting a Slot – 2

Direction of

Orbital Motion

Nominal

Fore

Nominal

j. Contracted Slot – 3

k. Reallocating from a Contracted Slot – 4

Direction of

Orbital Motion

Nominal

Direction of

Orbital Motion

Fore

Aft

l. Simultaneously Contracting and Reallocating – 1

Direction of

Orbital Motion

Nominal

Fore

Nominal

m. Simultaneously Contracting and Reallocating – 2

Aft

n. Simultaneously Contracting and Reallocating – 3

Direction of

Orbital Motion

Nominal

Fore

Aft

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Example non-standard configurations which provide equivalent-or-better performance than the

nominal configuration are graphically illustrated in Figure A.7-2. Mirror images of the illustrated

examples also provide equivalent-or-better performance.

The two contracting-and-reallocating sequences shown on Sheet 2 of Figure A.7-2 are likely

options for when one of the satellites in an expandable-24 slot must be moved elsewhere in the

orbital plane. The contract-then-reallocate sequence (h-i-j-k) and the simultaneous-contract-and-

reallocate sequence (l-m-n) both maintain slot availability during the moves.

A.7.3 Constellation Availability

The fraction of time that varying numbers of slots in the baseline 24-slot constellation are occupied

by satellites that are transmitting a trackable and healthy SPS SIS can be computed using a

simple binomial probability model with the two probabilities of 0.957 and 0.043. The results of

that computation are given in Table A.7-2 compared to the standard model for selected numbers

of occupied slots.

Table A.7-2. Constellation Availability

Number of Baseline Constellation Slots

Binomial Model:

Fraction of Time

Standard Model:

Fraction of Time

All 24 Slots

0.348

0.720

23 or More Slots

0.724

0.890

22 or More Slots

0.918

0.954

21 or More Slots

0.981

0.980

Notes:

1. The bottom line result of the simple binomial model and the standard model are both compatible

with the specified SPS SIS availability performance standard given in Section 3.7.

2. Additional terms in the binomial model are:

20 or more slots 0.99685

19 or more slots 0.99957

18 or more slots 0.99995

Operational constraints (see Section 3.7) preclude the number of occupied slots falling below 20.

3. For additional information on the standard model, see “RAIM Detection and Isolation Integrity

Availability With and Without CAG”, by M. Ananda, J. Leung, P. Munjal, and B. Siegel, in

Proceedings of ION GPS-94, the 7

International Technical Meeting of the Satellite Division of the

Institute of Navigation, Salt Lake City, September 1994.

A.7.4 Slot Occupancy

A satellite occupies a defined slot when the satellite’s footprint on the surface of the Earth overlaps

95% of the slot center’s footprint on the surface of the Earth averaged over an orbit revolution. A

conservative approximation for slot occupancy occurs when the following inequality is satisfied:

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Let:

ΔRAAN = RAAN

satellite

- RAAN

slot

semi-circles

ΔArgLat = ArgLat

satellite

- ArgLat

slot

semi-circles

The 95% requirement approximation is:

¼ x (ΔRAAN)

+ (ΔArgLat + 0.5736 x ΔRAAN)

≤ (0.0333 semi-circles)

Note:

1. The above inequality describes an ellipse whose center is at a nominal slot-center as defined in

Table 3.2-1. The semi-minor axis is along the nominal orbital plane (inclination 55˚). The semi-

major axis is along the great circle perpendicular to the nominal orbital plane (inclination 145˚). The

semi-major axis is 12° x sin(55˚) = 9.83˚ along the great circle. The semi-minor axis is 6˚ in

argument of latitude. Figure A.7-3 illustrates the slot ellipse when the nominal slot center is at the

equator:

Figure A.7-3. Nominal Slot Ellipse

Traditional CS management of the satellites in slots allowed for a ±4˚ tolerance in ArgLat (±2˚

GLAN). The ArgLat limit has been a best practice for minimizing PDOP outages. With the elliptical-

slot definition it is possible for the satellite to drift beyond the ±4˚ ArgLat tolerance. One possible

way for the CS to maintain both the 95% footprint and the ArgLat tolerance is as follows:

a) As RAAN drifts, adjust the target geographic longitude of ascending node (GLAN) by the

formula, ΔGLAN = 0.713 x ΔRAAN. This defines a moving target slot center as the RAAN

drifts.

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b) Limit the allowable ArgLat drift to the minimum of ±4˚ or the slot ellipse.

Based on the above diagram, as the RAAN shifts, the target slot center moves along the semi-

major axis of the slot ellipse. The ArgLat could then drift to either ±4˚ (the orange lines) or the

ellipse (blue curve), whichever is closer to the semi-major axis.

A pair of satellites occupy an expanded slot when the satellites’ combined footprint on the surface

of the Earth overlaps 95% of the baseline slot center’s footprint on the surface of the Earth

averaged over an orbit revolution. The nominal RAAN for each position in an expanded slot is in

the same orbit plane as the baseline slot to support constellation maintenance. However,

satellites in expanded slots can provide better coverage when the expanded slot RAANs are

slightly adjusted outside of the baseline slot’s orbit plane.

Note:

1. One way for the CS to manage satellites occupying expanded slots is to manage the satellite in the

forward position of an expanded slot separately from the satellite in the aft position of an expanded

slot. For satellites in the forward position of an expanded slot, that satellite may also be managed

by ensuring the satellite’s footprint on the surface of the Earth overlaps 95% of the footprint of the

nominal forward position’s center adjusted -5 degrees in RAAN, averaged over an orbit revolution:

¼ x (ΔRAAN + 5 degrees)

+ (ΔArgLat + 0.5736 x ΔRAAN)

≤ (0.03333 semi-circles)

For satellites in the aft position of an expanded slot, that satellite may also be managed by ensuring

the satellite’s footprint on the surface of the Earth overlaps 95% of the footprint of the nominal aft

position’s center adjusted +5 degrees in RAAN, averaged over an orbit revolution:

¼ x (ΔRAAN - 5 degrees)

+ (ΔArgLat + 0.5736 x ΔRAAN)

≤ (0.03333 semi-circles)

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SECTION A.8 Position/Time Domain

A.8.1 Relationship with Section 3.8

Section 3.8 contains the SPS performance standards for the position/time domain. This section

provides background information relative to those SPS position/time domain performance

standards.

Unlike the other SPS performance standards in Section 3 of this document, the SPS position/time

domain performance standards in Section 3.8 do not directly apply to the SPS SIS. The SPS

position/time domain performance standards are instead derived from the SPS SIS performance

standards in Sections 3.2 through 3.7, and the SPS position/time domain performance standards

apply only when interpreted through a specific set of user assumptions. (Compare the wording

difference between “SPS position/time domain performance standards” and “SPS SIS

performance standards”.) The specific user assumptions include the error exclusions identified

in paragraph 2.4.5 as well as the SPS SF L1 C/A-code receiver assumptions in Section 3.8.

These SPS position/time domain performance standards only apply to operations with the SF

C/A-code signal and the LNAV data stream. Future editions of this standard may add SPS

position/time domain performance standards applicable to operations with other SF SPS signals,

the CNAV data stream, and/or the DF/TF combinations of SPS signals.

Although these SPS position/time domain performance standards can be derived from the other

SPS performance standards in Sections 3.2 through 3.7 and are therefore somewhat “redundant”,

they are included in Section 3.8 because of their simplicity in defining and documenting SPS SIS

backward compatibility. The SPS position/time domain performance standards in this 5

Edition

of the SPS PS are backward compatible with the SPS position/time domain performance

standards in 4

Edition of the SPS PS. Similarly, the SPS position/time domain performance

standards in the 4

Edition of the SPS PS are backward compatible with the SPS position/time

domain performance standards in 3

Edition of the SPS PS; and so on and so forth.

A.8.2 Availability of Geometry (and PVT Determination)

The SPS performance standards for the position/time domain in Section 3.8 are dependent on

the availability of ‘good geometry’ of the in-view satellites relative to the assumed user. Without

‘good geometry’, it can sometimes be impossible for users to even determine a PVT solution. The

relative goodness of the satellites-to-user geometry is quantified by a set of metrics collectively

known as the “dilutions of precision” (DOPs). There are four DOP metrics applicable to Section

3.8. They are:

1. Position Dilution of Precision (PDOP) – spatial, three dimensions (3-D)

2. Horizontal Dilution of Precision (HDOP) – spatial, two dimensions (2-D)

3. Vertical Dilution of Precision (VDOP) – spatial, one dimension (1-D)

4. Time Dilution of Precision (TDOP) – temporal, one dimension (1-D)

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Due to their complexity and dependence on specific user assumptions, explanatory material on

the DOP metrics has been placed into a separate appendix (Appendix B). Using that DOP

explanation material as a prerequisite, background information on SPS position/time domain

performance standards subsequently follows in Appendix B. Much of the focus in Appendix B is

on how good the relative satellites-to-user geometry has to be in order to qualify as ‘good

geometry’ for accurate PVT determination purposes.

A.8.3 Availability of Geometry Impacts Due to Expandable-24 Slots

It can be shown that all 7 variations of the expandable-24 constellation defined by Tables 3.2-1,

3.2-2, and 3.2-3 provide global availability of PDOP/HDOP/VDOP/TDOP that is at least as good

as the fully-occupied baseline 24-slot constellation when all orbital locations in the expandable

24-slot constellation variation (baseline slots and expanded slots) are occupied by satellites

broadcasting a trackable and healthy SPS SIS. Expandable slots occupied by a pair of satellites

enhance the overall SPS SIS performance; but no credit can be taken for them relative to the

baseline 24-slot constellation performance standards. The SPS PS provides no standards for the

probabilities of any of the expandable slots being in their expanded configurations and occupied

by pairs of satellites broadcasting trackable and healthy SPS SISs.

A.8.4 Availability of Geometry Impacts Due to Auxiliary Satellites

The presence or absence of auxiliary satellites (i.e., operational satellites which are not occupying

a defined orbital slot in the baseline/expandable 24-slot constellation) does not affect the

availability of good PDOP/HDOP/VDOP/TDOP from the baseline/expandable 24-slot

constellation. If present and broadcasting trackable and healthy SPS SISs, auxiliary satellites

can be shown to enhance the provided availability of good PDOP/HDOP/VDOP/TDOP; but no

credit is taken for them relative to the baseline 24-slot constellation performance standards.

Similarly, the absence of auxiliary satellites has no adverse impact on the availability of good

PDOP/HDOP/VDOP/TDOP relative to the baseline 24-slot constellation performance standards.

There are no standards given for the probabilities of any number of auxiliary satellites being

present.

A.8.5 Position/Time Domain Time Derivatives

The position/time domain includes the time derivates of the position and time quantities. For

example, velocity is in the position/time domain because velocity is the time derivative of position.

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GLOBAL POSITIONING SYSTEM

STANDARD POSITIONING SERVICE

PERFORMANCE STANDARD

APPENDIX B

STANDARD POSITIONING SERVICE

POSITION, VELOCITY, AND TIME (PVT)

PERFORMANCE EXPECTATIONS

April 2020

Integrity - Service - Excellence

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TABLE OF CONTENTS

SECTION B.1 Introduction ....................................................................................... B-1

B.1.1 Historical Perspective .............................................................................................................. B-1

B.1.2 Global Utility Metaphor ............................................................................................................ B-3

B.1.3 Direct Use of the Information in Section 3 ............................................................................... B-3

B.1.4 Indirect Use of the Information in Section 3 (PVT Performance) ............................................ B-4

SECTION B.2 Computing PVT Accuracy ................................................................ B-5

B.2.1 Basic Equations for PVT Accuracy .......................................................................................... B-5

B.2.2 Basic Equation for Time Transfer Accuracy ............................................................................ B-6

B.2.3 UERE Values ........................................................................................................................... B-6

B.2.3.1 Specified UERE Values .................................................................................................. B-6

B.2.3.2 Derived UERE Values ..................................................................................................... B-6

B.2.3.3 Hypothetical UERE Values ............................................................................................. B-8

B.2.3.4 Specified URE Values ..................................................................................................... B-8

B.2.3.5 Higher-Fidelity UERE Values .......................................................................................... B-8

B.2.3.6 Dissimilar UERE Values .................................................................................................. B-8

B.2.4 DOP Values ............................................................................................................................. B-9

B.2.4.1 DOP Values at a Time-Space (T-S) Point ...................................................................... B-9

B.2.4.2 Computing DOP Values .................................................................................................. B-9

B.2.4.3 Receiver Algorithms for Selecting SISs/Measurements to be Used ............................... B-10

B.2.5 Combining UERE/UERRE and DOP Values ........................................................................... B-11

SECTION B.3 Availability of Geometry and PVT Accuracy ................................ B-12

B.3.1 PDOP6.0 Threshold for ‘Availability of Adequate Accuracy’ ................................................. B-12

B.3.2 Horizontal/Vertical Thresholds for ‘Availability of Accuracy’ ................................................... B-13

B.3.2.1 Sources of the SAT Values ............................................................................................. B-13

B.3.2.2 HDOP Distributions & VDOP Distributions ..................................................................... B-13

B.3.2.3 "Global Average" HDOP Distributions & VDOP Distributions ......................................... B-16

B.3.2.4 "Worst-Case" HDOP T-S Point & VDOP T-S Point ........................................................ B-16

B.3.2.5 Intermediate Population HDOP Distributions & VDOP Distributions .............................. B-17

B.3.2.6 3

Edition of the SPS PS "Global Average" and "Worst-Case Location"

DOP Distributions ......................................................................................................... B-17

B.3.2.7 Picking HDOP-AT & VDOP-AT Values for Computing SAT Values ............................... B-17

B.3.2.8 5

Edition SPS PS SATs and Position Service Availability Standards ........................... B-18

B.3.3 "Classic" Position Accuracy Statistics ..................................................................................... B-18

B.3.3.1 Reasons for Needing the DOP Distributions ................................................................... B-19

B.3.3.2 Basic Procedure for Computing Classic Position Accuracy Statistics ............................ B-22

B.3.3.3 Expanded Procedure for Computing Classic Position Accuracy Statistics ..................... B-23

B.3.3.4 Expanded Classic Position Accuracy Statistics .............................................................. B-24

B.3.4 "Current" Position Accuracy Statistics for Receiver Specification ........................................... B-25

B.3.4.1 Background ..................................................................................................................... B-25

B.3.4.2 “Current” Position Accuracy Statistics for Receivers ....................................................... B-25

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B.3.5 Position/Time Accuracy Standards in this Edition of the SPS PS ........................................... B-23

SECTION B.4 Customized PVT Performance Expectations ............................... B-27

B.4.1 Three Timeframes ................................................................................................................... B-27

B.4.2 Real-Time PVT Performance Expectations Directly from Your Receiver ............................... B-28

B.4.2.1 Real-Time Accuracy Estimates Directly from Your Receiver .......................................... B-28

B.4.2.2 Real-Time Integrity Estimates Directly from Your Receiver ............................................ B-30

B.4.3 In-Advance PVT Performance Expectations ........................................................................... B-34

B.4.3.1 General Rule -- Don't Worry About It .............................................................................. B-34

B.4.3.2 In-Advance PVT Performance Expectations Directly from Your Receiver…. ................. B-38

B.4.3.3 In-Advance PVT Performance Expectations from a Computer Model…. ....................... B-39

B.4.4 After-the-Fact PVT Performance Expectations ....................................................................... B-40

April 2020 GPS SPS PS

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List of Figures

Figure B.1-1. New GPS Organizational Perspective ............................................................................. B-1

Figure B.3-1. HDOP Distribution Curves ............................................................................................... B-14

Figure B.3-2. VDOP Distribution Curves ................................................................................................ B-14

Figure B.3-3. Vertical L95 for UERE=4.0 m 1-sigma and VDOP=1.80 ................................................. B-19

Figure B.3-4. Vertical L95 for VDOP=1.40 Sub-Population and VDOP=2.20 Sub-Population .............. B-20

Figure B.4-1. Three Time Frames .......................................................................................................... B-27

Figure B.4-2. HPL, HUL, HAL Relationships ......................................................................................... B-33

Figure B.4-3. Obscuration Angles versus SPS Receiver Antenna Height ............................................. B-37

Figure B.4-4. Obscuration Angles versus SPS Receiver Antenna Location ......................................... B-37

List of Tables

Table B.2-1. Typical UEE Error Budgets (95%) ..................................................................................... B-7

Table B.3-1. Global Ensemble DOPs for Weighted Mix of Constellation States ................................... B-25

Table B.4-1. EPE-to-FOM Correspondence .......................................................................................... B-30

Table B.4-2. RAIM Availability Examples .............................................................................................. B-34

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SECTION B.1 Introduction

B.1.1 Historical Perspective

GPS has often been described as having three main segments:

1. Control Segment,

2. Space Segment,

3. User Segment.

This is still true from the system architectural perspective. It really is no longer true from the

program organizational perspective. The new organizational perspective is shown in Figure B.1-

Figure B.1-1. New GPS Organizational Perspective

The USG owns the GPS Control Segment and the GPS Space Segment. The USSF operates

and maintains both of these GPS segments. The Space and Control Segments are part of the

GPS program organization. The USG cannot be said to own, operate, or maintain the User

Segment portion of GPS. The User Segment encompasses millions of GPS receivers. While the

USG does develop and procure military PPS receivers for U.S. and allied users, these make up

a very small fraction of the world’s total GPS receiver population.

USER SEGMENT

CONTROL SEGMENT

SPACE SEGMENT

GPS SIS

INTERFACE

K2DBK2DB

April 2020 GPS SPS PS

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Long ago, the GPS Joint Program Office (JPO), which has been reorganized within SMC, began

developing all three segments of GPS. Although the United States Air Force (USAF) was the

executive service responsible for the JPO, the GPS JPO was truly a "joint" program office in that

it included members from all branches of the U.S. military and later grew to include personnel

from allied nations (with particularly strong participation from the North Atlantic Treaty

Organization [NATO] countries) and from other Departments of the USG. The GPS JPO not only

developed the initial Control, Space, and User Segments of GPS; the GPS JPO also operated

and maintained them as well. Around the end of the 1970s, the GPS JPO either owned or directly

controlled virtually every GPS receiver in the entire world. Back then it made sense to specify

GPS requirements in terms of the PVT performance seen by the end user since all three segments

were under GPS JPO control. The GPS receiver's displayed PVT was the final interface at the

end of the GPS process.

During the 1980s, the GPS JPO developed and deployed the Operational Control System (OCS)

which it initially ran before turning it over to Air Force Space Command (AFSPC), now the USSF.

The GPS JPO developed and began deploying the operational Block II series of satellites which

were then handed over to AFSPC for on-orbit operations and maintenance. The GPS JPO also

developed and procured many different types of PPS receivers which were delivered to the Army,

Navy, and Air Force, as well as other federal agencies and allied governments. Not all PPS

receivers were developed or produced by the GPS JPO, however. Programs with specialized

applications that required unique capabilities began developing and producing their own PPS

receivers. Some NATO governments also initiated their own PPS receiver development efforts.

A few civil electronics manufacturers even started producing and selling commercial SPS

receivers. Since the bulk of the world's GPS receivers were still configuration managed by the

GPS JPO, specifying GPS requirements in terms of the PVT performance delivered to the end

user still made sense. But as evidenced by the PPS user equivalent range error (UERE) budgets

which appeared at the end of the 1980s, it had become necessary to specify the PPS SIS

performance to accommodate those PPS receivers not developed by the GPS JPO.

By the late 1990s, most PPS receivers were being bought by military system integrators directly

from the manufacturer for use as a sensor embedded in other products. The GPS JPO was still

developing and procuring a few types of stand-alone PPS receivers for domestic use and foreign

military sales, but those were only a very small fraction of the world's production of PPS receivers.

More and more, PPS receivers have become just another component in integrated systems. End

users often do not see PPS-based PVT, they instead see navigation signals based on integrated

PPS-inertial, PPS-Doppler, or PPS-terrain matching. The PPS receivers embedded in these

systems are purchased, operated, and maintained by organizations other than SMC. Neither

SMC nor the Joint Service System Management Office (JSSMO) nor USSF’s 50

Space Wing

(50 SW), who are SMC’s partners for maintaining SMC-procured PPS receivers and for operating

and maintaining the Control/Space Segments respectively), are responsible for the performance

of these PPS receivers. SMC, JSSMO, and 50 SW responsibilities end at the PPS SIS interface

as shown in Figure B.1-1. The same principle applies to the huge number of SPS receivers

produced in the 1990s -- those SPS receivers are not purchased, operated, or maintained by the

GPS program organization; and the GPS program organization's responsibility towards those

SPS receivers ends at the SPS SIS interface.

The GPS program organizational line of demarcation is at the SPS SIS interface (and PPS SIS

interface) as shown in Figure B.1-1. Operating and maintaining the Control/Space Segments to

produce the SPS SIS is the responsibility of the GPS program organization. The SPS User

Segment is not the responsibility of the GPS program organization.

April 2020 GPS SPS PS

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B.1.2 Global Utility Metaphor

GPS has been metaphorically described as a "global utility". This metaphor is seen in Figure B.1-

1 with the "electrical socket in the sky" representing the SPS SIS interface and the SPS receivers

shown "plugging in" to that interface. The SPS SIS ISs/ICDs (particularly IS-GPS-200 and IS-

GPS-705) are where the technical details like "AC, 60 Hz, 120 volts" are defined. Section 3 of

this standard is where the GPS utility's performance parameters are specified like:

a. The number of amperes each wall socket can deliver (accuracy)

b. The maximum probability of dangerous voltage spikes (integrity)

c. The mean time between unexpected blackouts (continuity)

d. The fraction of time unaffected by blackouts (availability)

e. The area served by the power company (coverage)

The metaphor is particularly appropriate when the GPS program organizations are described as

being "service providers". The metaphor does break down, however, when one tries to apply it

to cost; whereas utilities charge consumers for the services rendered, Federal law requires that

the Secretary of Defense provide for the sustainment and operation of the GPS SPS for peaceful

civil, commercial, and scientific uses on a continuous worldwide basis free of direct user fees (10

USC 2281(b)).

B.1.3 Direct Use of the Information in Section 3

The standards given in Section 3, along with the referenced ISs/ICDs, comprise a full and

complete description of the SPS SIS interface provided to the User Segment's SPS receivers.

Those standards and the ISs/ICDs provide the signal information needed by a manufacturer to

design a SPS receiver that will successfully interface with the SPS SIS. Some of the information

in Section 3 is needed by developers of augmentations systems (e.g., DGPS) to design the

parameters in the augmentation signal. The information in Section 3 is also directly applicable to

designing a system to integrate GPS with an inertial sensor; see Appendix R of RTCA/DO-229

for example.

The information in Section 3 is essential to determining whether the SPS SIS can satisfy maritime

user needs for a worldwide radionavigation system as expressed by the International Maritime

Organization (IMO) in Assembly Resolution A.953(23). The information is also essential for

establishing baseline GPS performance as an International Civil Aviation Organization (ICAO)

standardized Global Navigation Satellite System (GNSS) for both direct and augmented aerial

navigation for all phases of flight up to and including precision approach – see Annex 10 to the

Convention on International Civil Aviation, International Standards and Recommended Practices

(SARPs). For official U.S. Government purposes, Section 3 is the publication cited by the Federal

Radionavigation Plan that defines the specific capabilities provided by the SPS. Section 3 is the

only place where the detailed SPS SIS integrity information needed for SPS RAIM fault detection

(FD) algorithms is specified. The information in Section 3 was specifically required to address

integrity as well as the other Required Navigation Performance (RNP) parameters -- accuracy,

continuity, and availability -- to support a worldwide performance based navigation (PBN)

capability.

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B.1.4 Indirect Use of the Information in Section 3 (PVT Performance)

The standards in Section 3 describe the SPS SIS interface without constraining how the SPS SIS

is used. The SPS SIS standards are independent of the application of the SPS SIS information.

Although this independence is technically correct, there is a long-standing tradition in GPS system

specifications of addressing the implications of the SIS specifications to end users in the form of

PVT accuracies.

Section 3.8 in the main body of this standard continued this tradition by assuming hypothetical

"benchmark" User Equipment (UE) and used it to translate the SPS SIS specifications into user

PVT performance terms. This appendix of the SPS PS perpetuates the tradition by showing how

Section 3.8 used the preceding SPS SIS specifications and benchmark UE assumptions to derive

representative user PVT performance values given in Section 3.

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SECTION B.2 Computing PVT Accuracy

This section introduces the notion of using a computer model to translate GPS SIS performance

standards into user PVT performance expectations. It also describes some computer models that

have been found to be acceptable for translating the SPS SIS performance standards into SPS

PVT performance expectations.

B.2.1 Basic Equations for PVT Accuracy

The basic equation for PVT accuracy in GPS is:

Accuracy = UERE x DOP (B-1)

Equation B-1 is a simple approximation that has been found adequate for many applications. It

is appropriate when all pseudorange errors are zero mean, normally distributed, characterized by

the same UERE such that a single dilution of precision (DOP) number can be used. It is the same

equation used by Section 3.8 to derive representative end user PVT performance values from the

SPS SIS specifications and benchmark UE assumptions. See TOR S3-G-89-01 for additional

information regarding the use of this equation. See Section B.2.3 for more information on UERE

and Section B.2.4 for more information on DOP.

There are different variations of equation B-1 used for different accuracy values (e.g., horizontal

position accuracy, vertical velocity accuracy). The variations of equation B-1 of relevance to this

appendix are:

UHNE = UERE x HDOP (B-2)

UVNE = UERE x VDOP (B-3)

UHVE = UERRE x HDOP (B-4)

UVVE = UERRE x VDOP (B-5)

UTE = UERE x TDOP  c (B-6)

where:

UHNE = User Horizontal Navigation Error (RMS)

UVNE = User Vertical Navigation Error (RMS)

UHVE = User Horizontal Velocity Error (RMS)

UVVE = User Vertical Velocity Error (RMS)

UTE = User Time Error (RMS)

c = speed of light, m/sec

Note:

1. The UHNE and UVNE are called "navigation" errors instead of "position" errors for historical

reasons.

April 2020 GPS SPS PS

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B.2.2 Basic Equation for Time Transfer Accuracy

The basic equation for time transfer accuracy relative to UTC(USNO) in GPS is:

UUTCE = ((UERE x TTDOP  c)

+ (UTCOE)

)

(B-7)

where:

UUTCE = User UTC(USNO) Error (RMS)

TTDOP = Time Transfer Dilution of Precision

UTCOE = UTC(USNO) Offset Error (RMS)

Note:

1. The form of equation B-7 is the root-sum-square (RSS) of two root-mean-square (RMS) values.

The result is still an RMS value.

B.2.3 UERE Values

B.2.3.1 Specified UERE Values

When computing expected PVT accuracy for specification-compliance purposes, the UERE

values to use in equations B-2, B-3, B-6, and B-7, and the UERRE values to use in equations B-

4 and B-5, are the ones given in the appropriate GPS signal specification, system/segment

specification, or equivalent document for the particular circumstances being considered.

Appendix A of this SPS PS gives 3 different DF UERE values in Table A.4-1 and 3 different SF

UERE values in Table A.4-2 for 3 different age of data (AOD) circumstances. (Remember that

the UERE values in Tables A.4-1 and A.4-2 are only illustrations; the only standards given in

Section 3 of this document are for the SPS SIS.)

Notes:

1. Reserved.

2. Equations B-1 through B-7 are all formulated using RMS statistics. Care must be taken to ensure

that the UERE values (or URE values) and UTC(USNO) offset accuracy values used in these

equations are RMS statistics. UERE values, URE values, and UTC(USNO) offset accuracy values

expressed as 1-sigma statistics are equivalent to RMS statistics and can be used directly in

Equations B-1 through B-7. UERE values, URE values, and UTC(USNO) offset accuracy values

expressed as 95% statistics can be converted to RMS statistics for use in Equations B-1 through

B-7 by dividing them by a factor of 1.96 assuming that the errors are zero mean and normally

distributed.

B.2.3.2 Derived UERE Values

To compute expected PVT accuracy for long-term planning purposes for a particular type of GPS

receiver, the UERE values to use in equations B-2, B-3, B-6, and B-7 can be derived as the RSS

of the appropriate GPS SIS URE and the UEE for that particular GPS receiver under

April 2020 GPS SPS PS

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consideration. Recognize that not all SPS receivers are required to satisfy the same UEE

specification. Dating as far back as the late 1980s, the "traditional" UEE specification for a

medium quality SPS receiver is 5.5 m 95% (2.8 m 1-sigma). The commonly assumed "benign

conditions" UEE specification for a high quality SPS receiver is 1.6 m 95% (0.8 m 1-sigma). These

two different UEE values result in two different derived UERE values. For example, consider the

9.7 m 95% value for the SPS SIS URE specified in Table 3.4-1 at any AOD during normal

operations neglecting single-frequency (SF) ionospheric delay model errors (or equivalently, the

9.7 m 95% value obtained by root-sum-squaring (RSS-ing) the Space Segment and Control

Segment contributions to the SPS URE budget in Table A.4-1 in Appendix A of this SPS PS).

This is a "base" URE value to which the appropriate SPS receiver UEE value is root-sum-squared

(RSS-ed). If the UEE specification for the SPS receiver being considered is the traditional 5.5 m

95% value obtained by RSS-ing each of the User Segment contributions to the SPS UERE budget

in Table A.4-2 in Appendix A of this SPS PS, then the derived UERE value to use in equations B-

2, B-3, B-6, and B-7 would be computed as follows:

UERE = ((URE)

+ (UEE)

)

(B-8)

UERE = ((9.7 m 95%)

+ (5.5 m 95%)

)

UERE = 11.2 m 95%

UERE = (11.2 m 95%)  1.96

= 5.7 m 1-sigma

Taking this number and RSS-ing a conservative assumption for the SF ionospheric delay model

error contribution of between 9.8 m and 19.6 m 95% results in exactly the numbers shown in

Table A.4-2.

For reference, the error budgets comprising some typical UEE values applicable to airborne C/A-

code GPS receivers in normal operations are given in Table B.2-1. The SF ionospheric delay

compensation model performance does not appear in the SF receiver UEE budgets because its

accuracy is not under the control of the SF receiver. In contrast, the DF ionospheric delay

compensation process performance does appear in the DF receiver UEE budget because its

accuracy is under the control of the DF receiver. For the traditional specification for DF

ionospheric delay compensation accuracy, see Table A.4-1.

Table B.2-1. Typical UEE Error Budgets (95%)

Error Source

Traditional

Specification,

Improved

Specification,

Modern

Receiver,

Advanced

Receiver,

Ionospheric Delay Compensation

N/A

0.8

Tropospheric Delay Compensation

3.9

4.0

3.9

1.0

Receiver Noise and Resolution

2.9

2.0

0.4

Multipath

2.4

0.5

0.2

Other User Segment Errors

1.0

0.8

UEE (m), 95%

5.5

4.6

4.5

1.6

April 2020 GPS SPS PS

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B.2.3.3 Hypothetical UERE Values

In addition to specified and derived UERE values, it is also possible to compute hypothetical

UERE values and UERRE values to use in equations B-2 through B-7. Hypothetical UERE values

are often used in analytical "what if" studies. A common hypothetical UERE value used in many

studies is the UERE for a "perfect GPS receiver" with zero UEE. This UERE is obtained by setting

the UEE to zero in equation B-8. This is the same as using the SIS URE in lieu of the UERE.

The PVT accuracy using this "perfect GPS receiver" UERE is known as the "SIS-only PVT

accuracy". Another hypothetical UERE value neglects the SF ionospheric delay compensation

model errors (e.g., the example in the previous paragraph).

B.2.3.4 Specified URE Values

An important example of using the SIS-only URE in lieu of the UERE for computing expected PVT

accuracy was in the 3

Edition of the SPS PS, published in 2001. The 3

Edition of the SPS PS

restricted itself to just the SIS, specifically excluding the UERE contribution of ionospheric delay

compensation errors, tropospheric delay compensation errors, receiver tracking channel noise

and resolution errors, multipath errors, and other user segment errors. All of the expected

positioning and timing accuracy standards given in the 3

Edition of the SPS PS were SIS-only

PVT accuracy values based on a 6 m RMS URE over all AODs during normal operations, a

perfect GPS receiver, and neglecting the SF ionospheric delay compensation model errors.

B.2.3.5 Higher-Fidelity UERE Values

Higher-fidelity UERE values can be computed by observing the URA numbers contained within

the transmitted GPS SIS, averaging them over time, and using the results to compute higher-

fidelity "transmitted on-orbit average" URE values to use in equation B-8. Even higher-fidelity

URE values can be computed based on historical trends revealed by instantaneous URE

measurements produced by independent monitors such as differential GPS systems. Such

higher-fidelity URE values commonly reveal long-term variations between the satellites with the

SIS from some satellites consistently being more accurate than others. Caution must be

exercised in making use of any higher-fidelity UERE values computed these ways: (1) equations

B-2, B-3, B-6, and B-7 are only valid if the same UERE value applies to each and every SIS, and

(2) previous URE performance does not provide any guarantee of future URE performance.

B.2.3.6 Dissimilar UERE Values

Equations B-2, B-3, B-6, and B-7 are only valid when all pseudoranges are characterized by the

same UERE value. If the UERE values are different, then different equations must be used for

computing the expected PVT accuracy.

When the UERE values are different and there are pseudoranges available from more than four

visible satellites, most GPS receivers will compute a "weighted position solution". In a weighted

position solution, more or less trust (weight) will be placed on each pseudorange according to the

expected UERE value for that pseudorange. More weight will be placed on pseudoranges with

smaller expected UEREs, while less weight will be placed on pseudoranges with larger expected

UEREs.

There are still DOP values which apply to weighted position solutions, but these DOP values

depend on the specific set of weighting factors used to compute the weighted position solution as

well as the satellites-to-user geometry. To distinguish them from the simple DOP values of an

April 2020 GPS SPS PS

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unweighted position solution, these DOP values are known as "weighted DOPs". Weighted DOPs

are not discussed in this appendix due to their complexity.

B.2.4 DOP Values

B.2.4.1 DOP Values at a Time-Space (T-S) Point

Each particular satellites-to-user geometry has its own set of DOP values. Using the baseline 24-

slot constellation defined in Tables 3.2-1 and 3.2-3, the nominal satellites-to-user geometry can

be computed for any time at any point in the GPS coverage volume. Knowing the satellites-to-

user geometry at a specific time-space (T-S) point, and knowing which subset of the visible

satellite's SISs will be used in the PVT solution or time transfer solution at that specific T-S point,

allows the particular subset of DOP values to be computed for that specific T-S point.

B.2.4.2 Computing DOP Values

Although it is possible to compute the DOP values by hand for a specific T-S point, this is a very

tedious and time-consuming task. Computer models are therefore universally used for computing

the DOP values. Every GPS receiver that provides an output of the current DOP values has such

a computer model inside. RAIM availability prediction programs which are used in aviation

applications also use such a computer model. Stand-alone software for computing DOP values

are available from many sources; these programs all embody a computer model.

Typical computer models for computing the DOP values use the following inputs at a minimum:

a. An almanac data file, with data similar to that shown in Table A.2-1 in Appendix A which

defines the satellite constellation to be used in the computation. The almanac data file

customarily includes all parameters transmitted by the on-orbit satellites as part of their

broadcast almanac data set in the NAV messages, including the health bits. Some

almanac data files also include average URA values based on recent observations.

b. Operator-commanded overrides of the health settings built into the almanac data.

c. The operator-specified T-S point (or set of T-S points) for which the DOP values are to

be computed.

d. Parameters which describe the exact type of GPS receiver to be emulated, especially

the GPS receiver algorithm for selecting the subset of visible satellite SISs to be used in

the PVT solution or time transfer solution (described in the following section).

The output of the typical computer models, at a minimum, are the computed DOP values.

Note:

1. Several stand-alone software programs for computing DOP values and expected PVT accuracy

can be downloaded from the web. Many of these software programs use copies of the broadcast

almanac data in GPS System Effectiveness Model (SEM) format. A different software program for

computing predicted RAIM availability can be accessed at no cost from the web at

http://augur.ecacnav.com. This software program is known as AUGUR. The FAA maintains the

following website with related software https://sapt.faa.gov/default.php. There are many other

software programs available for computing DOP values and related information.

April 2020 GPS SPS PS

5th Edition Page B-10

B.2.4.3 Receiver Algorithms for Selecting SISs/Measurements to be Used

B.2.4.3.1 Satellite (SIS) Selection Algorithm

Most typical computer models allow the operator to control the "satellite selection" algorithm the

emulated GPS receiver will use to select the SISs used in the PVT solution or time transfer

solution. Many modern SPS receivers which can track and use up to a maximum of twelve SPS

SISs will select the SISs from highest twelve satellites in the sky. Advanced SPS receivers

typically have sufficient capability to track and use all SPS SISs in view.

B.2.4.3.2 Other Sensor Measurements

Some computer models allow the operator to control whether the emulated GPS receiver will

mimic the ability of a real GPS receiver to use information input by an aiding sensor. For aviation

use, most GPS receivers can take advantage of vertical position supplied by a barometric

altimeter. Computer models, which predict RAIM availability, emulate this capability by treating

the barometric altimeter measurements as a form of pseudoranges. For maritime use, most GPS

receivers can take advantage of the fact that their vertical position is at mean sea level; some

computer models also emulate this capability. Some GPS receivers can accept acceleration

information from an inertial measurement unit (IMU). The use of inputs from aiding sensors, and

the related computer modeling, is beyond the scope of this appendix. See TSO-C196 and

Appendix G of RTCA/DO-316 for further information on modeling the use of barometric altimeter

inputs for aviation.

B.2.4.3.3 Mask Angle

Most typical computer models allow the operator to control the minimum mask angle above the

local horizon which the emulated GPS receiver will use for determining whether a satellite is

visible (and therefore available). Some SPS receivers have a mask angle of 7.5 degrees, while

others have a mask angle of 5 degrees (see AC 20-138). Modern aviation receivers often use a

mask angle of 2 degrees. Time transfer receivers and some surveying receivers commonly use

a mask angle of 15 degrees. Some "all-in-view" GPS receivers do not have a mask angle per se;

their only limitation on satellite availability is the radio horizon. GPS receivers designed for space

applications usually have negative mask angles.

B.2.4.3.4 Maximum Number of SISs to be Used

Many computer models allow the operator to control the maximum number of SISs the emulated

GPS receiver will use in the PVT solution or time transfer solution. Many modern SPS receivers

can track and use up to a maximum of twelve SPS SISs at a time. If there are, say, exactly five

SPS SISs available at a given T-S point, the difference between the 4-SIS ("4-satellite") DOP

value and the 5-SIS ("5-satellite") DOP value is usually, but not always, substantial. Using five or

more SISs for the PVT solution or time transfer solution always results in DOP values that are at

least as good as the 4-SIS DOP values. Advanced DF SPS receivers typically have sufficient

capability to track and use all SPS SISs in view.

Note:

1. Although not always technically true, a GPS receiver that can track and use 12 SISs at a time is

commonly referred to as being an "all-in-view" (AIV) GPS receiver.

April 2020 GPS SPS PS

5th Edition Page B-11

B.2.5 Combining UERE/UERRE and DOP Values

With a uniform UERE value and UERRE value, the DOPs produced by a suitably configured

computer model can be simply scaled by those UERE and UERRE values according to equations

B-2 through B-6 to determine the expected PVT accuracy for the circumstances being considered.

Alternatively, the computer model may have the capability to use the UERE or UERRE values

and can automatically perform the scaling and output the expected PVT accuracy directly. The

same is also true for time transfer solutions; the operator can manually process equation B-7 or

the computer model can process equation B-7 using the UERE and UTCOE values.

When the UERE values and UERRE values are different across pseudoranges, a computer model

which automatically performs the scaling is essential for reliably determining the expected PVT

accuracy. Manually using equations B-2 through B-6, or equation B-7, is not practical. This is

doubly true if there are more than four pseudoranges available and if the subject GPS receiver

computes weighted position solutions.

April 2020 GPS SPS PS

5th Edition Page B-12

SECTION B.3 Availability of Geometry and PVT Accuracy

This section addresses the fundamental importance of having ‘good geometry’ for all discussions

of user PVT performance in the position/time domain. The availability of ‘good geometry’ from

the in-view satellites relative to the assumed GPS receiver is the critical factor which limits the

availability of accurate position determination. Without good geometry, it can be impossible for a

GPS receiver to even determine a PVT solution. The relationship between availability of ‘good

geometry’ and the availability of PVT accuracy is described herein.

B.3.1 PDOP6.0 Threshold for ‘Availability of Adequate Accuracy’

In the early years of GPS (the late 1970s), when only a few of the Block I series of satellites had

been launched into orbit, the constellation coverage was optimized to support daily GPS receiver

testing periods at various locations in the Southwest United States. For long intervals during the

day, there would be no satellites in view whatsoever at the testing locations. As time progressed

toward the testing period, first one satellite would rise into view followed by another until there

were at least four satellites in view (or, initially, a combination of four SIS transmitters in view

where those transmitters were a mix of on-orbit satellites and ground-based ‘pseudo-satellite’

[“pseudolite”] transmitters). In those days, a rule of thumb was adopted to the effect that the GPS

receiver testing ‘window’ opened as soon as the PDOP of the four SIS transmitters dropped below

a value of 6.0 and that it closed once the PDOP rose back above the value of 6.0.

Since those early years, the PDOP≤6.0 rule of thumb has evolved into the “PDOP Availability

Threshold” (PDOP-AT) criteria for defining ‘availability of adequate accuracy’. This PDOP-AT

value is also used extensively in GPS constellation design and maintenance planning. It is still

one of the primary position/time domain performance standards for backward compatibility – see

Table 3.8-1 in the main body of this SPS PS.

It can be shown that the baseline 24-slot constellation defined by Tables 3.2-1 and 3.2-3 in the

main body of this SPS PS provides a PDOP value of less than or equal to 6.0 continuously at all

points in the terrestrial service volume, under certain user receiving equipment assumptions,

when all orbital slots in the baseline 24-slot constellation are filled with satellites broadcasting a

trackable and healthy SPS SIS. Using this PDOP6.0 threshold as a metric for defining

“availability of adequate accuracy", the baseline 24-slot constellation is said to provide 100%

global availability of adequate accuracy when all orbital slots in the baseline 24-slot constellation

are occupied by satellites broadcasting a trackable and healthy SPS SIS.

Note:

1. For additional information on the use of the PDOP≤6.0 PDOP-AT value, see “The GPS

Constellation Design – Current and Projected”, by P. Massatt and M. Zeitzew, in Proceedings of

the 1998 National Technical Meeting of The Institute of Navigation, Long Beach, CA, January 1998.

2. The fraction of the terrestrial volume over time which satisfies the PDOP≤6.0 PDOP-AT value is

known as the “constellation value” (CV). When all orbital slots in the baseline 24-slot constellation

are occupied by satellites broadcasting a trackable and healthy SPS SIS, and certain user receiving

equipment assumptions are satisfied, the CV is equal to 1.000.

April 2020 GPS SPS PS

5th Edition Page B-13

3. Strictly speaking, the PDOP≤6.0 PDOP-AT value is properly a metric related to “availability of

adequate geometry for adequate accuracy” rather than to “availability of adequate accuracy”.

However, since “adequate accuracy” is qualitative rather than quantitative, a generic assumption

of “adequate URE” (which is generally true) is sufficient to allow the PDOP≤6.0 PDOP-AT value to

serve as a surrogate for “availability of adequate accuracy”. Quantitative “availability of accuracy”

performance standards which also take URE into account are covered in the following paragraph.

B.3.2 Horizontal/Vertical Thresholds for ‘Availability of Accuracy’

A pair of position accuracy values known as the "Horizontal Service Availability Threshold"

(HSAT) and the "Vertical Service Availability Threshold" (VSAT) are the logical evolution of the

PDOP-AT described in the previous paragraph. The HSAT and VSAT values are used by Table

3.8-2 in the main body of this SPS PS to describe the requirements for ‘service availability’ or

equivalently for ‘availability of accuracy’. The HSAT is a horizontal position accuracy of 15 m

95%, the VSAT is a vertical position accuracy of 33 m 95%. These two SAT values are not the

position service availability performance standards themselves, rather they are necessary

conditions (definitions) for specifying the position service availability performance standards. (The

actual position service availability performance standards are the ≥99% and ≥90% numbers in

the left-hand column).

B.3.2.1 Sources of the SAT Values

The HSAT of 15 m 95% and VSAT of 33 m 95% in Table 3.8-2 do not come from any operational

requirement. They are not user requirements. Instead, the HSAT and VSAT values are simply

the result of DOP values picked off a pair of DOP distribution curves multiplied by the 7.0 m 95%

(3.6 m 1-sigma) global statistic URE value during normal operations over all AODs specified in

Table 3.4-1 and a statistical conversion factor. These SAT values are therefore really basically

just the results of equations B-2 and B-3 where the URE value is the one specified in Table 3.4-

1. The HSAT value corresponds to a particular HDOP value and the VSAT value corresponds to

a particular VDOP value. In parallel with the PDOP-AT described in paragraph 3.2.1 above, the

particular HDOP value is known as the “HDOP Availability Threshold” (HDOP-AT) and the

particular VDOP value is known as the “VDOP Availability Threshold” (VDOP-AT). The process

for picking the particular HDOP-AT and VDOP-AT values is described below.

B.3.2.2 HDOP Distributions and VDOP Distributions

The process for picking the particular HDOP-AT and VDOP-AT values to use in equations B-2

and B-3 to compute the HSAT and the VSAT is based on an HDOP distribution and a VDOP

distribution like the ones illustrated in Figures B.3-1 and B.3-2.

Figure B.3-1 shows an HDOP distribution for the baseline 24-slot constellation (non-expanded) in

the form of two histogram curves, one direct and one cumulative. The range of HDOP values is

most easily seen from the direct distribution curve (the dotted one which looks vaguely like a bell-

shaped curve offset away from zero). The smallest HDOP value is 0.68, the biggest HDOP value

is 2.49. The most likely HDOP value (i.e., value with the maximum area under the direct

distribution curve) occurs at 0.91. The cumulative distribution curve (the solid one which starts at

0% at an HDOP of 0.00 and rises to 100% at an HDOP of 2.49) is the one that gives the "no

worse than" (NWT) percentages. This cumulative distribution curve shows 50.0% of the HDOP

April 2020 GPS SPS PS

5th Edition Page B-14

Figure B.3-1. HDOP Distribution Curves

Figure B.3-2. VDOP Distribution Curves

98.0

95.0

Percentage

100.0

90.0

80.0

70.0

60.0

50.0

40.0

30.0

20.0

10.0

00.0

HDOP Values

0.00 1.00 2.00 3.00 4.00 5.00 6.00

: Cumulative

: Direct

99.9

90.0

50.0

98.0

95.0

Percentage

100.0

90.0

80.0

70.0

60.0

50.0

40.0

30.0

20.0

10.0

00.0

VDOP Values

0.00 1.00 2.00 3.00 4.00 5.00 6.00

: Cumulative

: Direct

99.9

90.0

50.0

 1 Sidereal Day

 5 Minute Time Steps

 4x4 Degree Global Grid

 All-In-View Solution

 5 Degree Mask Angle

 No Aiding Sensors

 All 24 Baseline Satellites

 No Expanded Slots

 1 Sidereal Day

 5 Minute Time Steps

 4x4 Degree Global Grid

 All-In-View Solution

 5 Degree Mask Angle

 No Aiding Sensors

 All 24 Baseline Satellites

 No Expanded Slots

April 2020 GPS SPS PS

5th Edition Page B-15

values are NWT 0.94, 90.0% of the HDOP values are NWT 1.16, 95.0% of the HDOP values are

NWT 1.25, 98.0% of the HDOP values are NWT 1.37, and 99.9% of the HDOP values are NWT

1.80.

Figure B.3-2 shows the corresponding VDOP distribution, also in the form of two histogram

curves. Recognize that there really is only one HDOP distribution shown in Figure B.3-1 and one

VDOP distribution shown in Figure B.3-2. The direct distribution curve and the cumulative

distribution curve in each figure are just two different ways of looking at the same DOP distribution.

The HDOP distribution in Figure B.3-1 is only one out of a great many possible HDOP distributions

for the baseline 24-slot constellation (equivalently for the VDOP distribution in Figure B.3-2). That

is why it is said to be an HDOP distribution, not the HDOP distribution. The HDOP distribution in

Figure B.3-1 is specific to the particular conditions identified on the figure, namely:

1. 1 Sidereal Day

2. 5 Minute Time Steps

3. 4x4 Degree Global Grid

4. All-In-View Solution

5. 5 Degree Mask Angle

6. No Aiding Sensors

7. All 24 Slots Occupied by Usable Satellites

8. No Expanded Slots

Of these eight conditions, only the first one, "1 sidereal day", is standard because the baseline

24-slot constellation imposes it (the constellation geometry repeats every sidereal day, roughly

23 hours 56 minutes). The DOP distributions computed over any sidereal day are identical to the

DOP distributions computed over any other sidereal day for the baseline 24-slot constellation

provided the other conditions remain the same. This is not true for DOP distributions computed

over periods which are not an integer multiple of a sidereal day. The DOP distributions computed

over a solar day (exactly 24 hours) do not repeat from solar day to solar day.

The next two conditions, "5 minute time steps" and "4x4 degree global grid", define the set of T-S

points at which the DOP values are computed to go into the DOP distributions. 5 minute time

steps results in 287 independent time steps over a sidereal day. The 4x4 degree global grid refers

to the angular distance between the spatial points uniformly distributed around the equator. The

latitude spacing is uniform from pole to pole, but the longitude spacing varies with the cosine of

the latitude to ensure that each spatial point represents an equal area of the Earth's surface.

These two circumstances vary between different computer programs. For instance, the standard

T-S points used for civil aviation analyses (see RTCA/DO-316) are based on 5 minute time steps

and a 3x3 degree grid which only covers the Northern Hemisphere. Due to symmetry, the

Northern and Southern Hemispheres see the same DOP distributions every half sidereal day.

The fourth, fifth, and sixth conditions, "AIV solution", "5 degree mask angle", and "no aiding

sensors", define the assumptions about receiver algorithms for selecting SISs and measurements

(see paragraph B.2.4.3). These three conditions vary among different computer programs as

much as they do among different types of GPS receivers. These three conditions are the same

ones used in generating the horizontal and vertical SAT values.

April 2020 GPS SPS PS

5th Edition Page B-16

The seventh condition is that "all 24 slots in the baseline 24-slot constellation are occupied by

satellites which are operational and usable (i.e., transmitting a trackable GPS SIS and set

healthy)”. This is the circumstance that varies in picking the HDOP-AT and VDOP-AT values

used to compute the horizontal and vertical SAT values.

The final condition, that there are “no expanded slots”, is a conservative assumption.

B.3.2.3 "Global Average" HDOP Distributions and VDOP Distributions

The DOP distributions in the preceding paragraph are "global averages" in the sense that if one

randomly selects a time during the day and a place on the Earth's surface, then the probability

that one will find a DOP value that is NWT any particular value is given by the cumulative DOP

distribution. From Figure B.3-1 for example; at an "average" point in time and space, there is

50.0% probability of the HDOP being NWT 0.94, 90.0% of the HDOP being NWT 1.16, 95.0% of

the HDOP being NWT 1.25, and so on.

Given the DOP distributions shown in Figures B.3-1 and B.3-2, it is possible to compute the actual

mathematical average HDOP and VDOP values. From Figure B.3-1, the mathematical average

HDOP value is 0.96. This average HDOP value differs slightly from the 50.0% probability HDOP

value of 0.94 because the 50.0% probability HDOP value is actually the median value of the

distribution. The average value of a probability distribution and the median value of that

distribution are generally not equal. The average and the median are equal only for certain special

types of distributions; but those distributions are exceptions rather than the rule.

B.3.2.4 "Worst-Case" HDOP T-S Point and VDOP T-S Point

Instead of averaging over a full sidereal day over the entire globe as described above, one can

instead focus on the "worst-case" T-S point. From Figure B.3-1 for example; the worst-case T-S

point has an HDOP that is 2.49. From Figure B.3-2; the worst-case T-S point has a VDOP that

is 5.43.

Notes:

1. The worst-case T-S point for HDOP is generally not the same T-S point as the worst-case T-S point

for VDOP.

2. Because the satellite orbital period is exactly one-half sidereal day and because the orbits are

north-symmetric about the equator (i.e., near-circular orbits), the relative constellation-to-Earth

geometry repeats four times each sidereal day. With all else remaining the same (“ceteris paribus”),

the worst-case T-S point for HDOP will repeat twice each sidereal day in the Northern hemisphere,

separated by one-half sidereal day in time and by 180 degrees in longitude, and twice each sidereal

day in the Southern hemisphere, also separated by one-half sidereal day in time and by 180

degrees in longitude. The Northern-Southern repeats are out of phase with each other by one-

quarter sidereal day in time and 90 degrees in longitude. Similarly, there are also four identical

worst-case T-S points for VDOP each sidereal day with two occurring in the Northern hemisphere

and two occurring in the Southern hemisphere.

3. With a population of only one T-S point (actually four identical T-S points), it is unconventional to

use names like worst case HDOP “distribution” or worst case VDOP “distribution".

April 2020 GPS SPS PS

5th Edition Page B-17

B.3.2.5 Intermediate Population HDOP Distributions and VDOP Distributions

Between the "global-average" (global population) with all T-S points and the "worst-case" extreme

with only one (four) T-S point, it is possible to define intermediate populations of T-S points for

computing DOP distributions. One intermediate population of interest is a full sidereal day over

the Continental U.S. (CONUS). Another intermediate population is the "worst-case point in time

over a sidereal day" over the CONUS. There are many intermediate populations. With a suitable

computer model, the number of intermediate populations for which one could compute the DOP

distributions is virtually boundless.

For the “worst-case location”’ HSAT and VSAT values, Table 3.8-2 in the main body of this SPS

PS uses the ’worst-case point in space‘ over a day for its intermediate populations. These

populations are subsets of the global population. Before lumping all the DOP values for all the

space points together into the global population, the DOP distributions are computed for each

space point individually. These individual space point DOP cumulative distributions are then

sorted to find the "worst-case location" DOP distributions where "worst" is defined to be the DOP

cumulative distribution with the highest NWT value at a given probability.

B.3.2.6 3

Edition of the SPS PS "Global-Average" and "Worst-Case Location" DOP

Distributions

The analysis that went into the 3

Edition of the SPS PS computed the "global-average" (global

population) and "worst-case location" (single space point population) DOP distributions. These

computations covered the condition where all 24 slots are occupied by satellites which are

operational and usable, as well as the degraded conditions where each satellite in the

constellation is assumed to have either suffered a hard failure or been unusable (24 cases) and

where each pair of satellites in the constellation is assumed to have suffered a hard failure or

been unusable (276 cases).

Notes:

1. As discussed in Appendix B of the 4

Edition of the SPS PS, the DOP distribution results for the

"global-average" (global population) assumed the average pair of satellites down and the "worst-

case" (single space point population) assumed the worst (highest value) pair of satellites down.

B.3.2.7 Picking HDOP-AT & VDOP-AT Values for Computing SAT Values

For backward compatibility purposes, the 4

Edition of the SPS PS used the same HDOP-AT and

VDOP-AT values as the 3

Edition of the SPS PS. This 5

Edition of the SPS PS perpetuates

this use. The HDOP-AT and VDOP-AT values are:

HDOP-AT = 2.10

VDOP-AT = 4.53

The availabilities of HDOPs less than this HDOP-AT value and VDOPs less than this VDOP-AT

value are given as 90% or better at the worst case location, and 99% or better at the average

location, under the 3

Edition of the SPS PS conditions (i.e., the SIS from the worst-case two

slots are unavailable, or the SIS from the average two slots are unavailable).

April 2020 GPS SPS PS

5th Edition Page B-18

Note:

1. Since both the baseline 24-slot constellation definition and the SPS SIS availability performance

standards are unchanged since the 3

Edition of the SPS PS, it is reasonable that the HDOP-AT

and VDOP-AT values should be unchanged as well.

B.3.2.8 5

Edition SPS PS SATs and Position Service Availability Standards

In keeping with prior editions, this 5

Edition of the SPS PS converts these HDOP-AT and VDOP-

AT values into SIS-only HSAT and VSAT values assuming a 3.6 m RMS URE and a value of 2.0

for both the UHNE-to-R95 conversion factor and the UVNE-to-L95 conversion factor. Specifically:

HSAT = HDOP-AT x 3.6 m x 2.0

= 2.10 x 3.6 m x 2.0

= 15 m 95%

VSAT = VDOP-AT x 3.6 m x 2.0

= 4.53 x 3.6 m x 2.0

= 33 m 95%

B.3.3 "Classic" Position Accuracy Statistics

For its first quarter century, GPS position accuracies were always described in terms of a total

overall statistic. For example, the original 16 m spherical error probable (SEP) requirement – see

The American Practical Navigator for PPS users – was such a total overall statistic. This 16 m

SEP requirement meant that over all T-S points, 50% of the PPS user position fixes would have

a three-dimensional (3-D) accuracy equal to or better than 16 m. Equivalently stated, the 16 m

SEP specification meant that if a PPS user went out at a random point in time at random location

on the surface of the Earth, that user would have a 50% probability of getting a position fix with

3-D accuracy equal to or better than 16 m. This total overall statistic was one of the primary

sources of the term ‘global average accuracy’.

The 3

Edition of the SPS PS documented a radical paradigm shift in describing GPS position

accuracy. The focus went from "how good is GPS on average" to "how bad can GPS possibly

be". The 3

Edition of the SPS PS described GPS position service availability of accuracy

assuming the "worst-case" constellation (2 worst failed satellites) and the "worst-case" location

(any single point on the Earth). The position service availability of accuracy specifications at the

worst-case location are thus both extremely conservative ("worst-case" constellation and "worst-

case" location) and extremely liberal (excluding the worst 10% of the sidereal day as being

"unavailable"). The 4

and 5

Editions of the SPS PS have carried on with this paradigm shift.

This section of the current (5th) Edition of the SPS PS addresses the classic way of describing

GPS position accuracy as it is used in Table 3.8-3 in the main body. This classic way is also

known as the “global ensemble” description of GPS position accuracy. It is frequently referred to

as the “global average” description of GPS position accuracy, but the statistical probability level

is usually at the 95% level as seen in Table 3.8-3 rather than at the 50% as in the early 16 m SEP

requirement.

April 2020 GPS SPS PS

5th Edition Page B-19

B.3.3.1 Reasons for Needing the DOP Distributions

A global-average DOP value by itself is really not adequate from computing a global-average

position accuracy value. The actual DOP distribution must be taken into account in order to

compute an accurate accuracy value. The following simple example illustrates why this is so.

Note:

1. Because the probability conversion factors for the Gaussian (normal) distribution can be found in

any good statistics textbook, the following example uses GPS vertical position accuracy since the

vertical position accuracy follows a Gaussian distribution.

B.3.3.1.1 Global Average Accuracy Without DOP Distribution Information

Say one knows the global-average VDOP for some constellation condition (e.g., worst 2-satellite

failure) is exactly 1.80, but one does not know the distribution of the population of VDOP values.

One might just assume that all VDOP values are exactly 1.80. Under this assumption, for a 4.00

m 1-sigma URE and the more precise 95% conversion factor of 1.96 (instead of 2.0), one would

deduce that the 95% global-average SIS-only vertical accuracy is:

Vertical L95 = URE x VDOP x 1.96

= 4.0 m x 1.80 x 1.96

= 14.11 m 95%

This deduction is shown graphically in Figure B.3-3. Observe this figure shows only one Gaussian

distribution (Normal distribution with a zero mean) and that this Gaussian distribution has been

rotated 90 degrees from its usual orientation to better illustrate position fix errors in the vertical

dimension.

Figure B.3-3. Vertical L95 for URE=4.0 m 1-sigma and VDOP=1.80

0 m

+14.11 m

-14.11 m

95% of the Vertical

Position Fixes (L95)

April 2020 GPS SPS PS

5th Edition Page B-20

B.3.3.1.2 Global Average Accuracy With DOP Distribution Information

Say that one later finds better information which says that the distribution of the population of

VDOP values is such that half of the VDOP values are exactly 1.40 and the other half of the VDOP

values are exactly 2.20. The global-average VDOP is still exactly 1.80. In this case, with the

same URE assumption, one would deduce that the L95% global-average vertical accuracy for

each of the two sub-populations are:

50% Sub-Population with VDOP = 1.40

Vertical L95 = URE x VDOP x 1.96

= 4.0 m x 1.40 x 1.96

= 10.98 m 95%

50 % Sub-Population with VDOP = 2.20

Vertical L95 = URE x VDOP x 1.96

= 4.0 m x 2.20 x 1.96

= 17.25 m 95%

These two sub-populations are illustrated in Figure B.3-4. Observe that each sub-population is

shown only half as large as in the previous Figure B.3-3, which corresponds to each sub-

population having 50% of the total population.

Figure B.3-4. Vertical L95 for VDOP=1.40 Sub-Population and VDOP=2.20 Sub-Population

Taking the simple average of the 95% global-average vertical accuracies for each of these two

sub-populations will give the same result as before, namely 14.11 m 95%. However, this is not

correct because there is no mathematical basis for simply averaging two sub-populations. To

0 m

+10.98 m

-10.98 m

95% of the Vertical

Position Fixes (L95)

for VDOP = 1.40

Sub-Population

95% of the Vertical

Position Fixes (L95)

for VDOP = 2.20

Sub-Population

+17.25 m

0 m

-17.25 m

April 2020 GPS SPS PS

5th Edition Page B-21

illustrate the error, compare the sum of the two weighted fractions of each sub-population beyond

the 14.11 m 95% value against the 5% of the total population beyond the 14.11 m 95% value

which results from the simple average. For reference, note that the 1-sigma equivalents of each

sub-population distribution are:

10.98 m 95% = 5.60 m 1-sigma for VDOP=1.40 Sub-Population

17.25 m 95% = 8.80 m 1-sigma for VDOP=2.20 Sub-Population

50% Sub-Population with VDOP = 1.40

14.11 m 95% = 2.520-sigma relative to a 5.60 m 1-sigma distribution

0.0118 = Fraction of sub-population beyond 2.520-sigma (i.e., beyond ±14.11 m)

for a Gaussian distribution

0.0059 = Weighted fraction of total population beyond ±14.11 m given that this

sub-population is ½ of the total population

50% Sub-Population with VDOP = 2.20

14.11 m 95% = 1.604-sigma relative to an 8.80 m 1-sigma distribution

0.1088 = Fraction of sub-population beyond 1.604-sigma (i.e., beyond ±14.11 m)

for a Gaussian distribution

0.0544 = Weighted fraction of total population beyond ±14.11 m given that this

sub-population is ½ of the total population

Weighted Sum of Two Sub-Populations with 50% of VDOP = 1.40 and 50% of VDOP = 2.20

Fraction of total population beyond ±14.11 m = 0.0059 + 0.0544

= 0.0603

Equivalent accuracy statistic for total population = 14.11 m 93.97%

Using the simple fact that the global average VDOP = 1.80 will lead one to deduce that 5% of the

total population of vertical position fixes will be beyond ±14.11 m. But using better information

which defines the underlying VDOP distribution as being two equal sub-populations with VDOP =

1.40 and VDOP = 2.20 will reveal that actually 6.03% of the total population of vertical position

fixes will be beyond ±14.11 m. In this example, the statistical error introduced by using the global-

average VDOP value by itself instead of using the underlying VDOP distribution is thus slightly

greater than 1% in overall probability terms.

The error introduced by not using information about the underlying VDOP distribution is more

dramatic in scalar accuracy terms. For the same overall probability of 95%, using the better

VDOP information results in a scalar accuracy of 14.82 m which is 5% larger than the 14.11 m

value which results from using the simple global average VDOP. The numerology which

produced this result is as follows.

April 2020 GPS SPS PS

5th Edition Page B-22

50% Sub-Population with VDOP = 1.40

0.0080 = Unweighted fraction of VDOP=1.40 sub-population greater than or equal

to 14.82 m not accounting for the fact that this sub-population is ½

of the total population

0.0040 = Weighted fraction of total population beyond ±14.82 m given that this

sub-population is ½ of the total population

50% Sub-Population with VDOP = 2.20

0.0920 = Unweighted fraction of VDOP=2.20 sub-population greater than or equal

to 14.82 m not accounting for the fact that this sub-population is ½

of the total population

0.0460 = Weighted fraction of total population beyond ±14.82 m given that this

sub-population is ½ of the total population

Weighted Sum of Two Sub-Populations with 50% of VDOP = 1.40 and 50% of VDOP = 2.20

Fraction of total population beyond ±14.82 m = 0.0040 + 0.0460

= 0.0500

Equivalent accuracy statistic for total population = 14.82 m 95%

B.3.3.1.3 Procedure for Using DOP Distribution Information

Observe that the VDOP distribution is accounted for in this simple example by first computing the

position accuracy distribution for each sub-population VDOP value, generating the weighted sum

of the position accuracy distributions for each sub-population VDOP value using the probability of

that sub-population VDOP value occurring, and then finally determining the statistics for the total

position accuracy distribution for the full ensemble population.

The same procedure can be generalized for use with sub-population HDOP distribution

information, sub-population PDOP distribution information, sub-population TDOP distribution

information, sub-population TTDOP distribution information, and so on.

Note:

1. The total position accuracy distribution is often called the "global ensemble” position accuracy

distribution because it is the weighted-sum of many position accuracy sub-distributions.

B.3.3.2 Basic Procedure for Computing Classic Position Accuracy Statistics

The classic GPS position accuracy procedure is similar to the example in the preceding

paragraph, but the sub-populations are each individual T-S point by itself. Letting each T-S point

be its own sub-population simplifies the weighting since each sub-population is therefore simply

weighted by 1 over the total number of T-S points. It also accommodates different types of

position accuracy computations, particularly those where the basic “UERE x DOP” equation does

not apply (e.g., with aiding sensors, or with weighted solutions). The classic GPS position

accuracy procedure is:

April 2020 GPS SPS PS

5th Edition Page B-23

1. The geometry at each T-S point over a sidereal day and across the Earth is computed for

the particular circumstances being considered.

2. The solution matrix is computed for the geometry at each T-S point. (This solution matrix

is the same one a GPS receiver would compute based on that geometry given the same

circumstances.)

3. A Monte Carlo simulation is run for each T-S point geometry where simulated

pseudorange error samples drawn from a Gaussian distribution with a 1-sigma value equal

to the specified UERE are deterministically converted via the solution matrix to produce

simulated position error samples (e.g., horizontal, vertical, spherical). The position error

samples at each T-S point represent the position accuracy at that T-S point.

4. The position error samples produced by the Monte Carlo simulation for each T-S point

geometry are summed together to produce a very large ensemble of position error

samples from all T-S points.

5. The ensemble of position error samples from all T-S points is then sorted to find the 95

percentile (or 50

percentile, 90

percentile, 98

percentile, 99.9

percentile, etcetera)

statistics. These statistics are the classic total overall GPS position accuracy values.

B.3.3.3 Expanded Procedure for Computing Classic Position Accuracy Statistics

The basic procedure in paragraph B.3.3.2 applies to the circumstances being considered, such

as assuming a particular set of 2 satellites are failed out of the baseline 24-slot constellation. The

basic procedure can be expanded to cover multiple circumstances by appropriately weighting and

summing the ensembles of position error samples from all T-S points for each circumstance being

considered into a super ensemble (an “ensemble of ensembles”).

One of the main applications for this expanded procedure is addressing the probabilities of being

in different constellation conditions. For example, consider the standard model for constellation

availability described in Table A.7-2. The standard model has the baseline 24-slot constellation

fully populated with 24 usable satellites transmitting a trackable and healthy SPS SIS 72.0% of

the time, 23 usable satellites transmitting a trackable and healthy SPS SIS 17.0% of the time, 22

usable satellites transmitting a trackable and healthy SPS SIS 6.4% of the time, 21 usable

satellites transmitting a trackable and healthy SPS SIS 2.6% of the time, and 20 or fewer usable

satellites transmitting a trackable and healthy SPS SIS 2.0% of the time. The appropriate

weightings for each ensemble of position error samples is the constellation condition probability

divided by the number of possible combinations making up each constellation condition.

Specifically:

April 2020 GPS SPS PS

5th Edition Page B-24

1 ensemble for the full 24-satellite constellation weighted by 0.720, plus

24 ensembles for all possible 23-satellite constellations, each weighted by 0.170/24, plus

276 ensembles for all possible 22-satellite constellations, each weighted by 0.064/276, plus

2,024 ensembles for all possible 21-satellite constellations, each weighted by 0.026/2,024, plus

10,626 ensembles for all possible 20-satellite constellations, each weighted by 0.020/10,676.

B.3.3.4 Expanded Classic Position Accuracy Statistics

Following the expanded procedure in the preceding paragraph with 5 minute time steps, with a

4x4 degree grid, with an AIV solution, with a 5 degree mask angle, with no aiding sensors, with

all 12,951 ensembles weighted as described in the preceding paragraph, and with the 6.0 m 1-

sigma (11.8 m 95%) SPS SIS-only URE value in the 3

Edition of the SPS PS for any trackable

and healthy SPS SIS, the resulting classic GPS position accuracy statistics would have been:

10.7 m = 95% Horizontal Position Accuracy

19.8 m = 95% Vertical Position Accuracy

Note:

1. The 3

Edition of the SPS PS uses the geometry which results when the SIS from the average two

slots are unavailable. The weighted ensemble geometry used here is different.

The classic GPS position accuracy statistics for the 4.0 m 1-sigma (7.8 m 95%) SPS SIS-only

URE value in the 4

Edition of the SPS PS would have been:

7.1 m = 95% Horizontal Position Accuracy

13.2 m = 95% Vertical Position Accuracy

The corresponding classic GPS position accuracy statistics for the 3.6 m 1-sigma (7.0 m 95%)

SPS SIS-only URE value over all AODs during normal operations in this 5th Edition of the SPS

PS are:

6.4 m = 95% Horizontal Position Accuracy

11.8 m = 95% Vertical Position Accuracy

And the classic GPS position accuracy statistics for the combination of: the 1.0 m 1-sigma (2.0 m

95%) SPS SIS-only URE value over all AODs during normal operations, the modern SF SPS

receiver UEE of 2.3 m 1-sigma (4.5 m 95%) in Table B.2-1, and an assumed SF ionospheric delay

compensation error of 2.5 m 1-sigma (4.9 m 95%) under benign ionosphere conditions in the mid

latitudes; or a total UERE of 3.6 m 1-sigma (7.0 m 95%), are:

6.4 m = 95% Horizontal Position Accuracy

11.9 m = 95% Vertical Position Accuracy

Note:

1. The above position accuracies all scale linearly with the UERE or SIS-only URE.

April 2020 GPS SPS PS

5th Edition Page B-25

B.3.4 "Current" Position Accuracy Statistics for Receiver Specifications

B.3.4.1 Background

The “current” position accuracy statistics for receiver specifications typically use some of the

concepts from the classic expanded position accuracy statistics discussed in the previous section.

They use a global ensemble (“global average”) and use the same constellation condition

probability weighting for all possible 24- through 20-satellite constellations. Unlike the classic

expanded position accuracy statistics, the “current” position accuracy statistics will frequently

ensemble the DOP distributions from each T-S point rather than ensembling the position fix error

distributions. The DOP results for the weighted mix of all possible 24- through 20-satellite

constellations is shown in Table B.3-1.

Table B.3-1. Global Ensemble DOPs for Weighted Mix of Constellation States

Percentile

HDOP

VDOP

PDOP

50%

0.945

1.535

1.815

60%

0.985

1.625

1.905

67%

1.015

1.695

1.975

75%

1.055

1.795

2.075

80%

1.095

1.865

2.155

90%

1.205

2.085

2.325

95%

1.315

2.305

2.605

97%

1.405

2.475

2.795

98%

1.485

2.625

2.945

99%

1.655

2.925

3.305

99.9%

2.655

5.055

5.595

Notes:

1. Weighted based on 24 satellites 72.0% of the time, 23 satellites 17.0% of the time, 22 satellites

6.4% of the time, 21 satellites 2.6% of the time, and 20 or fewer satellites 2.0% of the time.

2. 5 degree mask angle assumed.

B.3.4.2 “Current” Position Accuracy Statistics for Receivers

Since many “current” receiver specifications use a given availability of 99% for the position

accuracy statistics, the corresponding HDOP value from Table B.3-1 is 1.655 and the VDOP value

is 2.925. These HDOP and VDOP values are used basically as shown in equations (B-2) through

(B-6) given earlier in this section to develop position accuracy statistics. With UERE values

expressed as 1-sigma quantities, the summary equations are:

99% Worst DOP, Horizontal R95 = UERE x HDOP x 1.73

= UERE x 1.655 x 1.73 (B-9)

99% Worst DOP, Vertical L95 = UERE x VDOP x 1.96

= UERE x 2.925 x 1.96 (B-10)

April 2020 GPS SPS PS

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Typical “current” UERE specifications are:

a. 4.5 m 1-sigma for DF SPS UERE over all AODs during normal operations, assuming a

URE of 7.0 m 95% (3.6 m 1-sigma) and a UEE of 5.5 m 95% (2.8 m 1-sigma), and intentionally

ignoring the contribution of the SF ionospheric delay model errors.

b. 4.4 m 1-sigma for SF SPS UERE over all AODs during normal operations, assuming a

URE of 7.0 m 95% (3.6 m 1-sigma) and a UEE of 5.0 m 95% (2.6 m 1-sigma), and intentionally

ignoring the contribution of the SF ionospheric delay model errors.

Substituting each of these UERE values into equations (B-9) and (B-10), and rounding as

appropriate, produces position accuracy statistics as follows.

a. For the 4.5 m 1-sigma DF UERE:

99% Worst DOP, Horizontal R95 = 12.9 m 95%

99% Worst DOP, Vertical L95 = 25.8 m 95%

b. For the 4.4 m 1-sigma SF UERE:

99% Worst DOP, Horizontal R95 = 12.6 m 95%

99% Worst DOP, Vertical L95 = 25.2 m 95%

B.3.5 Position/Time Accuracy Standards in this Edition of the SPS PS

To clearly demonstrate backward compatibility, the position/time domain statistics in Section 3.8

of this edition of the SPS PS differ from those in the 3

Edition of the SPS PS and 4

Edition of

the SPS PS in only two ways: (1) the assumed 1-sigma URE is now 3.6 m, and (2) there are no

additional margin factors – such as the square root of 2 used in the computation of the 3

Edition

HSAT and VSAT values – beyond the traditionally rounded-up statistical conversion factors as

illustrated below:

Horizontal R95 (SIS only) = URE x HDOP x 2

= 3.6 m RMS x 1.1 x 2

= 8 m 95%

Vertical L95 (SIS only) = URE x VDOP x 2

= 3.6 m RMS x 1.8 x 2

= 13 m 95%

April 2020 GPS SPS PS

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SECTION B.4 Customized PVT Performance Expectations

This section describes some of the methods which can be employed to obtain PVT performance

expectations customized to the particular circumstances of an actual "real world" mission. These

methods are general suggestions for typical SPS users and applications. They are meant to be

informative in the sense of being recipes that can optionally be followed to obtain the desired

information. They are not prescriptive in the sense of being procedures that should or must be

complied with.

B.4.1 Three Timeframes

There are three time frames over which customized PVT performance expectations are typically

desired. They are: (1) in advance of the mission, (2) during the mission, and (3) after the mission.

The three time frames, along with the primary reasons customized PVT performance expectations

are desired, are illustrated in Figure B.4-1.

Figure B.4-1. Three Time Frames

Of the three time frames for PVT performance expectations, the most important one is almost

always the real-time one. In-advance PVT performance expectations can be important for

mission planning (or for interpreting specifications like the SPS PS). After-the-fact PVT

performance expectations can be important for determining whether maintenance actions are

necessary (e.g., if your SPS receiver suddenly lost accuracy during a mission, was it because the

SPS receiver failed or was it because you encountered an unexpected DOP hole?). But real-time

PVT performance expectations are almost always the most important because they will alert you

when unexpected conditions occur -- particularly conditions which can make the output PVT data

unreliable -- and thereby help you safely accomplish your mission using your SPS receiver.

Time Frame

Reason

In-Advance

Real-Time

After-the-Fact

Now

Mission

Period

Now

Mission

Period

Now

Mission

Period

Planning

Safety

Maintenance

April 2020 GPS SPS PS

5th Edition Page B-28

B.4.2 Real-Time PVT Performance Expectations Directly from Your Receiver

The best (and simplest) course of action is to use the real-time PVT performance expectations

produced by your SPS receiver whenever possible for your "real world" mission.

B.4.2.1 Real-Time Accuracy Estimates Directly from Your Receiver

Virtually all SPS receivers automatically generate real-time PVT accuracy estimates and output

them for your use. This is simple thing for a SPS receiver to do since it already has all the

information it needs to generate those PVT accuracy estimates whenever the receiver is turned

on and producing a PVT solution: it knows exactly where each satellite is in the sky, which SPS

SISs it is tracking, and which SISs/satellites it is using to produce the PVT solution at each instant

in time. Using its own estimate of its current antenna position directly from the PVT solution, the

SPS receiver precisely computes the current satellite-to-receiver geometry (all SPS receivers

must precisely compute the current satellite-to-receiver geometry in order to produce their PVT

solutions). Having already precisely computed the satellite-to-receiver geometry, it only takes a

few additional equations to compute and output precise DOP values in real time.

To produce as accurate a PVT solution as possible, a modern SPS receiver will place more weight

on its more accurate measurements and less weight on its less accurate measurements. The

weighting factors it uses basically amount to real-time estimates of the UERE for each SPS SIS

being used in the PVT solution. The SPS receiver begins by using the URA number transmitted

by each satellite (see Subframe 1 in Figure 2.2-1 and paragraph A.5.2 in Appendix A) as the best

available estimate of the current URE provided by that satellite's SPS SIS. Following equation B-

8, these estimates of the current UREs are then RSS-ed with receiver-developed estimates of the

current UEEs to produce estimates of the current UERE for each SPS SIS.

Notes:

1. The currently transmitted URA number is the best estimate of the current URE available to a SPS

receiver. The currently transmitted URA number automatically takes the time since last upload into

account -- see the "graceful degradation effect" and the normal variations in URE as a function of

AOD described in Section A.4. Using the current URA number provides a higher fidelity estimate

of the SPS SIS URE than any other method (e.g., using the SPS SIS URE performance standard

in Section 3.4). DGPS systems are not a source of better SPS SIS URE estimates. DGPS systems

do not broadcast URE estimates, they instead broadcast corrections for the instantaneous UREs

along with estimates of the accuracy of those differential corrections (i.e., differential URE

estimates, commonly known as “User Differential Range Error” [UDRE] estimates). DGPS

correction UDRE estimates are analogous to SPS SIS URE estimates, and DGPS receivers use

the broadcast UDRE estimates the same way that a SPS receivers use the transmitted URA

numbers.

2. The SPS receiver computation of the current estimated UEE varies significantly from receiver to

receiver, but all SPS receivers should address at least the first four components of the UEE shown

in Table B.2-1; namely: (1) ionospheric delay compensation errors, (2) tropospheric delay

compensation errors, (3) receiver tracking channel noise and resolution errors, and (4) multipath

errors.

With the current satellite-to-receiver geometry and current UERE estimates computed, it is a

simple matter for the SPS receiver to perform the multiplications indicated by equations B-2 and

B-3 to compute the current UHNE and UVNE values. (Note that equations B-2 and B-3 are not

actually used by SPS receivers because the real-time UERE estimates are generally not identical

across all pseudoranges, but the basic principle still applies and the process will be discussed in

April 2020 GPS SPS PS

5th Edition Page B-29

terms of equations B-2 and B-3 for simplicity.) For historical reasons, some SPS receivers do not

use UHNE and UVNE; they use the following terminology instead:

EHE = UHNE = UERE x HDOP (B-11)

EVE = UVNE = UERE x VDOP (B-12)

EPE = (EHE

+ EVE

)

(B-13)

EPE = UNE = UERE x PDOP (B-14)

where:

EHE = Estimated Horizontal Error (2-D, RMS, meters)

EVE = Estimated Vertical Error (1-D, RMS, meters)

EPE = Estimated Position Error (3-D, RMS, meters)

and

UNE = User Navigation Error (3-D, RMS, meters)

Note:

1. In addition to the EHE, EVE, and EPE values, many SPS receivers will also output the full set of

numbers which result from the multiplication of the satellite-to-receiver geometry and the individual

UERE estimates. This full set of numbers, often called a "covariance matrix", is output over a digital

interface. Covariance matrix type outputs are typically used for integrating the output SPS PVT

solution with the outputs of another sensor system like an IMU. Covariance matrix type outputs

are too complicated to be of use to a human operator. They are therefore beyond the scope of this

appendix.

As seen from equations B-11 through B-14, the SPS receiver does all the work for you in real

time. The EHE, EVE, and EPE values output by the SPS receiver in real time are your customized

PVT performance expectations. Even if you never need to worry about customized PVT

performance expectations in advance or after the fact, it is still important to keep an eye on the

EHE, EVE, and EPE values output/displayed by your SPS receiver in real time. If something

unanticipated should happen -- like a surprise DOP hole caused by multiple satellite failures, SPS

SIS obscuration due to an unforeseen obstruction, or loss of SPS SIS tracking due to RFI (e.g.,

jamming) -- the EHE, EVE, and EPE values will let you know about it in real time.

In fact, since DOP holes are the most likely cause of an unexpectedly bad PVT solution, and since

SPS receivers are so good (reliable) at reporting any DOP holes via the EHE, EVE, and EPE

values, the output EHE, EVE, and EPE values are actually the first line of defense for integrity

warnings. An unexpectedly bad PVT solution is defined to be an integrity failure unless it is

accompanied by a timely warning. The real-time EHE, EVE, and EPE values provide a timely

warning whenever an unexpectedly bad PVT solution is caused by a surprise DOP hole. The

EHE, EVE, and EPE values are thus what keep surprise DOP holes from becoming integrity

failures.

Many GPS receivers will output a Figure of Merit (FOM) instead of or in addition to the EPE value

(and/or the EHE and EVE values). The FOM is actually a simplified version of the EPE. For

example, the correspondence between computed EPE value from a particular type of GPS

April 2020 GPS SPS PS

5th Edition Page B-30

receiver and the displayed FOM value is shown in Table B.4-1. Many other GPS receivers also

display EPE and FOM this same way.

Some GPS receivers, notably those which comply with RTCA/DO-236B, will output a parameter

called the estimated position uncertainty (EPU). The EPU is closely related to the EHE, and may

even be equal to the EHE in certain GPS receiver implementations.

Table B.4-1. EPE-to-FOM Correspondence

EPE Value

Displayed FOM Value

EPE  25 m

25 m < EPE  50 m

50 m < EPE  75 m

75 m < EPE  100 m

100 m < EPE  200 m

200 m < EPE  500 m

500 m < EPE  1,000 m

1,000 m < EPE  5,000 m

5,000 m < EPE

Note:

1. A widely used rule of thumb is to only rely on the output PVT solution when the FOM value equals

1. For a SF SPS receiver, the UERE can be assumed to be on the order of 16.3 m 95% (URE over

all AODs, average ionosphere). The corresponding 1-sigma UERE value is 8.3 m. For a FOM

value of 1 (or equivalently an EPE value less than or equal to 25 m) with this UERE, the PDOP

value would have to be less than or equal to 3.0. This gives rise to a related rule of thumb which

can be used if the FOM/EPE values are unavailable: "Only rely on a SF SPS receiver’s output PVT

solution if the PDOP is less than or equal to 3.0."

B.4.2.2 Real-Time Integrity Estimates Directly from Your Receiver

In addition to automatically generating and outputting real-time PVT accuracy estimates, many

modern SPS receivers will also automatically generate and output real-time PVT integrity

estimates using a RAIM algorithm (also known as a Fault Detection (FD) algorithm) when

possible. There are two parts to every RAIM algorithm: (1) the non-measurement part, and (2)

the measurement part.

The non-measurement part of a modern SPS receiver's RAIM algorithm is similar to the receiver's

PVT accuracy estimate computation. The inputs are the same: the computed satellite-to-receiver

geometry and the current estimated UERE for each SPS SIS. The non-measurement part of the

RAIM algorithm determines whether the geometry and the UERE will be good enough to allow

the receiver to reliably detect a SPS SIS integrity failure if one were to occur. This basically comes

down to a determination whether RAIM is available or not. The geometry and the UERE are used

to compute and output a quantity commonly known as the horizontal protection level (HPL). The

HPL is the radius of a circle in the horizontal plane which the RAIM algorithm will be able to assure

contains the true horizontal position with a very high probability (see RTCA/DO-316).

April 2020 GPS SPS PS

5th Edition Page B-31

Notes:

1. The HPL does not depend on the actual pseudoranges. The HPL does depend on the receiver

tracking and using the SISs from at least 5 satellites unless additional sources of aiding information

are available.

2. The assurance level for a typical RAIM algorithm is set to a miss detection probability of 99.99999%

(1 - 10

-7

) per hour, with a false alert probability of 0.00001% (10

-5

) per hour, based on the SPS SIS

standards given in Section 3.

3. Some receivers will also compute and output the corresponding vertical protection level (VPL)

and/or time protection level (TPL).

The measurement part of a SPS receiver's RAIM algorithm is where the actual pseudoranges

from the SPS SISs are used to determine whether a SPS SIS integrity failure has occurred or not.

The inputs to the measurement part of the RAIM algorithm are the computed satellite-to-receiver

geometry, the current estimated UERE, and the current pseudoranges. Some receiver's RAIM

algorithms will only detect whether a SIS integrity failure is present or not. Other RAIM algorithms

go a step further by computing and outputting a quantity known as the horizontal uncertainty level

(HUL). The HUL is similar to the HPL except the HUL reflects the actual errors in the

pseudoranges (see RTCA/DO-316).

Notes:

1. The HUL depends on the receiver tracking and using the SISs from at least 5 satellites unless

additional sources of aiding information are available.

2. Some receivers will also compute and output the corresponding vertical uncertainty level (VUL)

and/or time uncertainty level (TUL).

3. Rather than simply using a RAIM algorithm for fault detection (FD), many modern receivers will

also use their RAIM algorithm for fault detection and exclusion (FDE). FDE processing requires

the receiver to track and use the SISs from at least 6 satellites unless additional sources of aiding

information are available.

There are three basic definitions which govern the integrity implications of the HPL and the HUL

with respect to a known horizontal alert limit (HAL) for a particular mission phase (e.g., an aircraft

conducting a non-precision approach where the HAL is defined based on the presence of nearby

obstacles). These three basic definitions are:

1. RAIM is defined to be available to provide integrity for a particular mission phase whenever

the HPL is less than or equal to the HAL for that mission phase (i.e., HPL  HAL).

2. A SPS SIS integrity fault is defined to be detected whenever the HUL is greater than or

equal to the HPL (i.e., HUL  HPL).

3. A mission-critical SPS SIS integrity fault is defined to be detected for a particular mission

phase whenever the HUL is greater than or equal to the HAL for that mission phase (i.e.,

HUL  HAL).

April 2020 GPS SPS PS

5th Edition Page B-32

The three basic definitions governing the integrity implications of the HPL and HUL values output

by a SPS receiver with respect to the HAL for a particular mission phase are illustrated in Figure

B.4-2 for a variety of situations. The illustrated situations are as follows:

a. The normal situation where HUL < HPL < HAL is illustrated by panel "a" at the top of Figure

B.4-2. In this situation, RAIM is available to provide integrity for this mission phase

because HPL < HAL. No SPS SIS integrity fault has been detected because HUL < HPL.

These two integrity implications combine to give an "all systems go" result which is

symbolized by the green light on the stoplight icon.

b. Panel "b" shows a situation where RAIM is not strictly available to provide integrity for this

mission phase because HPL > HAL. Even though RAIM is not strictly available, it is still

working well enough to determine that no SPS SIS integrity fault is detected because HUL

< HPL. The combination of these two integrity implications gives an "exercise caution"

result symbolized by the yellow light on the stoplight icon.

c. Panel "c" in Figure B.4-2 shows a situation where RAIM is available because HPL < HAL,

and where a SPS SIS integrity fault has been detected because HUL > HPL. The fact that

HUL < HAL means that the detected SPS SIS integrity fault is defined as not being mission

critical. The combination of these three integrity implications gives a "weak should not

use" result symbolized by the dim red light on the stoplight icon.

d. Panel "d" shows a slightly different situation than panel "c". RAIM is still available because

HPL < HAL, and a SPS SIS integrity fault has been detected because HUL > HPL. The

difference from panel "c" is that HUL > HAL which means the detected SPS SIS integrity

fault is mission critical. The combination of these three integrity implications gives a

"strong do not use" result symbolized by the bright red light on the stoplight icon.

Different SPS receivers implement their RAIM algorithms in different ways and have different

displays for the real-time integrity results. SPS receivers for aviation applications often have HAL

values stored in their database for different phases of flight and will provide simple indications

(like with the stoplight icon in Figure B.4-2) using flags on the pilot's navigation display. Some

handheld SPS receivers let you enter a HAL value and provide simple indications based upon

that HAL. Other SPS receivers only output the HPL and HUL values; they leave it up to you to

compare those values against whatever HAL you decide is appropriate for your mission phase.

April 2020 GPS SPS PS

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Figure B.4-2. HPL, HUL, HAL Relationships

HAL

HUL < HPL < HAL

HPL

HUL

B. Normal Situation

HAL

HUL < HAL < HPL

HPL

HUL

b. RAIM Not Available for this HAL

HAL

HPL < HUL < HAL

HPL

HUL

c. Fault Detected, Non-Critical for this HAL

HAL

HPL < HAL < HUL

HPL

HUL

d. Fault Detected, Critical for this HAL

April 2020 GPS SPS PS

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The performance of a RAIM algorithm is highly dependent on the specific assumptions made by

the algorithm designer about the characteristics of the UERE and on the actual characteristics of

the UERE that the RAIM algorithm experiences. Most RAIM algorithm designers tend to err on

the conservative side in their assumed UERE compared to the actual UERE. For example, some

RAIM algorithms assume a conservative URE of 5.7 m 1-sigma based on an assumed SPS URA

index of 3 (see Table A.5-1 in Appendix A) and others assume a conservative UEE of 5.5 m 95%

(see Table B.2-1 in Appendix B). For RAIM algorithms in SPS receivers, the dominant factor in

the assumed UERE is typically the residual error due to the SF ionospheric delay model.

Conservative assumptions for the SF ionospheric delay model errors, such as the ones given in

RTCA/DO-316, can effectively establish the limits of RAIM availability almost independent of the

assumed SPS URA or assumed UEE. A well-designed aviation grade SPS receiver with a RAIM

algorithm designed and evaluated in accordance with the conservative assumptions in RTCA/DO-

316 will be limited to the RAIM availabilities given in Table B.4-2.

Table B.4-2. RAIM Availability Examples

Phase of Flight

Availability of Detection

Availability of Exclusion

En Route

(HAL = 2.0 nm)

99.95%

99.30%

Terminal

(HAL = 1.0 nm)

99.90%

98.45%

Non-Precision Approach

(HAL = 0.3 nm)

99.80%

93.10%

No matter how a SPS receiver implements its RAIM algorithm, if you are using PVT solution from

that receiver for any safety critical application -- it is vitally important that you pay heed to the real-

time PVT integrity information provided by your receiver. That real-time PVT integrity information

will alert you when unexpected conditions occur which make the output PVT solution unreliable

and potentially unsafe.

B.4.3 In-Advance PVT Performance Expectations

B.4.3.1 General Rule -- Don't Worry About It

As a general rule, most SPS users do not need customized PVT performance expectations in

advance of a mission. There are three main reasons for this general rule.

B.4.3.1.1 Good SPS PVT Performance

The SPS SIS provided by the satellites is robust enough, and sufficient satellites are kept usable

in the on-orbit constellation, that good SPS PVT performance can be reasonably be assumed any

time of day anywhere in the world. For example, paragraph B.3.3.4 describes the classic position

accuracies at a random time, random location, any AOD for a SPS user with conservative SF

UERE assumptions as 15.5-22.0 m 95% horizontal averaged over all constellation conditions

(from 24 satellites transmitting a trackable and healthy SPS SIS to only 20 of 24 satellites

transmitting a trackable and healthy SPS SIS).

High availability of good accuracy is borne out by the global average DOP values shown in Figure

B.3-1. The usual constellation condition has all 24 satellites transmitting a healthy SIS (or 23 out

of 24 baseline satellites transmitting a healthy SIS combined with a few auxiliary satellites which

are also transmitting a healthy SIS). With this constellation condition, Figure B.3-1 shows that

April 2020 GPS SPS PS

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99.9% of all the HDOP values will be less than 1.80. Substituting this HDOP value into equation

B-2 along with a very conservative 12.3 m 1-sigma UERE value, and translating to an R95 value

gives:

UHNE = UERE x HDOP

= 12.3 m x 1.8

= 22.1 m RMS

R95 = UHNE x 1.73

= 22.1 m RMS x 1.73

= 38.2 m 95%

A 99.9% availability of a horizontal accuracy of 38.2 m 95% or better any random time at any

random location is pretty good odds. Furthermore, 38.2 m 95% is also quite accurate -- it is more

than adequate for many real-world missions. While it is certainly possible to search the entire

world to find a location with worse accuracy, those locations are the 1-in-2,500 exceptions rather

than the rule.

If you don't require horizontal accuracy better than 15.5-22.0 m 95% on average, 38.2 m 95%

with high availability, or if you can coast along for a few minutes if you should accidentally

encounter one of those rare DOP holes, it isn't worth worrying about customized in-advance PVT

performance expectations. The odds are heavily stacked in your favor.

B.4.3.1.2 Repetitive Constellation Geometry

Another good reason for not worrying about customized in-advance PVT performance

expectations is prior success using the SPS SIS in your particular area of operations. Remember

that the constellation geometry repeats every sidereal day (i.e., 4 minutes earlier each succeeding

day because a sidereal day is shorter than a solar or "wall clock" day). Unless something drastic

happens -- like a satellite suddenly failing to transmit a trackable and healthy SPS SIS -- the PVT

performance expectations for your operational area will not significantly change from sidereal day

to sidereal day. If there is a temporary DOP hole due to a satellite outage, that same DOP hole

will repeat every sidereal day until the satellite is restored or the outage is repaired. The SPS

performance you got yesterday is a very good predictor of the SPS performance you will get

today.

The best, and easiest, way to keep current on satellite status changes is by subscribing to the

NANUs issued by the Control Segment. The NANUs -- both for satellite status changes that are

scheduled in advance and for after-the-fact surprises -- are sent directly via e-mail almost the

instant they are issued. Civil SPS users can subscribe at https://www.navcen.uscg.gov. This

web site also posts the NANUs for subsequent downloading on demand.

April 2020 GPS SPS PS

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B.4.3.1.3 Receiver and Mission Characteristics

Certain types of SPS receivers make worrying about customized in-advance PVT performance

expectations unnecessary because the expected PVT performance just doesn't vary all that

much. Certain types of missions also make worrying about customized in-advance PVT

performance expectations impractical because it takes too much effort to develop reliable

expectations. Some representative illustrations include:

a. Time Transfer Receivers. Time transfer receivers, which operate from a known location,

are affected by TTDOP rather than TDOP. Fortunately for time transfer performance

expectations, the TTDOP variations over time are much smaller than the TDOP variations.

So long as the SPS SIS is available from at least two visible satellites (a virtual certainty),

the TTDOP will be adequate to give excellent time transfer performance.

b. Waterborne Receivers. SPS receivers used for waterborne missions can normally take

advantage of aiding information in the vertical direction when they encounter a DOP hole.

For example, the SPS receiver on a ship in the middle of the ocean knows the (calibrated)

height of its antenna above sea level. The ability to use this information as an extra

measurement effectively "fills in" any DOP holes. As a result, waterborne SPS receivers

are not usually subject to significant swings in expected PVT performance.

c. Land Navigation in Obstruction-Rich Environments. Land navigation in an environment,

which offers a clear view of the sky in all directions (e.g., flat desert terrain) is one thing,

but trying to navigate in an environment with nearby buildings, trees, or other obstructions

is another thing altogether. There can be so many obstructions around that they

completely block every SPS SIS from reaching your SPS receiver's antenna. Even when

there is only one nearby obstruction that only blocks one SPS SIS, the loss of that SPS

SIS can radically alter the DOP values. In such obstacle-rich environments, it is difficult

to try to predict in advance which satellite's SPS SIS will be blocked and when that

blockage will start or end. Obscuration angles are very sensitive to small changes in SPS

receiver antenna height and location as shown in Figures B.4-3 and B.4-4 for a nearby

obstacle.

Although it is possible to compute obscuration angles for situations like those in Figures B.4-3

and B.4-4, doing so is generally a wasted effort. Note how accurately you would have to know

the height and location of your SPS receiver antenna in order to precisely compute the

obscuration angle. If you knew in advance where your SPS receiver antenna was going to be

that accurately, then you would already have better height and location information than you are

probably going to get from your SPS receiver! Computing obscuration angles and expected DOP

values in advance may be a waste of time in these situations, but computing them in real-time is

important (as described previously).

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Figure B.4-3. Obscuration Angles versus SPS Receiver Antenna Height

Figure B.4-4. Obscuration Angles versus SPS Receiver Antenna Location

16 m

0.9 m

1.5 m

1.2 m

9

10

11

4.0 m

1.5 m = Holding SPS Receiver Head-High

1.2 m = Holding SPS Receiver Chest-High

0.9 m = Holding SPS Receiver Waist-High

16.0 m

1.2 m

9

10

11

4.0 m

17.5 m = Holding SPS Receiver Away From the Building

16.0 m = Holding SPS Receiver Straight Out in Front

14.5 m = Holding SPS Receiver Towards the Building

17.5m

14.5 m

April 2020 GPS SPS PS

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B.4.3.2 In-Advance PVT Performance Expectations Directly from Your Receiver

Certain missions which rely on the GPS SPS may be safety-related or critical enough that it is

worthwhile to take a simple in-advance look to be sure that the SPS will be available at some

future time at some future place to support mission accomplishment. SPS availability is high, but

it is not always 100% available everywhere. Accuracy is more available than integrity.

B.4.3.2.1 In-Advance Accuracy Expectations Directly from Your SPS Receiver

Some SPS receivers include provisions to let you define a future time and location and will

respond back with in-advance accuracy expectations (properly called in-advance "accuracy

predictions").

Generally speaking, being limited to predicted PDOP values is not too significant for mission

planning purposes in the field. In-advance predictions can never do better than use the

"transmitted on-orbit average" URE values (see paragraph B.2.3.4) for each satellite's SPS SIS.

Since averaging the satellite-transmitted URA numbers over time is difficult to do under field

conditions, the normal approximations are: (1) that all SPS SISs have the same UERE value,

and (2) that an appropriate 1-sigma UERE value can be developed from Table 3.4-1 and Table

B.2-1 for the particular type of SPS receiver in use. Under these approximations, it is easy enough

to simply follow equation B-14 and multiply the predicted PDOP value from the SPS receiver by

the appropriate 1-sigma UERE value.

B.4.3.2.2 In-Advance Integrity Expectations Directly from Your SPS Receiver

Some SPS receivers -- particularly avionics SPS receivers -- include provisions to let you define

a future time and location and will respond back to you with in-advance integrity predictions. This

capability is usually called "predictive RAIM" (see AC 20-138 for example).

For in-advance use, an avionics SPS receiver can use its RAIM algorithm to compute predicted

HPL values but it cannot compute predicted HUL values. The HUL computation requires actual

pseudoranges which are obviously not available in advance. The HPL computation only requires

satellite-to-receiver geometry and UERE estimates. Just for accuracy predictions, the satellite-

to-receiver geometry can be computed in-advance from the almanac data stored in the receiver's

internal memory. The UERE estimates for the particular type of SPS receiver can be developed

in a manner comparable to Table B.2-1 and stored in the receiver's internal memory as a uniform

number to be applied to all SPS SISs. Most avionics SPS receivers which provide this predictive

RAIM capability allow you to select your destination airport and your estimated time of arrival

(ETA), and will respond with an automatic "RAIM YES/NO" determination over a window of time

surrounding your ETA. The automatic "RAIM YES/NO" determination is made by comparing the

predicted HPL value against a HAL value of 0.3 nm for the non-precision approach phase of flight.

Notes:

1. Using a hard-coded, uniform UERE value is not as realistic as using "transmitted on-orbit average"

URE values along with receiver-specific UEE values, but it has been found to be sufficient for in-

the-cockpit predictions.

2. For additional information on this predictive RAIM capability, see MSO-C129, TSO-C145, MSO-

C145, TSO-C146, and TSO-C196.

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B.4.3.3 In-Advance PVT Performance Expectations from a Computer Model

If you must worry about high-fidelity in-advance PVT performance predictions, whether for

accuracy or for integrity, then you can use a computer model. There are customized computer

models and general purpose computer models.

B.4.3.3.1 Customized SPS Computer Model

If your mission requires you to frequently use high-fidelity PVT performance predictions, then

odds are that you will have been provided with a computer model that is set up specifically for

your particular SPS receiver (e.g., see AC 20-138). Your computer model may very well be an

integral part of the mission planning system you use -- there is a natural synergy between the two

because your mission planning system already knows when and where you want to go which is

the same as knowing the T-S points for which the PVT performance predictions must be

computed. For the highest fidelity PVT performance expectations, your computer model may be

integrated with portions of the actual SPS receiver software.

You will need to provide the customized computer model with the appropriate almanac data for

the satellites along with the satellite status in effect during the time window of interest. The daily

almanac data is available for downloading from https://www.navcen.uscg.gov . This web site also

has the NANUs which give the dates and times for projected changes in the satellite status

settings. For higher fidelity (if possible), you may also want to provide the computer model with

the "transmitted on-orbit average" SPS URE values (see paragraph B.2.3.4) for each satellite's

SPS SIS. Estimated SPS SIS URE values averaged over a day are built into the SEM-formatted

almanacs available at https://www.navcen.uscg.gov ; these URE values can be input to a moving

average filter to compute the higher-fidelity "transmitted on-orbit average" SPS URE values. You

won't need to know too much about the SPS receiver itself (e.g., satellite selection algorithm, UEE

values, mask angle), since that information will have been built directly into your customized

computer model.

Notes:

1. The daily almanac data from many web sites is available in two formats: the SEM format and the

Yuma format. Although the information is basically the same as what the satellites broadcast via

the SPS SIS, the two formats are not interchangeable. Be sure to download the right format for

your customized computer model.

2. The SEM format almanac data from both the indicated web sites already has the "transmitted on-

orbit average" SPS URA values built into it.

3. The highest fidelity will generally be achieved with the almanac data time tagged just prior to the

window of interest. Using almanac data from preceding days or following days not recommended

due to the potential for satellite repositioning events which will invalidate previous valid almanac

data.

B.4.3.3.2 General-Purpose GPS Computer Model

If you don't have a customized computer model for your SPS receiver, you can use a general-

purpose GPS computer model; however, you'll have to know some details about your SPS

receiver to set up the computer model right to get good results. In addition to the proper almanac

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data, SIS health settings, and URE estimates for the satellites, you will also need to know some

details about your SPS receiver which were described in general in Section B.2. Those SPS

receiver details are:

a. The SPS receiver algorithm for selecting the subset of visible satellite SISs to be used

in the PVT solution or time transfer solution.

b. Reserved.

c. Whether the SPS receiver can use aiding data (especially details about using vertical

aiding).

d. The SPS receiver's internal mask angle (or its satellite visibility algorithm).

e. The maximum number of SPS SISs that can/will be used in the PVT solution or time

transfer solution.

B.4.4 After-the-Fact PVT Performance Expectations

If you need after-the-fact PVT performance expectations customized to a particular circumstance,

then you can use a computer model like those described in the preceding section. The process

is usually the same. Occasionally, there are a few exceptions.

For high-fidelity after-the-fact PVT performance expectations, you will want to provide the

computer model with the actual URA numbers from the SPS SISs during the time window of

interest. Most modern SPS receivers use those URA numbers to compute the weighting factors

which modify the position solution as well as the effective satellites-to-user geometry. Without

the actual URA numbers, the computer model will not be able to replicate what the SPS receiver

did in computing its weighted DOPs.

Depending on the reason for needing after-the-fact PVT performance expectations, it may be

necessary to simulate actual SPS SIS pseudorange errors (instantaneous UREs) if the problem

being investigated is related to a SPS SIS integrity failure. Sometimes, short-duration SPS SIS

outages (e.g., short periods of non-standard C/A-code) may need to be simulated. Occasionally,

a computer model which handles terrain blockage or unusual receiving antenna orientations may

be required. Experience has shown that RFI is often a cause of unexpected PVT performance;

specialized computer models which address SPS SIS signal strength can be useful in these

cases.

April 2020 GPS SPS PS

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GLOBAL POSITIONING SYSTEM

STANDARD POSITIONING SERVICE

PERFORMANCE STANDARD

APPENDIX C

KEY TERMS, DEFINITIONS,

ABBREVIATIONS AND ACRONYMS

April 2020

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TABLE OF CONTENTS

SECTION C.1 Key Terms ......................................................................................... C-1

SECTION C.2 Definitions ......................................................................................... C-7

SECTION C.3 Abbreviations and Acronyms .......................................................... C-8

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SECTION C.1 Key Terms

Alarm. An indication requiring an immediate response (e.g., to preserve integrity).

Alert. Generic term encompassing both alarm and warning.

Alerted Misleading Signal-in-Space Information (AMSI). The pseudorange data set (e.g.,

pseudorange measurement and NAV data) provided by a SPS SIS provides alerted MSI (AMSI)

when the instantaneous URE exceeds the SIS URE NTE tolerance but a timely alert (alarm or

warning) is provided.

Auxiliary Satellite. An operational satellite that is not occupying a defined orbital slot in the

baseline 24-slot constellation or the expandable 24-slot constellation. Auxiliary satellites are

typically either newly launched satellites waiting to take their place in the baseline/expandable 24-

slot constellation, or are older satellites which are nearing the end of their useful lives and have

been shifted out of the baseline/expandable 24-slot constellation. The SPS SIS broadcast by an

auxiliary satellite is not required to meet all of the standards in Section 3.

Baseline 24-Slot Constellation. Operational satellites deployed in the 24 defined baseline

orbital slots. Each orbital slot is characterized by a near one-half sidereal day period such that

the orbit ground trace repeats each sidereal day. The orbital slots are organized by the orbit

plane, with each orbit plane having multiple slots and each slot having a unique orbital ground

trace. In the baseline 24-slot constellation, there are six orbit planes, each with four slots.

Baseline Satellite. An operational satellite occupying a defined orbital slot in the baseline 24-

slot constellation. Not all operational satellites occupy slots in the baseline 24-slot constellation,

and not all slots in the baseline 24-slot constellation are necessarily occupied by an operational

satellite. The SPS SIS broadcast by a baseline satellite is required to meet all of the standards

in Section 3. In contrast, the SPS SIS broadcast by an auxiliary satellite is not required to meet

all the performance standards in Section 3.

Block II Satellites. The current deployed operational constellation consists entirely of Block II

series satellites (the IIA, IIR, IIR-M, and IIF).

Dilution of Precision (DOP). The magnifying effect on GPS position error induced by mapping

URE into a position solution within the specified coordinate system, through the relative satellite-

to-receiver geometry. The DOP may be expressed in any user local coordinate system desired.

Examples include HDOP for local horizontal, VDOP for local vertical, PDOP for local horizontal

and vertical together, and TDOP for time.

Expandable 24-Slot Constellation. Operational satellites deployed in a constellation with at

least one of the defined-as-expandable orbital slots in its expanded configuration.

Expanded PRN Satellite. An operational satellite broadcasting PRN code signals numbered 33

or higher. Expanded PRN satellites are not eligible for satisfying the availability performance

standards in Section 3.7 due to backward compatibility limitations.

Expanded Slot Satellite. An operational satellite occupying a defined-as-expandable orbital slot

in its expanded configuration. There can be up to 12 expanded satellites at any one time.

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Full Operational Capability (FOC). Full Operational Capability (FOC) was achieved on 27 April

1995 when the GPS satellite constellation met all of its specified requirements. The DoD formally

announced the achievement of FOC to the public on 17 July 1995. For SPS users, the 1995 FOC

was limited to just the C/A-code signal. As of the date of this 5

Edition of the SPS PS, there are

no FOCs declared for any of the other SPS signals.

Geometric Range. The difference between the location of a satellite (true origin of the

pseudorange measurement) and the location of a GPS receiver.

Global Average. The typical (average, mean, RMS) value of a performance metric or

characteristic (e.g., dilution of precision) over the specified coverage (e.g., surface of the Earth,

satellite footprint).

Global Statistic. The statistical (95%, RMS, 1-sigma) value of an algebraically signed

performance metric or characteristic (e.g., instantaneous URE) over the specified coverage (e.g.,

satellite footprint). Formerly, but improperly, also called “global average”.

GPS Time. A continuous time scale maintained by the GPS Control Segment which began at

midnight on the night of 5/6 January 1980 on the Coordinated Universal Time (UTC) scale as

established by the U.S. Naval Observatory (USNO).

Hazardously Misleading Information (HMI). The errors in the position solution output by a SPS

receiver exceed the user’s particular tolerance for error in the current application.

Healthy Satellite. A satellite which is transmitting at least one trackable and healthy SPS SIS.

Healthy SPS Signal-in-Space (SIS). A trackable SPS SIS which: (a) is not subject to a SPS

alert indication, (b) indicates the SPS SIS is healthy in the navigation (NAV) message data, (c)

does not indicate a URA index value of 8 or greater, and (d) does not indicate a User Range

Accuracy (URA) alert.

Initial Operational Capability (IOC). Initial Operational Capability (IOC) was declared on 8

December 1993 when the DoD formally made the SPS available to the DOT. For SPS users, the

1993 IOC was limited to just the C/A-code signal. As of the date of this 5

Edition of the SPS PS,

there are no IOCs declared for any of the other SPS signals.

Instantaneous User Range Error (URE). An instantaneous URE is the difference between the

pseudorange measured at a given location assuming a receiver clock that is perfectly calibrated

to GPS time and the expected pseudorange as derived from the NAV message data for the given

location and the assumed receiver clock. The instantaneous SIS URE includes only those

pseudorange data set error budget components assigned to the GPS Space and Control

Segments (i.e., not including the error budget components assigned to the GPS User Segment

such as the troposphere delay compensation error, multipath, and receiver noise).

Integrity Assured User Range Accuracy (IAURA). The IAURA is a conservative representation

of the upper bound on each satellite's expected RMS URE performance over the curve fit interval

represented by the NAV data from which the URA is read. The IAURA is a key parameter in

determining the SIS URE not-to-exceed (NTE) tolerance for integrity. The equations for

determining the LNAV IAURA based on the LNAV URA index are given in IS-GPS-200, while the

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equations for determining the CNAV IAURA based on the CNAV URA

, URA

NED0

, URA

NED1

, and

URA

NED2

, indexes are given in both IS-GPS-200 and IS-GPS-705.

Major Service Failure. A condition during which a trackable and healthy SPS SIS's

instantaneous URE exceeds the SIS URE not-to-exceed (NTE) tolerance without a timely alert

(alarm or warning) being provided. Also known as a UMSI event and/or an integrity failure.

Misleading Signal-in-Space Information (MSI). The pseudorange data set (e.g., pseudorange

measurement and NAV data) provided by a SPS SIS provides Misleading Signal-in-Space

Information (MSI) when the instantaneous URE exceeds the SIS URE NTE tolerance.

Modernized Block III Satellites. This edition of the SPS PS does not address the future open

access signals (e.g., L1C) that will be transmitted by Modernized Block III satellites.

Navigation. The process of planning, recording, and controlling the movement of a craft or

vehicle from one place to another. Navigation is an application of position, velocity and time

information.

Navigation (NAV) Message Data. The data provided to a GPS receiver via each satellite's SIS

containing the satellite's predicted clock correction polynomial ("clock"), the satellite's predicted

orbital elements ("ephemeris"), the satellite's predicted healthy and accuracy parameters

("integrity"), optionally a reduced-precision subset of the clock/ephemeris/integrity data for all

operational satellites in the constellation ("almanac"), optionally pseudorange correction data,

parameters relating GPS time to UTC, SF ionospheric correction model parameters, and other

system information. The clock, ephemeris, and integrity information comprise a matched

collection of information known as the CEI data set, The C/A-code signal provides the ‘legacy’

version of the NAV data (LNAV) while the CM-code signal and I5-code signal provides the ‘civil’

version of the NAV data (CNAV). Detailed definitions of the NAV data are provided in IS-GPS-

200 and IS-GPS-705.

Non-Standard Code. The non-standard codes (NSCs) are used to protect the user from SIS

malfunctions. Non-standard codes are not trackable by SPS receivers which are compliant with

the GPS ISs/ICDs.

Occupied. A slot is occupied by a satellite when the satellite’s footprint on the surface of the

Earth overlaps 95% of the slot center’s footprint on the surface of the Earth averaged over an

orbit revolution.

Operational Satellite. A satellite which is capable of transmitting, but is not necessarily currently

transmitting, a trackable ranging signal. For the purposes of these performance standards, any

satellite in the transmitted navigation message almanac is considered an operational satellite.

Percentile (95%) URE. A statistical measurement of the instantaneous URE performance

sampled over some interval. The 95% URE can apply to the SPS SIS from an individual satellite

or to the SPS SISs from an ensemble of satellites (e.g., all usable satellites in the constellation).

PPS Signals. A subset of the electromagnetic signals originating from a satellite. The PPS

signals consist of a Pseudorandom Noise (PRN) C/A-code with NAV data to support the PVT

solution generation process on the GPS L1 frequency; a PRN P(Y)-code with identical NAV data

to support the PVT solution generation process on the GPS L1 frequency; and a PRN P(Y)-code

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with identical NAV data to support the PVT solution generation process on the GPS L2 frequency.

Depending on the particular satellite, PPS signals may also include M-code signals.

Precise Positioning Service (PPS). The GPS broadcast signals based on the L1 P(Y)-codes,

L1 C/A-codes, L2 P(Y)-codes, and optionally M-codes, as defined in the GPS ISs/ICDs, providing

constellation performance to authorized users, as established in the PPS Performance Standard

(PPS PS), in accordance with U.S. Government (USG) policy.

Pseudorange. Depending on context, “pseudorange” may refer to a full pseudorange data set

or just to a pseudorange measurement by itself.

Pseudorange Data Set. The matched combination of a corrected pseudorange measurement

and a pseudorange origin, or equivalently the matched combination of a raw pseudorange

measurement and the associated NAV data. In vector terms, a pseudorange data set comprises

an origin and a scalar magnitude – but no orientation information.

Pseudorange Measurement. The difference between the PRN code time of reception (as

defined by the SPS receiver’s clock) and the PRN code time of transmission (as defined by the

satellite’s clock) multiplied by the speed of light. May also be a short-hand reference to the

corrected pseudorange measurement where the raw pseudorange measurement is adjusted to

GPS system time using the broadcast satellite clock correction polynomial and related data. The

corrected pseudorange measurement is also known as an SPS signal ranging measurement.

Pseudorange Origin. The point in space from which a particular pseudorange measurement

originates (e.g., Earth-Centered, Earth-Fixed Cartesian coordinates), typically derived from the

broadcast satellite ephemeris portion of the NAV data.

PVT Solution. The use of pseudorange data sets from at least four SISs to solve for three

position coordinates and time offset relative to GPS time, plus three velocity coordinates and

frequency offset relative to GPS time. In cases where the altitude is known (e.g., maritime GPS

receivers), the PVT solution only requires the use of pseudorange data sets from at least three

SISs to solve for two position coordinates and time offset relative to GPS time, plus two velocity

coordinates and frequency offset relative to GPS time.

Receiver Autonomous Integrity Monitoring (RAIM). RAIM is an algorithm used by a GPS

receiver to autonomously monitor the integrity of the output position/time solution data. There are

many different RAIM algorithms. RAIM algorithms are sometimes also known as fault detection

(FD) algorithms. All RAIM algorithms are based on the consistency of redundant measurements.

Root Mean Square (rms or RMS) URE. A statistical measurement of the instantaneous URE

performance sampled over some interval. The RMS URE can apply to an individual satellite or

to an ensemble of satellites (e.g., all operational satellites in the constellation).

Satellite Outage. A satellite outage occurs when a satellite either stops transmitting a trackable

SPS SIS or the SPS SIS becomes unhealthy.

SatZap. A manual technique used by the Control Segment to temporarily remove a satellite from

service by commanding the satellite to substitute transmission of an unused PRN code number

(typically PRN-37) for its standard PRN code number. May also refer to the Control Segment

commanding the satellite to switch to NSC transmission.

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Selective Availability (SA). Protection technique employed by DoD in the past to deny full

system accuracy. On May 1, 2000, President Clinton announced the discontinuance of SA

effective midnight 1 May 2000. The effects of SA went to zero at 0400 UTC on 2 May 2000.

Service Interruption. A condition over a time interval during which one or more SPS

performance standards are not satisfied.

SIS URE. The SIS URE includes only those pseudorange data set error budget components

assigned to the GPS Space and Control Segments. The SIS URE can be expressed in different

ways; e.g., on an instantaneous basis (see the definition of instantaneous URE) or on a statistical

basis (see the definition of RMS URE).

Space Service Volume. One of the two spatial volumes addressed by this SPS PS. The space

service volume extends from 3,000 km above the surface of the earth up to and including 36,000

km above the earth’s surface.

SPS Signal Ranging Measurement. The difference between the PRN code time of reception

(as defined by the SPS receiver's clock) and the PRN code time of transmission (as defined by

the satellite's clock) adjusted to GPS system time using the satellite clock correction polynomial

and related data contained within the satellite's NAV data multiplied by the speed of light. Also

known as the corrected pseudorange measurement.

SPS Signals. A subset of the electromagnetic signals originating from a satellite. The SPS

signals consist of at least a Pseudorandom Noise (PRN) C/A-code with LNAV data to support the

PVT solution generation process on the GPS L1 frequency. Depending on the particular satellite,

the SPS signals may also include a PRN CM-code with CNAV data, a PRN CL-code without

CNAV data, a PRN I5-code with CNAV data, and/or a PRN Q5-code without CNAV data to support

the PVT solution generation process on the GPS L1/L2/L5 frequencies.

SPS SIS. See the definition of SPS signals.

Standard Positioning Service (SPS). The GPS broadcast signals, as defined in IS-GPS-200

and IS-GPS-705, providing constellation performance to peaceful civil, commercial, and scientific

users, as established in the SPS Performance Standard (SPS PS), in accordance with U.S.

Government (USG) policy.

Terrestrial Service Volume. One of the two spatial volumes addressed by this SPS PS. The

terrestrial service volume covers the entire surface of the Earth up to an altitude of 3,000

kilometers. The terrestrial service volume is thus global.

Trackable SPS Signal. An SPS signal that can be preprocessed and categorized as either

healthy, marginal, or unhealthy by an SPS receiver.

Unalerted Misleading Signal-in-Space Information (UMSI). The pseudorange data set (e.g.,

raw pseudorange measurement and NAV data) provided by a SPS SIS provides unalerted MSI

(UMSI) when the instantaneous URE exceeds the SIS URE NTE tolerance without a timely alert

(alarm or warning) being provided.

Usable Satellite. A satellite that is broadcasting a usable SPS signal.

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Usable SPS Signal. An SPS signal that is trackable and is either healthy or marginal. A usable

SPS signal may be used to form the PVT solution.

User Range Accuracy (URA). The URA is a conservative representation of each satellite's

expected RMS URE performance over the curve fit interval represented by the NAV data from

which the URA is read. The URA is a coarse representation of the expected URE statistic, in that

it is quantized to the levels represented by the LNAV URA index "N" as defined in IS-GPS-200 or

to the levels represented by the CNAV URA

, URA

NED0

, URA

NED1

, and URA

NED2

, indexes as

defined in IS-GPS-200 and IS-GPS-705.

Warning. An indication requiring prompt attention (e.g., to preserve integrity).

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SECTION C.2 Definitions

Accuracy. Accuracy is defined to be the statistical difference between the estimate or

measurement of a quantity and the true value of that quantity. For the purposes of this SPS PS,

the SPS SIS quantities are the pseudorange, the pseudorange rate (velocity), and the

pseudorange acceleration (rate rate). The statistical differences are expressed either as 95

percentile (95%) differences or as RMS differences.

Availability. Availability is defined as the percentage of time that the SPS SISs are available to

a SPS receiver. Availability can be expressed in different ways; e.g., on a per-satellite basis or

on per-constellation basis.

Backward Compatibility. Backward compatibility is defined as the set of SIS characteristics

which: (a) enables existing IS-GPS-200 compliant SF C/A-code receivers to continue operating

and obtaining the performance specified in this standard as well as in prior editions of this

standard, (b) enables new IS-GPS-200 compliant SF or DF receivers to continue operating and

obtaining the performance specified in this standard, and (c) enables new IS-GPS-200 and/or IS-

GPS-705 compliant SF, DF, or TF receivers to continue operating and obtaining the performance

specified in this standard, .

Continuity. Continuity is defined to be the probability that a trackable and healthy SPS SIS will

continue to be trackable and healthy without unscheduled interruption over a specified time

interval. SPS SIS continuity is directly related to SPS SIS reliability.

Coverage. The surface area or spatial volume where the SPS SISs are intended to be provided

in a manner to meet the specified level of accuracy. The coverage for the SPS SISs is the

terrestrial service volume.

Ergodic Period. The time span containing the minimum number of samples such that the sample

statistic is representative of the population statistic. A common first-order approximation is the

minimum time span containing 30 independent random samples.

Integrity. Integrity is a measure of the trust which can be placed in the correctness of the

information supplied by the total system. Integrity includes the ability of the SPS SIS to provide

timely alerts (alarms or warnings) to receivers when the SPS SIS should not be used. SPS SIS

integrity is directly related to SPS SIS reliability

Reliability. Reliability is the ability of a SPS SIS to perform its required functions over a specified

time interval. Reliability includes continuity and integrity.

UTC(USNO) Accuracy. The SPS SIS UTC(USNO) time accuracy is defined to be the statistical

difference, at the 95

percentile, between the parameters contained in the SPS SIS which relate

GPS time to UTC as maintained by the USNO and the true value of the difference between GPS

time and UTC(USNO). Also known as the UTC Offset Error (UTCOE).

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SECTION C.3 Abbreviations and Acronyms

- A -

A Alongtrack

ABAS Aircraft Based Augmentation System

ADS-B Automatic Dependent Surveillance – Broadcast

AFSPC Air Force Space Command

AII Accuracy Improvement Initiative

AIV All-In-View (GPS receiver SIS tracking capability)

AMCS Alternate Master Control Station

AMSI Alerted Misleading Signal-in-Space Information

AOD Age of Data (with regards to NAV message data)

AOO Area of Operations

ArgLat Argument of Latitude (satellite orbital parameter)

- B -

BC Backwards Compatibility

bps Bits Per Second

BPSK Bi-Phase Shift Key

- C -

C Crosstrack

c Speed of light (2.99792458x10

m/sec)

C/A-code Coarse/Acquisition PRN ranging code modulating the carrier signal at L1

CEI Clock, Ephemeris, Integrity (data set) – LNAV or CNAV

CL-code The Civil Long PRN ranging code, which is half of the L2C signal modulating

the carrier signal at L2

CM-code The Civil Moderate PRN ranging code which is half of the L2C signal modulating

the carrier signal at L2

CNAV Civil Navigation (as in “CNAV data” or "CNAV message")

CONUS Continental U.S.

cps Chips Per Second

CS Control Segment

CUT Contingency Upload Threshold

CV Constellation Value

- D -

1-D One-Dimensional

2-D Two-Dimensional

3-D Three-Dimensional

dBW Decibels with respect to one Watt

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DF Dual Frequency

DFMC Dual-Frequency Multi-Constellation

DFSC Dual-Frequency Single-Constellation

DGPS Differential GPS

DoD Department of Defense

DOP Dilution Of Precision

DOT Department of Transportation

- E -

EOL End of Life

ETA Estimated Time of Arrival

EHE Estimated Horizontal Error

EPE Estimated Position Error

EPU Estimated Position Uncertainty

EVE Estimated Vertical Error

- F -

FAA Federal Aviation Administration

FD Fault Detection (see RAIM)

FOC Full Operational Capability

FOM Figure of Merit

FRP Federal Radionavigation Plan (DOT-VNTSC-OST-R-15-01)

- G -

GA Ground Antenna (part of the CS)

GBAS Ground-Based Augmentation System

GDOP Geometric Dilution Of Precision

GEC Groundtrack Equatorial Crossing (satellite orbital parameter)

GLAN Geographic Longitude of the Ascending Node (satellite orbital parameter)

GNSS Global Navigation Satellite System (GPS is one of many different GNSSs)

GPS Global Positioning System (or Navstar Global Positioning System)

- H -

HAL Horizontal Alert Limit (for RAIM)

HDOP Horizontal Dilution Of Precision

HDOP-AT Horizontal Dilution Of Precision - Availability Threshold (a DOP limit)

HMI Hazardously Misleading Information

HOW Handover Word (part of the NAV message)

HPL Horizontal Protection Level (from RAIM)

HSAT Horizontal Service Availability Threshold (an accuracy limit)

HUL Horizontal Uncertainty Level (from RAIM)

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- I -

I5 In-phase PRN ranging code modulating the carrier signal at L5

IAURA Integrity Assured User Range Accuracy

ICAO International Civil Aviation Organization

ICD Interface Control Document

IFR Instrument Flight Rules

IMU Inertial Measurement Unit

IOC Initial Operational Capability

IODC Index of Data Clock (part of the LNAV message)

IODE Index of Data Ephemeris (part of the LNAV message)

IS Interface Specification

ISB Inter-Signal Bias (a characteristic of the SIS)

ISC Inter-Signal Correction (part of the CNAV message)

- J -

JPO Joint Program Office (predecessor of SMC Production Corps and PNT Mission

Integration)

JSSMO Joint Service System Management Office

- K -

- L -

L1 The SIS centered at the 1575.42 MHz frequency

L1C A future civil signal centered at L1 (not addressed in this SPS PS)

L2 The SIS centered at the 1227.60 MHz frequency

L2C The civil signal centered at L2

L2C-code The chip-by-chip multiplexed combination of CM-code and CL-code

L5 The SIS centered at the 1176.45 MHz frequency

L95 95

percentile of a Linear distribution (e.g., vertical position error)

LNAV Legacy Navigation (as in “LNAV data” or "LNAV message")

LOAN Launch On Anticipated Need (a philosophy for constellation sustainment)

LON Launch On Need (a philosophy for constellation sustainment)

LSB Least Significant Bit

LT Long Term

LTS Long Term Scheduled (type of outage)

LTS Launch To Sustain (a philosophy for constellation sustainment)

LTU Long Term Unscheduled (type of outage)

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- M -

M-code Encrypted Precise PRN ranging code modulating the carrier signals at

L1 and L2

Mcps Mega chips per second

MCS Master Control Station (part of the CS)

MNAV Military Navigation (as in “MNAV data” or "MNAV message")

MS Monitor Station (part of the CS)

MSB Most Significant Bit

MSI Misleading Signal-in-Space Information

MSO Military Standard Order (DoD document)

MT Message Type (for CNAV)

MT-3x Message Type 30/31/32/33/etcetera (for CNAV)

MTBF Mean Time Between Failure

MTBLOC Mean Time Between Loss of Continuity

- N-

NANU Notice Advisory to Navstar Users

NATO North Atlantic Treaty Organization

NAV Navigation (as in “NAV data” or "NAV message", a generic term applying to

LNAV, CNAV, and MNAV data and/or LNAV, CNAV, and MNAV

messages)

NGA National Geospatial-Intelligence Agency

NH-code Neuman-Hofman Code

NOTAM Notice to Airmen

ns Nanosecond

NSC Non-Standard Code

NSTB National Satellite Test Bed (FAA sponsored)

NTE Not To Exceed (i.e., a tolerance limit)

NWT No Worse Than

- O -

OCS Operational Control System

OCX Next Generation Operational Control System

O&M Operations and Maintenance

- P -

P-code Unencrypted Precise PRN ranging code

const

Probability of multiple satellites being in a common-cause MSI faulted state

at a given time

PBN Performance Based Navigation

PDOP Position Dilution of Precision

PDOP-AT Position Dilution of Precision - Availability Threshold (a DOP limit)

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PNT Positioning, Navigation, and Timing

PPS Precise Positioning Service

PRN Pseudorandom Noise (a characteristic of the SIS ranging codes)

PS Performance Standard (as in PPS PS or SPS PS)

sat

Probability of a satellite being in an MSI faulted state at a given time

PSK Phase Shift Key

PVT Position, Velocity, and Time

P(Y)-code Precise PRN ranging code (unencrypted or encrypted) modulating the

carrier signals at L1 and L2

- Q -

Q5 Quadrature-phase PRN ranging code modulating the carrier signal at L5

- R -

R Radial

R95 95

percentile of a Radial distribution (e.g., horizontal position error)

RAAN Right Ascension of the Ascending Node (satellite orbital parameter)

RAC Radial-Alongtrack-Crosstrack (orbital coordinate system)

RAIM Receiver Autonomous Integrity Monitoring

RCM Requirements Correlation Matrix

RF Radio Frequency

RFI Radio-Frequency Interference

RHCP Right-Hand Circularly Polarized

RMS Root-Mean-Square

RNP Required Navigation Performance

RSS Root-Sum-Square

- S -

SA Selective Availability

SARPs Standards and Recommended Practices (see ICAO)

SAT Service Availability Threshold (an accuracy limit)

SBAS Satellite-Based Augmentation System

SEM System Effectiveness Model

SEP Spherical Error Probable (3-D accuracy, 50

percentile)

SF Single Frequency

SFSC Single-Frequency Single-Constellation

SIS Signal In Space

SMC Space and Missile Systems Center

2 SOPS 2

Space Operations Squadron (USSF)

SPS Standard Positioning Service

sps Symbols Per Second

SS Space Segment

ST Short Term

STS Short Term Scheduled (type of outage)

STU Short Term Unscheduled (type of outage)

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SV Space Vehicle (e.g., satellite)

- T -

TDOP Time Dilution Of Precision

TF Triple Frequency

TFSC Triple-Frequency Single-Constellation

Group Delay Time correction (for SF receivers)

TLM Telemetry Word (part of the LNAV message)

Almanac reference time (part of the LNAV message)

Propagation time of week (part of the CNAV message)

TPL Time Protection Level (from RAIM)

T-S Time-Space (description of a point)

TSO Technical Standard Order (FAA document)

TTA Time to Alert

TT&C Telemetry, Tracking, and Command

TTDOP Time Transfer Dilution of Precision

TUL Time Uncertainty Level (from RAIM)

- U -

UE User Equipment (i.e., GPS receiver, antenna, display system, etcetera)

UEE User Equipment Error (pseudorange inaccuracy due to the receiver)

UERE User Equivalent Range Error (total pseudorange inaccuracy)

UERRE User Equivalent Range Rate Error (total pseudorange rate inaccuracy)

UHNE User Horizontal Navigation Error (user horizontal position error)

UHVE User Horizontal Velocity Error

UMSI Unalerted Misleading Signal-in-Space Information

UNE User Navigation Error (user 3-D position error)

URA User Range Accuracy (a parameter in the NAV messages)

URAE User Range Acceleration Error (total pseudorange acceleration inaccuracy)

URE User Range Error (pseudorange inaccuracy due to the SIS)

URRE User Range Rate Error (pseudorange velocity inaccuracy due to the SIS)

U.S. United States (of America)

U.S.C. United States Code (U.S. law)

USAF U.S. Air Force

USG U.S. Government

USNO U.S. Naval Observatory

USSF U.S. Space Force

UTC Coordinated Universal Time (the acronym comes from the French)

UTCOE UTC(USNO) Offset Error (relative to GPS time)

UTE User Time Error (relative to GPS time)

UUTCE User UTC(USNO) Error

UVNE User Vertical Navigation Error (user vertical position error)

UVVE User Vertical Velocity Error

- V -

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VDOP Vertical Dilution Of Precision

VDOP-AT Vertical Dilution Of Precision - Availability Threshold (a DOP limit)

VPL Vertical Protection Level (from RAIM)

VSAT Vertical Service Availability Threshold (an accuracy limit)

VUL Vertical Uncertainty Level (from RAIM)

- W -

WGS 84 World Geodetic System 1984

Almanac reference week (part of the LNAV message)

- X -

- Y -

Y-code Encrypted Precise PRN ranging code modulating the carrier signals at

L1 and L2

- Z -

ZAOD Zero Age Of Data (a categorization of error or accuracy)