GLOBAL POSITIONING SYSTEM
WIDE AREA AUGMENTATIONSYSTEM (WAAS)PERFORMANCE STANDARD

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Approved for public release; distribution is unlimited.

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TABLE OF CONTENTSSECTION 1.0 The GPS WAAS Service ........................................................ 1
1.1 Purpose .......................................................................................................................... 1
1.2 Scope.............................................................................................................................. 1
1.3 GPS SPS Definition ........................................................................................................ 2
1.4 WAAS Service Definition ................................................................................................ 2
1.5 Key Terms and Definitions ............................................................................................. 2
1.6 Global Positioning System Overview.............................................................................. 2
1.6.1 GPS Space Segment ............................................................................................. 2
1.6.2 GPS Control Segment............................................................................................ 4
1.7 Wide Area Augmentation System (WAAS) Overview .................................................... 6
1.7.1 GPS WAAS Air Navigation User Segment............................................................. 6
1.7.2 GPS WAAS Corrections and Integrity .................................................................... 7
1.7.3 WAAS Architecture and Operational Environment................................................. 9

SECTION 2.0 WAAS SIS Characteristics..................................................... 12
2.1 WAAS SIS Interface Requirements................................................................................12
2.2 Overview of WAAS SIS Interface Characteristics ..........................................................122.2.1 WAAS SIS RF Characteristics ...............................................................................122.2.2 WAAS Message Characteristics ............................................................................12
2.3 Overview of WAAS SIS Performance Characteristics....................................................132.3.1 WAAS SIS Accuracy ..............................................................................................132.3.2 WAAS SIS Integrity ................................................................................................13
2.3.3 WAAS SIS Continuity .............................................................................................142.3.4 WAAS SIS Availability ............................................................................................15

SECTION 3.0 WAAS SIS Performance Standards ...................................... 16
3.1 WAAS SIS Coverage......................................................................................................16
3.2 WAAS SIS Performance Requirements .........................................................................19
3.3 WAAS SIS Performance.................................................................................................20
3.4 WAAS Current Services and Availability ........................................................................20
3.4.1 Mapping Functions to Operational Capability ........................................................21

SECTION 4.0 References.............................................................................. 25
4.1 Government Documents.................................................................................................25
4.2 Non-Government Documents.........................................................................................27

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List of Figures
Figure 1.6-1 GPS SIS Generation and Transmission................................................................ 3
Figure 1.6-2 The GPS Operational Control System .................................................................. 4
Figure 1.7-1 WAAS Augmented and GPS Only Operations...................................................... 7
Figure 1.7-2 WAAS Integrity Protection Cylinder ...................................................................... 8
Figure 1.7-3 WAAS Architecture and Operational Environment................................................ 9
Figure 1.7-4 WAAS Data Processing Path ................................................................................ 10
Figure 3.1-1 WAAS Zone 1 Coverage Area .............................................................................. 16
Figure 3.1-2 WAAS Zone 2 Coverage Area .............................................................................. 17
Figure 3.1-3 WAAS Zone 3 Coverage Area .............................................................................. 17
Figure 3.1-4 WAAS Zone 4 Coverage Area .............................................................................. 18
Figure 3.1-5 WAAS Zone 5 Coverage Area .............................................................................. 18
Figure 3.4-1 WAAS Ranging Function, Satellite Status and Differential Correction Function .. 22
Figure 3.4-2 WAAS Service Area for LNAV............................................................................... 23
Figure 3.4-3 WAAS Service Area for Localizer Precision with Vertical Guidance (LPV) Approach(HAL = 40m; VAL = 50m)............................................................................................................ 24

List of Tables
Table 2.1-1 WAAS Message Types...........................................................................................12
Table 3.2-1 Navigation Performance Standards .......................................................................19
Table 3.3-1 WAA SIS Performance ...........................................................................................20

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List of Appendices

Appendix A: WAAS Service Special Topics
Appendix B: 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 spacebased positioning, navigation, and timing (PNT) signals generated from space vehicles orbiting theearth and delivered free of direct user fees for civil, commercial, and scientific uses worldwide. The
Wide Area Augmentation System (WAAS) provides an augmentation signal to GPS, delivered freeof direct user fees, that provides correction and integrity information intended to improve
positioning navigation and timing (PNT) service over the United States (U.S.) and portions ofCanada and Mexico. WAAS is the first operational implementation of an International Civil Aviation
Organization (ICAO) compliant Space Based augmentation System (SBAS). This WAAS
Performance Standard (WAAS PS) specifies the levels of navigation performance that will beavailable to suitably equipped users who use both the GPS SPS broadcast signals and the WAAS
augmentation signal. The U.S. Government is committed to meeting the minimum levels of service
specified in this WAAS PS. Refer to the GPS SPS PS for specific information regarding the USG
commitment for the GPS SPS.
Since WAAS was commissioned in 2003, actual performance has typically met and exceeded theminimum accuracy, integrity, continuity, and availability performance requirements specified in this
WAAS PS and users can therefore generally expect improved performance over the minimum
levels described here. Actual real-time performance, statistical performance and real-time data areprovided on-line by the Federal Aviation Administration (FAA). Quarterly Performance Analysis
Reports for both GPS and WAAS are also available at the FAA Technical Center WAAS Test Bed
web site (http://www.nstb.tc.faa.gov/). Interested readers are encouraged to refer to this site andother sources for updated GPS and WAAS performance information. Note that the number of
WAAS-based Localizer Performance with Vertical (LPV) guidance procedures now exceeds thenumber of Instrument Landing System (ILS) procedures in the United States.
GPS will provide three new modernized civil signals in the future: L2C, L5, and L1C. With theadditional signal on L5, airborne receivers will be able to correct for the line of sight ionosphericpropagation delay error. This dual frequency (L1/L5) mode of operation will allow changes to bemade in the delivery of GPS-based augmentation services, such as WAAS, but this PerformanceStandard does not consider those future changes. This WAAS Performance Standard only applies
to WAAS augmented GPS SPS users of the L1 (1575.42 MHz) Coarse/Acquisition (C/A) signal andthe WAAS Signal-In-Space (SIS) broadcast by a geostationary satellite (GEO).
The WAAS PS will be updated as required to reflect substantial changes to WAAS augmentation
services. In addition to the WAAS PS, readers are referenced to the GPS SPS PS and FAA
Technical Standard Order (TSO)-145/146 for details in both the fundamental GPS SPS service and
WAAS receiver equipment. WAAS also meets or exceeds the ICAO Annex 10, Standards andRecommended Practices (SARPs) for Global Navigation Satellite System (GNSS) Satellite Based
Augmentation System (SBAS).

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SECTION 1.0 The GPS WAAS Service
The Navstar Global Positioning System, hereinafter referred to as GPS, is a space-based
radionavigation system owned by the United States Government (USG) and operated by theUnited States Air Force (USAF). The Wide Area Augmentation System (WAAS) is owned by the
USG and operated by the Federal Aviation Administration (FAA) to provide regional GPS
augmentation services coverage in North America.
The FAA is responsible for the design, development, operation, and sustainment of the WAAS.
The WAAS Program Office is located at FAA headquarters in Washington, D.C. Second levelengineering and logistics are located at Mike Monroney Aeronautical Center in Oklahoma City, and
the WAAS performance monitoring center is located at the FAA Technical Center in Atlantic City,
N.J.
stThis 1 Edition WAAS Performance Standard serves as a companion document to the GPS SPS
Performance Standard to define the performance of WAAS augmented GPS SPS. This WAAS
Performance Standard follows the same format and structure as the GPS SPS PS.

1.1 Purpose
This 1st Edition of the WAAS Performance Standard (WAAS PS) defines the levels of signal-inspace (SIS) performance to be provided by the USG to the WAAS user community. A user willattain this performance with a properly installed receiver compliant with TSO-C145/146.
This WAAS PS consists of a main body and two appendices. The WAAS PS provides an overview
of the WAAS and GPS SPS services, how they are used, and an overview of the aviation
operations that are enabled by WAAS augmentation. It then provides the performance standards
for the WAAS SIS. It concludes with the relevant reference documents. The appendices provide
additional information that quantifies and illustrates WAAS SIS performance. Provided below is adefinition of each appendix's purpose.
•

Appendix A: Service Special Topics. This appendix discusses special topics of interest
to the WAAS service and/or flight operations.

•

Appendix B: Definitions. This appendix provides a list of key terms, definitions,abbreviations and acronyms used in this WAAS PS.

1.2 ScopeThis WAAS PS defines standards for the WAAS SIS performance. Section 3 specifies the WAAS
SIS performance standards in terms of performance metrics. This WAAS PS employs standard
definitions and relationships between the performance parameters such as availability, continuity,
integrity, and accuracy.
This WAAS PS only applies to the WAAS SIS as it exists on the publication date of this document.
This document does not address services relying on future civil signals (L1C, L2C or L5), that will
be broadcast by modernized GPS satellites. It should be noted that WAAS receivers certified foraviation use must comply with FAA TSO-145/146.

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1.3 GPS SPS Definition
The GPS Standard Positioning Service (SPS) is defined in the SPS PS as follows:
The SPS is a positioning and timing service provided by way of ranging signals broadcast at the
GPS L1 frequency. The L1 frequency, transmitted by all satellites, contains a coarse/acquisition(C/A) code ranging signal, with a navigation data message, that is available for peaceful civil,
commercial, and scientific use.

1.4 WAAS Service Definition
The WAAS Service is defined as follows:
The WAAS is a Satellite Based Augmentation System (SBAS) for North America that augments
GPS SPS by broadcasting differential GPS (DGPS) correction messages from GEO satellites.
The WAAS Service provides augmentation of GPS integrity via integrity data included on the
WAAS message broadcasts. The WAAS Service is specifically designed to meet high accuracy,
integrity, continuity, availability standards of aviation users, but is an open service that has thecapability to support other applications as well. WAAS provides a ranging function throughout theentire satellite footprint that improves the availability of GPS positioning for SBAS users. WAAS
also provides differential corrections as well as satellite status for GPS satellites.

1.5 Key Terms and Definitions
Terms and definitions that are essential to understanding the WAAS PS are provided in Appendix
B. A list of abbreviations and acronyms is also provided in Appendix B.

1.6 Global Positioning System Overview
The GPS comprises three segments: the Control, Space, and User Segments. The Control andSpace Segments are described below. Together they provide two types of service, the SPS SIS
and the PPS SIS. The SPS SIS is available to all users, but the PPS SIS is encrypted and isreserved for authorized users. For further information on the SPS SIS or PPS SIS, refer to the
SPS PS (http://pnt.gov/public/docs/2008-spsps.pdf)or PPS PS (http://pnt.gov/public/docs/2007pps-unclassified.pdf). The WAAS SIS is added as an augmentation to the SPS SIS by separate
broadcast from geostationary earth orbiting (GEO) satellites. Together, the WAAS SIS and theSPS SIS entail the WAAS service. The WAAS SIS interface is described in Section 2. This GPS
description is taken from the GPS SPS PS.

1.6.1 GPS Space Segment
The GPS baseline 24-slot constellation consists of 24 slots in six orbital planes with four slots perplane. Three of the 24 slots are expandable. The baseline satellites will occupy these slots. Any
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surplus satellites that exist on orbit will occupy other locations in the orbital planes. There are no apriori specified slots for surplus satellites. The nominal semi-major axis is 26,559.7 kilometers.
The SPS SIS generation and transmission process for a Block IIA satellite is illustrated in Figure
1.6-1. The Atomic Frequency Standard (AFS) generates a nominal 10.23 MHz clock signal. The
signal is distributed by the Frequency Synthesizer and Distribution Unit (FSDU) to other payload
subsystems. The Navigation Data Unit (NDU) receives the uploaded navigation (NAV) data fromthe Control Segment (CS) through the Telemetry, Track, and Command (TT&C) subsystem. The
Navigation Baseband generates the pseudorandom noise (PRN) ranging codes and adds the NAV

GPS SIS
Right-Hand Circularly Polarized
1575.42 MHz & 1227.6 MHz

NAV DATAUPLOAD
FROM CSATOMICFREQUENCY
STANDARD

TT&C
SUBSYSTEMFREQUENCY SYNTHESIZER
AND DISTRIBUTION UNIT
• 10.23 MHz synthesized
digital clock

Helix Array
Antenna

NAVIGATIONDATA UNIT

NAVIGATIONBASEBAND• NAV & control
data checks• 50 Bits Per Sec
NAV data• P(Y)-code generation• C/A-code generation• Modulo-2 addition ofcodes and NAV data

L-BANDSUBSYSTEM• Spread spectrum
modulation of 1575.42
MHz and 1227.6 MHz
L-band carriers

Figure 1.6-1. GPS SIS Generation and Transmission
data message. The L-Band subsystem modulates the binary sequences onto the L1 (1575.42
MHz) and L2 (1227.6 MHz) L-band carriers which are then broadcast by the helix array antenna.
Each satellite broadcasts three PRN ranging codes: the precision (P) code, which is the principal
ranging code; the Y-code, used in place of the P-code whenever the anti-spoofing mode ofoperation is activated; and the coarse/acquisition (C/A) code which is used for acquisition of the P(or Y) code (denoted as P(Y)) and as a civil ranging signal. A NAV message based upon data
periodically uploaded from the Control Segment is provided by adding the NAV data to both the1.023 MHz C/A-code sequence and the 10.23 MHz P(Y)-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. The entire set of code-plus-data sequences and carriers is referred to as the
PPS SIS. A subset of the PPS SIS, the SPS SIS, comprises only the 1.023 MHz code-plus-data
sequence on the L1 carrier. Collectively, the PPS SIS and the SPS SIS are known as thesatellite's navigation signals (or navigation SIS or GPS SIS).
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The satellites are designed to provide reliable service over a 7.5- to 11-year design life, dependingon the production block, through a combination of space qualified parts, multiple redundancies forcritical subsystems, and internal diagnostic logic. The satellites require minimal interaction with theground and allow all but a few maintenance activities to be conducted without interruption to thebroadcast SIS. Periodic uploads of NAV message data are designed to cause no interruption to
the SIS, although Block IIA satellites may experience a 6- to 24-second interruption during the
upload.

1.6.2 GPS Control Segment
The Operational Control System (OCS) is comprised of four major subsystems: a Master Control
Station (MCS, soon to be replaced by a New Master Control Station [NMCS]), a Backup Master
Control Station (BMCS, soon to be replaced by 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 OCS is provided in Figure 1.6-2.
Figure 1.6-2. The GPS Operational Control System (OCS)
TT&C
Link

GPS
SIS

GROUND ANTENNASMONITOR STATIONSMASTER CONTROL STATION• Commands• Telemetry• Navigation Uploads• Ground Antenna Control• Ground Antenna Status• Data Traffic Management• Navigation Data
• Ranging Measurement Data
• Monitor Station Control• Monitor Station Status

Performance Assess Function

Satellite Control Function

Positioning & Timing Function• Check performance against
specifications• Alarm if failure detected• Command Generation• Telemetry Processing• Upload Generation & Control• Ranging Measurement Processing• Satellite State Estimation/Prediction• Navigation Data Generation

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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 weekthroughout the year. The MCS is responsible for all aspects of constellation command and control,
to include:
•
•
•
•
•

Routine satellite bus and payload status monitoring
Satellite maintenance and anomaly resolution
Management of GPS SIS performance in support of all performance standards (SPS PS
and PPS PS)
NAV message data upload operations as required to sustain performance in accordancewith accuracy and integrity performance standards
Detecting and responding to GPS SIS failures

In the event of a prolonged MCS outage, GPS operations can be moved to the BMCS (or from the
NMCS to the AMCS).
The OCS's four ground antennas (GAs) provide a near real-time TT&C interface between thesatellites and the MCS. The monitor stations (MSs) provide near real-time satellite pseudorange
measurement data and recovered NAV message data to the MCS and support continuous
monitoring of constellation performance. The current OCS monitor stations provide 100% global
coverage with the inclusion of National Geospatial-Intelligence Agency (NGA) stations

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1.7 Wide Area Augmentation System (WAAS) Overview
The primary mission of WAAS is to augment GPS SPS for air navigation. The WAAS signal
provides the integrity, accuracy, availability, and continuity required for instrument approach
operations in terminal, en route (ER), and final approach phases of flight in all signal coverage
areas defined in Section 3. In the primary area of coverage, the WAAS SIS also supports verticallyguided instrument approach operations called Localizer Performance with Vertical (LPV) guidanceand enables an aircraft to descend as low as 200 foot height above touchdown (HAT).
WAAS provides an en route navigation and approach landing capability (lateral and verticalguidance) that; (a) improves safety by reducing controlled flight into terrain on approach, (b)increases the number of candidate runway ends that can have vertically guided approachprocedures, and (c) supports area navigation (RNAV) en route for aircraft that are not equippedwith inertial navigation and/or flight management systems. WAAS provides the additional accuracy,
availability, continuity and integrity necessary to enable users to rely on GPS for all phases of flight,
from en route through approaches with vertical guidance, at all qualified airports within the WAAS
LPV coverage area. This WAAS service enables development of more standardized approachprocedures, missed approach procedures, and departure guidance for numerous runway ends andheliport/helipads in the National Airspace System (NAS).
Section 1.7.1 describes both GPS-only and GPS WAAS operations. GPS-only operations dependon the receiver-based integrity monitoring technique called receiver autonomous integrity
monitoring (RAIM). GPS WAAS operations add an additional WAAS SIS broadcast from ageostationary earth orbit (GEO) satellite to improve the accuracy of the GPS SPS SIS and toprovide integrity by alerting when not to use satellites as part of the WAAS solution. The GEO
broadcast may also be used as an additional ranging signal to improve the availability of theaugmented GPS service. Section 1.7.2 describes the WAAS approach for ensuring integrity
performance. Section 1.7.3 describes the WAAS architecture in detail including its operational
environment. Further details about how the WAAS user receiver interprets the protection levels
that provide integrity are described in Appendix A. The WAAS SIS interface is described in Section
2, and performance standards in Section 3.
1.7.1 GPS WAAS Air Navigation User Segment
GPS WAAS is a combination of ground-based and space-based equipment that augments the
GPS SPS. A network of ground monitoring stations with precisely surveyed GPS antennas is
strategically positioned to collect GPS satellite data across the NAS including sites in Alaska,
Hawaii, and Puerto Rico. Additional stations are located in Canada and Mexico to further improveavailability in the NAS and extend coverage to most of Canada and Mexico. Using sitemeasurements of the GPS SIS, GPS WAAS calculates errors in the GPS signal and then provides
users with correction data to compensate for these GPS SIS errors as well as integrity information
associated with those corrections. WAAS GEO satellites, used to relay the correction and integrity
information to users, also provide an additional GPS-like ranging capability that further improves
the availability of GPS positioning. Hence, WAAS provides for increased availability and accuracy
in position reporting, enabling more uniform and high-quality NAS-wide air traffic management.
The end result is a GPS WAAS service that supports navigation from the en route phase of flight toapproach with ceiling and visibility minimums equivalent to Category (CAT) I.
GPS WAAS augments the GPS SPS SIS with integrity and corrections to make the GPS SPS atrusted air navigational aid. Figure 1.7-1 depicts aircraft with a GPS-only operation with an FAA
Technical Standard Order (TSO) C129 compliant receiver and an aircraft with TSO-C145/146
compliant WAAS receiver.
The GPS-only receiver compliant with TSO-C129 uses RAIM to assure the integrity of its solution.
A GPS receiver solves for position and time using signals from a minimum of four GPS satellites.

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RAIM requires a fifth satellite, or barometric aiding, to perform a consistency check to detect a fault
on a single satellite.
The GPS WAAS receiver compliant with TSO-C145/146 also uses RAIM for instances when theaugmentation signal becomes unavailable. The WAAS receiver adds a fault detection & exclusion(FDE) feature requiring a minimum of 6 satellites to detect and exclude a faulted satellite. Insteadof declaring GPS SPS service unusable with a RAIM alert, RAIM/FDE excludes the bad satellite
and continues to provide an integrity-assured solution provided the geometry of the remaining
satellites in view is sufficient. The WAAS GEO broadcast also provides an additional ranging
source for improved availability of navigation services. When a WAAS receiver is using thecorrections and integrity messages broadcast by the GEO, only four GPS or GEO satellites areneeded, which increases the availability of service versus RAIM or RAIM/FDE.

GEO Satellite

GPS Satellite
User with
GPS/WAAS
ReceiverGPS-only
Receiver(TSO -C145/146)(TSO -C129)

WAAS

RunwayUser’s calculated
protection levelsignificantly reducedwith integrity.

User’s calculated
protection level
Is very large
Ground Level

Figure 1.7-1 WAAS Augmented and GPS-Only Operations

1.7.2 GPS WAAS Corrections and Integrity
The WAAS receiver uses the WAAS broadcast-corrections in conjunction with GPS SPS signals to
calculate an accurate position with high-confidence error bounds on this position. Correction data
is used to differentially correct errors in the broadcast GPS range measurements. Integrity data isused to calculate integrity bounds called protection levels. Depending on the flight operation, theuser equipment may either simply compute a Horizontal Protection Level (HPL) or both a HPL anda Vertical Protection Level (VPL). The user receiver compares the computed protection levels withthe alert limit thresholds established for the selected phase of flight. If one of the protection levels
exceeds the corresponding alert limit, the receiver provides an annunciation to the pilot. The
overall system design ensures that the user receiver does not exceed a time-to-alert of 6.2
seconds.
WAAS estimates GPS satellite clock and ephemeris errors and ionosphere delays for satellites andionospheric grid points that are adequately viewed by WAAS wide-area reference stations,calculates corrections for those errors, and broadcasts the corrections through the GEO satellites.
Three types of corrections are broadcast: 1) Fast Corrections (FCs) for each GPS satellite clock’s
rapid, short-term errors; calculated and broadcast every 6 seconds for each satellite, 2) Long Term
Corrections (LTCs) for each GPS satellite clock’s slow drift errors and slow ephemeris errors;
calculated every 256 seconds and broadcast at least every 120 seconds, and 3) Ionospheric Grid
Point (IGP) Corrections (ICs) for the estimated ionosphere signal propagation delays; calculated forevery 5 degrees latitude and longitude grid point over the LPV coverage area and broadcast every
5 minutes.

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WAAS calculates integrity data associated with its generated corrections at the required level ofintegrity for the intended flight operation. Integrity data is provided in the form of error bounds
which are used to compute the protection levels taking all relevant error sources into account. The
integrity data consists of the User Differential Range Error (UDRE) and the Grid Ionospheric
Vertical Error (GIVE). UDRE characterizes the residual error in the FC and LTC. The UDRE istransmitted with the FC message. GIVE characterizes the residual error in the IC for the estimatedionosphere signal delays calculated for IGPs, defined in the IGP mask and broadcast every 5
minutes. The GIVE is transmitted with the IC message.
If using WAAS vertical guidance, the user receiver utilizes this integrity data to calculate a“protection cylinder” as defined in the RTCA Inc. (formerly Radio Technical Commission for
Aeronautics) Minimum Operational Performance Standard (MOPS) (RTCA/DO-229), incorporated
by reference, in TSO-C145/146. A simplified depiction is shown in Figure 1.7-2. The user receiverapplies the various WAAS corrections described above and calculates a user position using the
WAAS corrected range and ephemeris data. Then, the user receiver applies the UDRE, GIVE (andother error characteristics for residual troposphere delay and receiver errors) values to calculate
VPL and HPL. These protection limits can be envisioned as defining a protection cylinder, depicted
as the dark cylinder in Figure 1.7-2.

Vertical and Horizontal Alert Limit cylinder are defined by the phase of flight
The aircraft’s calculated
position based on the
WAAS corrections is
the center of thecylinder.
The aircraft’strue position

VAL

VPL
HPL
HAL

Protection level cylinder:
VPL and HPL arecomputed by the user’s
receiver from error
bounds calculated by
WAAS (UDRE and
GIVE).

Figure 1.7-2 WAAS Integrity Protection CylinderThis protection cylinder, centered on the user’s calculated position of the aircraft, represents theuncertainty of that calculated position. The user’s receiver calculates and compares the VPL and
HPL against the Vertical Alert Limit (VAL) and Horizontal alert Limit (HAL) values, respectively.
These alert limits have values that are fixed for each flight operation and can be thought of as
defining the clear alert limit cylinder in Figure 1.7-2. The alert limit cylinder is centered on theaircraft’s WAAS calculated position, just as the protection level cylinder. Normally, the aircraft’strue position is within the dark protection level cylinder.
If the WAAS integrity data results in a protection level cylinder that is too large to be contained
within the alert limit cylinder for a particular flight operation, the user receiver will indicate to thepilot that the operation is not available. For example, if the alert limit for LPV is exceeded, thereceiver will notify the pilot that LPV is no longer available. It is likely in this scenario that theLateral Navigation (LNAV) alert limit, which is much larger, will not be exceeded and therefore thereceiver indication of LNAV operation would still be available.
Hazardously Misleading Information (HMI) exists when the user’s position error exceeds theprotection levels for a period longer than the time-to-alert (TTA). If the protection cylinder is too
small, it will not enclose the aircraft’s true position. This is an underbound condition. When theunderbound condition persists for longer than the TTA, this event is then called HMI. An
underbound condition cannot be detected by the receiver so the WAAS integrity algorithms are
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designed to ensure that UDRE and GIVE will not cause an underbound condition under any
conditions of flight operation.
1.7.3 WAAS Architecture and Operational Environment
The GPS WAAS architecture and operational environment are shown in Figure 1.7-3. The spacesegment consists of the GPS and GEO satellites. The GPS constellation nominally consists of 24
satellites located in orbital slots distributed in 6 different orbital planes (an average of approximately
8-10 satellites are in view to a user at any time). The GPS SPS PS contains a complete
description of the GPS constellation. Each GPS satellite transmits signals on two frequencies thatare used by WAAS. The Link 1 (GPS L1) 1575.42 MHz Coarse Acquisition (C/A) coded signal
contains the orbital and timing parameters that a receiver requires to calculate the user’s position.
The L2 signal is the 1227.60 MHz P(Y) coded signal. Currently, the WAAS user receiver receives
only the L1 signal.

Figure 1.7-3 WAAS Architecture and Operational Environment
The WAAS is used to produce a Satellite Based Augmentation System (SBAS) signal-in-space(SIS) that contains GPS corrections and integrity data messages. The WAAS includes Wide-area
Reference Stations (WRSs), Terrestrial Communication Network (TCN), Wide-area Master Stations(WMSs), Ground Earth Stations (GESs), and GEO satellites. WAAS is controlled from two

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operations & maintenance (O&M) workstations located at the National Operations and Control
Center (NOCC) and Pacific Operations Control Center (POCC).
Figure 1.7-4 shows the WAAS data processing path. WAAS receives the GPS satellite signals andprocesses the GPS satellite data to determine the corrections and integrity data for each satellite.
The resultant correction and integrity data for each GPS satellite is referred to as the WAAS
message. The WAAS message is uplinked to the GEO satellites for broadcast on the GPS L1
frequency. The user receives the signal with a unique GEO satellite pseudo-random noise (PRN)
number coding on the SIS. The user receiver extracts the various corrections and integrity data for
the GPS and GEO satellites to calculate an accurate position and associated protection level(s).
WAAS also controls the timing of the GEO satellite signal and generates the GEO satellite
navigation message data (GEO satellite position and clock) that enables use of the GEO satellites
as an additional GPS-like ranging source.
GEO Satellite

GPS Satellites
GPS L1 Signal

WAAS SIS (GEO L1 Signal)

WAAS SIS(GEO L1 Signal)

GPS L1 & L2 Signals

U
-

U
-

Subsystem Monitoring
and Recording
O&M

WRE C
WRE B
WRE A

U
-

WAAS
Receiver

Data Collection
Processor

WRS
Data
Collection

C 5up

WAAS SIS (GEO L1 Signal)

GPS L1 Signal

TCN

CalculateCorrections
Processor #2
Corrections
Processor #1

O&M Subsystem

Antenna andGUS ReceiverT
C
N

Calculate
Integrity Data andMonitor Integrity
Data

GUS
Processor

WAAS
Messages

RFU and Signal
Generator

Safety Computer
GUS Subsystem

WMS
TCN

L5

Build WAAS
Message

Safety Computer
C&V Subsystem

Data
Processing

C 1up

GES
WAAS Message
Transmission

Data Communication

Figure 1.7-4 WAAS Data Processing Path
The WAAS consists of the following configuration items.
Wide-area Reference Stations (WRS): There are 38 WRSs located through the Contiguous UnitedStates (CONUS), Hawaii, Puerto Rico, Alaska, Canada, and Mexico. Each WRS contains threesets of Wide-area Reference Equipment (WRE). Each WRE consists of a dual frequency (L1 & L2)
WAAS Receiver, cesium clock, and a Data Collection Processor (DCP). L2 signal processing isaccomplished using a semi-codeless tracking technique. The DCP collects, packages, andforwards the satellite data to the WMS for processing. Each WRE, referred to as WRE-A, -B, and C, independently collects satellite data. On September 23, 2008, the DoD announced its intent tono longer support semi-codeless or codeless receiver capability after December 31, 2020. The
FAA plans to upgrade the WRSs with receivers capable of processing the new civil signals. A
substantial effort will be required to transition the WAAS software and algorithms. After thistransition, WAAS will no longer be able to monitor satellites that do not transmit L5.
Terrestrial Communications Network (TCN): The TCN provides the communications link between
the WAAS subsystems for transmitting WAAS data. The TCN consists of two dedicated,
redundant, and diverse networks to provide highly reliable communications.
Wide-area Master Station (WMS): There are three WMSs and any one can perform the WAAS
corrections & integrity processing function. In each WMS, there is one corrections & verification(C&V) subsystem. The C&V receives the satellite data from each WRE. The C&V consists of two
Corrections Processors (CP1 and CP2) and one Safety Computer (SC) that contains two Safety
Processors (SPs) and a hardware comparator. Each CP calculates clock and ephemeris
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corrections from WRE data. The SC calculates ionospheric corrections and high-confidence error
bounds for clock, ephemeris, and ionospheric corrections and ensures that the outputs from the CP
have not been corrupted. The data is then forwarded to the Ground Earth Station (GES) via theTCN.
WMS Operations and Maintenance (O&M): Each of the two O&M subsystems located at the OCCs
(NOCC and POCC) provide subsystem monitoring and recording. Each O&M workstation monitors
the status of the other WAAS subsystems and allows the single controlling operator to monitor thestate of the system and recording. WAAS is a fully automated system that requires minimal
operator involvement.
Ground Earth Station (GES): Each GES is a location where the function of transmitting the WAAS
message to the GEO is performed. At the physical GES location, there may be one or more colocated GEO Uplink Subsystems (GUSs). Each GEO is served by two GUSs located at different
GESs in order to retain geographic diversity for the uplink sources. Each GUS has a Signal
Generation Subsystem (SGS) and an RF Uplink (RFU). The SGS contains a GUS Processor (GP)and a WAAS Message Processor (WMP). The GP provides network and RS-232 communications
interfaces for the WMP and the WMP formulates the WAAS user message for transmission to the
GEO. The RFU uplinks the WAAS signal to the GEO satellite.
Geostationary Earth Orbit (GEO) satellites. WAAS currently relies on the service of two leased
GEOs positioned at 107W and 133W longitude. A third GEO is planned to ensure dual coverage
requirements are met. The WAAS objective is for a user receiver to have a minimum of two GEOs
in view during LPV operations. Each GEO receives 2 uplinked C-Band signals (C1up, C5up ) fromone of two RFUs (primary & backup) and uses bent-pipe transponders that frequency translateeach signal to broadcast WAAS signals on L1 and L5. Each GEO satellite transmits the WAAS
message in an L1 signal (1575.42 MHz) coded in a similar manner as a GPS satellite C/A codetransmission. The L5 signal is currently used internally to determine ionosphere line of sight delays
at the GUS, which are needed to properly control the timing of the L1 SIS. Currently, both L1 andL5 broadcast the L1 integrity and correction messages. In the future, L5 will broadcast correction
and integrity messages specifically designed for dual frequency (L1 & L5) users.

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SECTION 2.0 WAAS SIS Characteristics
This section provides an overview of the WAAS SIS interface characteristics, WAAS SIS
performance characteristics, and the assumptions made to arrive at the performance standards in
Section 3.0.

2.1 WAAS SIS Interface Requirements
The WAAS SIS complies with the technical requirements related to the interface between theSpace Segment and the WAAS user receivers as established by the current versions of the WAAS
Specification, FAA-E-2892 as well as RTCA DO-229 and associated TSOs (TSO-C145, TSOC146). In the event of conflict between the WAAS SIS interface characteristics described in thisdocument and the TSOs, the TSOs take precedence.

2.2 Overview of WAAS SIS Interface Characteristics
This section provides an overview of the WAAS SIS interface characteristics. WAAS SIS interface
characteristics are allocated to two categories: (1) carrier and modulation radio frequency (RF)characteristics, and (2) the structure, protocols, and contents of the WAAS user message.
2.2.1 WAAS SIS RF Characteristics
The GEO satellites transmit right-hand circularly polarized (RHCP) L-band signals. At present, only
the WAAS L1 signal is intended for airborne equipment use. The received L1 power level out of a
3 dBil linearly polarized user antenna at worst normal orientation is greater than -128.5 dBm (158.5 dBW) at elevation angles greater than 5 degrees within the intended service volume. The
maximum received power level is -120 dBm (-150 dBW) for such an antenna. For future WAAS
GEOs the maximum received power will be -122.5 dBm (-152.5 dBW). WAAS SIS RF
characteristics are further defined in the FAA-TSO-145/146.
2.2.2 WAAS Message Characteristics
WAAS transmits a number of different message types. The Table below summarizes the message
types and their purpose. For format and information content of the message types and informationon the use of the WAAS messages, refer to the FAA-TSO-145/146.
Message
Type
0

Title

Table 2.1-1 WAAS Message Types
PurposeDon’t Use for Safety Applications

1
2-5
6

PRN Mask Assignments
Fast Clock Corrections
Integrity Information

7

Fast Correction Degradation

9
17

WAAS Satellite Navigation Message
WAAS Satellite Almanac

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Discard data from the transmitting PRN signal.
Used for testing or if problem in data is
discovered.
PRNs for GPS and GEOs
Fast corrections & UDREIUDREI for Fast Corrections and Long Termcorrections for all satellites in one message
Clock error bound acceleration and Timeout
interval for Fast Corrections
WAAS geostationary satellite orbit information
Almanac data for WAAS satellites

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18
24

Ionosphere grid point (IGP) mask
Fast & Long Term Clock Corrections

25
26

Long Term Clock Corrections
Ionospheric vertical delay and GIVEI
for each IGP in message 18
WAAS Service Message
Clock-Ephemeris Covariance Matrix

27
28

GPS WAAS PS

Mask for IGP corrections
Mixed message consisting of fast & long term
corrections
Correction to the broadcast ephemeris
Ionospheric corrections and error boundindicators
Differential UDRE factors for user locations
UDRE multiplier

2.3 Overview of WAAS SIS Performance Characteristics
The WAAS SIS performance characteristics are described below for accuracy, integrity, continuity,and availability.
This overview of the WAAS SIS performance characteristics addresses WAAS service
performance. The service relies on GPS SPS service and the WAAS GEO broadcast SIS. The
five performance attributes (metrics) are used to characterize performance for all air navigation
services, though their application to WAAS service is defined by parameters that are unique to the
WAAS service in this PS. The overview addresses each of these performance characteristics andexplains those parameters in both their fundamental physical definition and their effect on flight
operations.
This section states the system level performance requirements for GPS WAAS. Requirements aredefined in terms of horizontal and vertical 95% accuracy, integrity risk (VAL, HAL and TTA),
continuity risk, and availability. Integrity is defined in terms of probability of HMI and associatedtimely notification.

2.3.1 WAAS SIS Accuracyth
WAAS SIS accuracy is defined for both vertical and horizontal accuracy as the 95 percentile(95%) of the error in the vertical domain or in the horizontal plane. This assumes the proper
application of the WAAS Fast Corrections, Long Term Corrections, and Ionospheric Corrections to
the GPS SPS SIS to obtain a position solution. Horizontal accuracy is used to ensure an aircraft’s
horizontal approach position is aligned with the runway/airway centerline, the vertical accuracy isused to ensure an aircraft’s vertical approach position is aligned with the approach glidepath, andalong-track position is needed to ensure that an aircraft turns in the correct location. Without
WAAS corrections, the GPS SPS by itself is not capable of providing vertical accuracy with integritythat can support vertically guided approach operations defined in Section 3.2 of the PS. With
WAAS provided corrections, the horizontal and vertical accuracy meets or exceeds therequirements for vertically guided landing operations as low as 200 HAT for LPV procedures.

2.3.2 WAAS SIS Integrity
Integrity is synonymous with trust. For the Global Navigation Satellite System (GNSS), theInternational Civil Aviation Organization (ICAO) defines integrity as “A measure of the trust that canbe placed in the correctness of the information supplied by the total system. WAAS integrity
includes the ability of a system to provide timely and valid warnings to the user (alerts).”
Probability of HMI: HMI exists when the position error exceeds the protection level for a longer
period than the time-to-alert without an annunciation to the pilot. To support safety-of-life flight
operations, the probability of HMI must be exceedingly small. Certification regulations define
several levels of hazard with an associated acceptable level of risk. Some navigation services
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have a hazardous consequence on flight operations in the presence of an integrity failure. Thislevel of consequence is associated with an inability to remain within obstacle clearance surfaces onapproach, missed approach, and en-route operations. The likelihood that a hazardous condition is
encountered during any single flight operation must be extremely remote to comply withcertification regulations. Extremely remote is interpreted to be less than 1 × 10-7 per hour of enroute flight or per approach (150 seconds) during instrument approach operations.
Alert Limits: For an instrument approach operation, the alert limit provides a threshold that will not
be exceeded without issuing an alert. The HAL is used to determine whether indicated horizontal
position places the pilot in a horizontal position adequate to successfully execute the landing
phase. A 40 meter HAL was determined to meet the localizer performance characteristics requiredfor LPV approach operations. The vertical alert limit is used to determine whether an aircraft typecan avoid penetrating the obstacle clearance surface during vertically guided approaches. The 35
meter VAL was evaluated extensively for all types of aircraft to determine whether the pilot could
react adequately to avoid penetrating the Obstacle Clearance Surfaces (OCS) defined by Terminaland En Route Instrument Procedures (TERPS) for LPV approach and missed approach operations.
For LPV approach operations, it was determined that this vertical alert limit, in combination with thesmall WAAS vertical position error distribution, supported a vertically guided approach down to 200’
HAT and missed approach for all categories of aircraft.
Time-to-alert (TTA): TTA is defined by ICAO as “The maximum allowable time elapsed from theonset of the navigation system being out of tolerance until the equipment enunciates the alert.” In
the case of WAAS, this requirement is satisfied by alerting that a particular satellite is not to be
used in the WAAS solution by a “not monitored” indication or not to be used in any navigation
solution by a “do not use” indication. Lateral navigation only (LNAV) approach operations set thisvalue at 10 seconds and Localizer performance with vertical (LPV) approach operations down to
200 HAT requires 6 seconds. The WAAS ground and space segments meet a 6.2 second alert
time and the receiver is allocated 0.8 seconds in LNAV/VNAV, LPV and LP modes to receive,
process, and display an alert. For En route through LNAV operations, the TTA allocation to thereceiver is 2 seconds.
The potential hazard caused by this TTA does not constitute increased safety risk for approach(LPV) operations. Early tube-type ILS systems could lose a channel inducing a “hard over”
condition in an approaching aircraft. This is the worst control condition an aircraft can encounter
during the most critical phase of flight. These early systems with unreliable monitors had difficultyin supporting any better than a 6 second TTA, so the pilot had to fight the hard over condition until
either the ILS system shut down the signal or the pilot disengaged the autopilot. Later, modem
technology ILSs reduced this time to, nominally, 2 seconds, but it took modern digital ILS design toeliminate the susceptibility to an equipment failure that could cause a hard over condition. WAAS
HMI would not cause a hard over condition because the receiver has a 100 second smoothing
delay. Instead, it manifests itself as a bias error. The 6.2 seconds limits the amount of exposure
time to a potential bias error.
Issuance of the WAAS alert is typically better than 6.2 seconds for flight operations. In the case ofa failed satellite, it is removed from the solution with the alert indication and all further guidancemeets the integrity assurances. During LPV operations, if the HPL exceeds the HAL while theaircraft is on the final approach segment, a flag is displayed and the receiver will discontinue
guidance or revert to LNAV-only guidance.
2.3.3 WAAS SIS Continuity
WAAS continuity is defined as the probability that the WAAS SIS performance level will continue tobe available throughout one flight hour for en route through LNAV operations or 15 seconds for
LNAV/VNAV, LPV, and LP operations given that the service was available at the beginning of thespecific exposure time, unless the loss of service was the subject of a prior notice to airmen(NOTAM). For en-route operations, this is defined as a probability of loss of service per hour. For
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approach operations, this is defined as a probability of loss of service over any 15 second period.
WAAS continuity is directly dependent on GPS SPS SIS continuity because both are required inorder to provide the service. For LPV operations, additional external effects, such as ionosphericstorms, can interrupt the service.

2.3.4 WAAS SIS Availability
WAAS availability is the probability that the WAAS SIS performance level is available to support theintended flight operation. This produces different levels of availability for en-route operations andLPV operations because the signal performance level is more stringent for LPV operations.
Although the WAAS availability is directly dependent on GPS SPS SIS availability, the probability
that WAAS can support an en-route flight operation is higher than the probability that GPS alone
can support that operation because the WAAS availability calculation adds the GEO ranging
signals and corrections, which enable the aircraft to tolerate fewer numbers of healthy GPS
satellites and still meet the minimum required level of performance.

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SECTION 3.0 WAAS SIS Performance Standards
3.1 WAAS SIS Coverage
WAAS coverage characterizes the airspace where the WAAS service is available. The WAAS
Program defines five areas of coverage for CONUS, Alaska, Hawaii, Puerto Rico, and thegeographic area over all U.S. territory. The service level expectations for availability and continuity
are different for these coverage areas primarily because the ability to site reference stations toadequately monitor the ionosphere does not exist for Hawaii and Puerto Rico. Alaska is affectedboth by being in the Northern latitudes at the edge of GEO coverage and by the ability to locate
enough reference stations. For clarity, the descriptions and maps of these coverage areas areincorporated below:
Zone 1 - Zone 1 is defined as the region from the surface up to 100,000 feet above the surface ofthe 48 contiguous states, extended to 30 nautical miles (nm) outside of its borders.

Figure 3.1-1 WAAS Zone 1 Coverage Area

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Zone 2 – Zone 2 is defined as the region from the surface up to 100,000 feet above the surface of
Alaska, including 30 nm outside its borders.

Figure 3.1-2 WAAS Zone 2 Coverage Area
Zone 3 – Zone 3 is defined as the region from the surface up to 100,000 feet above the surface of
Hawaii, including 30 nm outside its borders.

Figure 3.1-3 WAAS Zone 3 Coverage Area

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Zone 4 – Zone 4 is defined as the region from the surface up to 100,000 feet above the surface of
the Caribbean islands shown in Figure 2.3-4.

Figure 3.1-4 WAAS Zone 4 Coverage Area
Zone 5 – Zone 5 is defined as the region from the surface up to 100,000 feet above Mean Sea
Level (MSL) of the region defined by the following points (see table below) , excluding the volume
covered by zones 1-4.
Lat.
Lon

50N
61W

50N
122W

70N
140W

70N
165W

68N
169W

20N
164W

17N
160W

17N
155W

30N
120W

16N
75W

16N
61W

50N
61W

Figure 3.1-5 WAAS Zone 5 Coverage Area

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Subsequently, the service areas were expanded in some areas when the number of WRSs wasincreased from 25 to 38 WRS through the addition of four WRSs in Alaska (in addition to theoriginal three), four in Canada, and five in Mexico.

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3.2 WAAS Signal-In-Space Performance Requirements
The WAAS SIS performance requirements are specified in this section. Performance is predicatedon a user receiver compliant with TSOs 144 antenna and 145/146 user equipment based on GPS
providing the SPS in full compliance with the SPS PS. These performance requirements are
subdivided into En-route, Terminal, LNAV, LNAV/VNAV, LPV, and LPV-200 flight operations. The
requirements are based on the navigation performance required to support the flight operation andare presented in Table 3.2-1
Table 3.2-1 WAAS Navigation Performance Requirements
En Route

Terminal

LNAV

TTA
HAL
VAL
Probability of HMI

15 s
2 nm
N/A
10-7
per hour

15 s
1 nm
N/A
-7
10
per hour

10 s
556 m
N/A
-7
10
per hourZone 1 Continuity

1-10-5per hour
0.4 nm1-10
per hour
0.4 nm1-10
per hour
220 m

N/A

N/A

0.99999(100%)
0.999(100%)
0.999(100%)
0.999(100%)
0.99999(100%)

0.99999(100%).999(100%).999(100%).999(100%).999(100%)

Horizontal Accuracy(95%)
Vertical Accuracy(95%)
Availability(Zone 1 Coverage)
Availability(Zone 2 Coverage)
Availability(Zone 3 Coverage)
Availability(Zone 4 Coverage)
Availability(Zone 5 Coverage)

-5

LPV

LPV 200

1–5.5 x 10
/15 seconds
220 m

6.2 s
40 m
50 m
-7
2 x 10per approach(150 seconds)-6
1–8 x 10/15 seconds
16 m

6.2 s
40 m
35 m
-7
2 x 10per approach(150 seconds)-6
1–8 x 10/15 seconds
16 m

N/A

20 m

20 m

4m

0.99999(100%).999(100%).999(100%).999(100%).999(100%)

0.99(100%)
.95
(75%)
N/A

0.99
(80-100%)
0.95
(75%)
N/A

0.99
(40-60%)
N/A

N/A

N/A

N/A

N/A

N/A

N/A

-5

LNAV
/VNAV
10 s
556 m
50 m
-7
2 x 10per approach-5

N/A

1. Real-time and statistical performance results for WAAS and GPS are available at the FAA
Technical Center WAAS Test Bed web site at http://www.nstb.tc.faa.gov. For real-time
performance maps, select the type of service (LNAV/VNAV, LPV, or LPV 200) andcoverage area (CONUS, Alaska, etc.). For WAAS statistical performance, select the
WAAS Performance Analysis Reports. Note that GPS SPS Performance Analysis Reports
are also available.
2. The continuity exposure time for LNAV is one flight hour rather than the duration of one
approach in order to assure a high probability that an aircraft is able to land after divertingfrom the destination to the alternate airport (in case of unpredicted loss of approachcapability, after departure, at the destination airport) and conduct an instrument approachat the alternate airport.
3. WAAS continuity is the probability that service is supported throughout an hour in en-route,terminal, and LNAV operations, and throughout 15 seconds for vertically guided approach
operations, given that service was supported at the beginning of the interval and predicted
to be supported throughout the interval (i.e., a prior NOTAM was not issued 48 hours inadvance stating that service would not be supported).

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3.3 WAAS Signal-In-Space Performance
WAAS SIS performance meets or exceeds the aviation requirements. The actual WAAS
performance has been measured and analyzed by the FAA Technical Center throughout the period
it has been in service.
The WAAS delivers the level of service based on the most demanding flight operation, LPV-200.
This level of service is available throughout the coverage area, as indicated on the web site. In
areas where that level of service is not available, the WAAS signal indicates the reduced level ofservice that is available (LPV or LNAV/VNAV). The level of service available is displayed on thereceiver. The WAAS design delivers an integrity alert within a TTA of 6.2 seconds. Otherwise, thereceiver RAIM/FDE alerts within 8 seconds. The probability of HMI analysis for the WAAS design-7
is less than 1 × 10 per approach (150 seconds). Continuity is not tracked. Accuracy nominal isapproximately 1.6 m vertical or horizontal 95% of the time. The maximum observed error is lessthan 12 m when the VPL is less than 50 m.. Treating this as a 6σ limit, a conservative 95%
maximum error is the same as the vertical accuracy requirement of 4 m. Availability maps areillustrated real-time and historically on the FAA Technical Center web site at www.nstb.tc.faa.gov.
Table 3.3-1 presents WAAS SIS performance based on the system design, analysis, and actual
performance.
Table 3.3-1 WAAS SIS Performance
PerformanceComment
TTA
6.2 s
Same as requirement
TTA receiver RAIM/FDE
8s
TSO-C145/146
Probability of HMI<1 × 10-7 per approachLess than requirement(150 seconds)
Continuity
1 – 8 × 10-6/15 seconds
Same as requirement
Horizontal Accuracy nominal
1.6 m (95%)
Performance Analysis Report
Horizontal Accuracy maximum 12 m (maximum observed)
Horizontal Accuracy limit
4 m (conservative 95% limit)
Vertical Accuracy nominal
1.6 m (95%)
Performance Analysis Report
Vertical Accuracy maximum
12 m (maximum observed)
Vertical Accuracy limit
4 m (conservative 95% limit)
Availability
real-time availability maps
www.nstb.tc.faa.gov

3.4 WAAS Current Services and Availabilitya) WAAS provides a ranging function throughout the entire satellite footprint, as shown in Figure
1. GPS/SBAS avionics are designed to incorporate this ranging signal to improve RAIM/FDE
availability, even when no other SBAS service is available.
b) WAAS provides the GNSS satellite status and differential corrections function for all satellites
that are in view of the ground network, for any user within the footprint of one of thegeostationary satellites. GPS/SBAS avionics are designed to use this service for en route
through LNAV approach navigation when the resulting integrity bound is better than thatprovided by RAIM/FDE. This function must be provided on at least four satellites for thereceiver to use the differential correction function for integrity. When this function is available
for fewer than four satellites, the GPS/SBAS equipment can still use the corrections andintegrity information that is available to improve the performance of RAIM/FDE. At a given
location, the number of satellites which receive this service varies during a typical day assatellites orbit the Earth. This function is provided on at least four satellites throughout atypical day in the light blue area shown in Figure 3.4-1.

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It should be noted that it has been the FAA’s policy that WAAS must meet the integrity
requirements over the entire coverage area. This policy ensures that GPS/SBAS receivers canbe used throughout the WAAS satellite footprint with the assurance that the navigation solution
will have integrity whether based on WAAS augmentation or on RAIM/FDE.
c) WAAS provides the ionosphere corrections function within the dark blue region shown in
Figure 3.4-1. GPS/SBAS avionics use a standard ionospheric model outside this region, andno LPV capability is available.

3.4.1 Mapping Functions to Operational Capability
It is important to recall that the availability of service for a given flight operation is not determined by
the location of a user, inside or outside a particular pre-defined service volume, but by the user
equipment. Service availability is determined by the GNSS satellites the receiver is able to track,
the availability of WAAS corrections and magnitudes of the error bounds for these satellites, and,for vertically guided approach operations, the availability of ionosphere corrections and magnitudesof their error bounds.
Figures 3.4-2 and 3.4-3 show the operational capability of GPS/WAAS.
•

Figure 3.4-2 shows the availability of LNAV approach capability. Note that this capability
exists even outside the WAAS footprint, as the GPS/WAAS avionics automatically revert toGPS-only operation.

•

Figure 3.4-3 shows the availability of LPV approach capability.

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Figure 3.4-1 WAAS Ranging Function, Satellite Status and Differential Correction Function
Legend:• The ranging function is available throughout the footprints of the two WAAS Geostationary
satellites located at 133W and 107W.
• The satellite status and clock/ephemeris differential correction function is available on atleast four satellites throughout the entire day within the light blue region.
• All four services, including ionosphere correction function, are available for at least four
satellites during an entire typical day within the dark blue region.
• The figure shows average performance over 288 5-minute epochs (24 hours).
• The figure assumes a 24 satellite GPS constellation without failures and the two WAAS
GEOs.

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Figure 3.4-2 WAAS Service Area for LNAV
Legend:• The figure shows average LNAV availability performance over 288 5-minute epochs (24hours).
• The different colors correspond to different levels of LNAV service availability as shown by
the color bar on the right side of the figure.
• SBAS receivers will use either SBAS or Fault Detection and Exclusion to provide en route
through LNAV service; this explains that the figure shows service in areas where the
WAAS signal cannot be received (e.g., Greenland).
• The figure assumes a 24 satellite GPS constellation without failures and two WAAS GEOs
with ranging function.

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Figure 3.4-3 WAAS Service Area for Localizer Precision with Vertical Guidance (LPV)
Approach (HAL = 40 m; VAL = 50 m)
Legend:• The figure shows average LPV availability performance over 288 5-minute epochs (24hours).
• The different colors correspond to different levels of LPV service availability as shown by
the color bar on the right side of the figure• The figure assumes a 24 satellites GPS constellation without failure and two WAAS GEOs
with ranging function.

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SECTION 4.0 References
This section identifies the Government documents and non-Government documents explicitly
referenced in or related to the content of this WAAS PS.

4.1 Government Documents
SPECIFICATIONS:
Federal
23 February 2007

Global Positioning System Precise Positioning Service
Performance Standard, 1st Edition

30 September 2008

Global Positioning System Standard Positioning Service
Performance Standard, 4th Edition

13 August 2001

WAAS Specification FAA-E-2892b(C2)Military
None
STANDARDS:
FederalTSO-C129
10 December 1992

Technical Standard Order (TSO), Airborne Supplemental
Navigation Equipment Using the Global Positioning System (GPS)TSO-C129a
20 February 1996

Technical Standard Order (TSO), Airborne Supplemental
Navigation Equipment Using the Global Positioning System (GPS)TSO-C144a
30 March 2007

Technical Standard Order (TSO) Passive Airborne Global
Positioning Antenna

TSO-C145a
19 September 2002

Technical Standard Order (TSO), Airborne Navigation Sensors
Using the Global Positioning System (GPS) Augmented by
the Wide Area Augmentation System (WAAS)

TSO-C145b
14 February 2007

Technical Standard Order (TSO), Airborne Navigation Sensors
Using the Global Positioning System (GPS) Augmented by
the Satellite Based Augmentation System

TSO-C145c
2 May 2008

Technical Standard Order (TSO), Airborne Navigation Sensors
Using the Global Positioning System (GPS) Augmented by theSatellite Based Augmentation System

TSO-C146a
19 September 2002

Technical Standard Order (TSO), Stand-Alone Airborne
Navigation Equipment Using the Global Positioning System (GPS)Augmented by the Wide Area Augmentation System (WAAS)

TSO C146b

Technical Standard Order (TSO), Stand-Alone Airborne

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14 February 2007

Navigation Equipment Using the Global Positioning System (GPS)Augmented by the Satellite Based Augmentation System

TSO C146c
9 May 2008

Technical Standard Order (TSO), Stand-Alone Airborne
Navigation Equipment Using the Global Positioning System (GPS)Augmented by the Satellite Based Augmentation System

TSO C190
20 March 2007

Technical Standard Order (TSO) Active Airborne Global
Navigation Satellite System (GNSS) Antenna

Program
IS-GPS-200
Current Revision

Navstar GPS Space Segment / Navigation User
Interfaces

OTHER PUBLICATIONS:
Plans
DOT-VNTSC-RITA-05-12
5 January 2006

2005 Federal Radionavigation Plan (Additionally numbered as
DoD-4650.5)

DOT-VNTSC-RSPA-01-3.1
19 March 2002

2001 Federal Radionavigation Systems (Additionally numbered
as DOD-4650.5)

Miscellaneous
N 8110.60
2 February 1997

Federal Aviation Administration Notice
GPS as a Primary Means of Navigation in Remote/
Oceanic Operations

AC 20-138A
22 December 2003

Federal Aviation Administration Advisory Circular (AC)Airworthiness Approval of Global Satellite Navigation System(GNSS) Equipment

AC 90-96A
13 January 2005

Federal Aviation Administration Advisory Circular (AC)Approval of U.S. Operators and Aircraft to Operate Under
Instrument Flight Rules (IFR) in European Airspace Designatedfor Basic Area Navigation (BRNAV) and Precision Area Navigation(PRNAV)

AC 90-100A
1 March 2007

US. Terminal and En Route Area Navigation (RNAV) Operations

AC 90-101
15 December 2005

Approval Guidance for RNP Procedures with SAAAR

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4.2 Non-Government Documents
SPECIFICATIONS:
None

STANDARDS:
SARPs Annex 10
Amendment 83
20 July 2008

International Civil Aviation Organization (ICAO)
Annex 10 to the Convention on International Civil Aviation,
International Standards and Recommended Practices,
Aeronautical Telecommunications, Volume 1,
Radio Navigation Aids

RTCA/DO-229D
13 December 2006

RTCA Document, Special Committee 159
Minimum Operational Performance Standards for Global
Positioning System/Wide Area Augmentation System
Airborne Equipment

RTCA/DO-236B
28 October 2003

RTCA Document, Special Committee 181
Minimum Aviation System Performance Standards:
Required Navigation Performance for Area Navigation

RTCM Paper 136-2001/SC104-STD
August 2001

RTCM Document, Special Committee 104
RTCM Recommended Standards for Differential
Navstar GPS Service, Version 2.3

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GLOBAL POSITIONING SYSTEM
WIDE AREA AUGMENTATIONSYSTEM (WAAS)PERFORMANCE STANDARD
APPENDIX A
WAAS SERVICESPECIAL TOPICS

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TABLE OF CONTENTSSECTION A.1 Introduction............................................................................ A-1
SECTION A.2 Integrity Protection Levels.................................................... A-2
A.2.1 Background – Protection Levels ................................................................................. A-2
A.2.2 Actual Error – Compared to Protection Levels............................................................ A-2
A.2.3 Effect on Flight Operations.......................................................................................... A-5

SECTION A.3 Semi-Codeless GPS Receiver Transition ............................ A-6
A.3.1 Background – Semi-Codeless Receivers ................................................................... A-6
A.3.2 Semi-Codeless Receiver Requirements ..................................................................... A-7
A.3.3 Effect on WAAS Users ................................................................................................ A-7
A.3.4 Semi-Codeless Federal Register Notice..................................................................... A-8

List of Figures
Figure A.2-1. Vertical Triangle Chart for Kansas City................................................................ A-3
Figure A.2-2. Histogram of WAAS Vertical Navigation Signal Error (VNSE) ............................ A-3

List of Tables
Table A.2-1. Example Protection Levels from WAAS Performance Analysis Report................ A-2
Table A.2-2. Maximum Vertical Position Error........................................................................... A-4

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SECTION A.1 Introduction
This Appendix provides background information about topics that affect WAAS performancestandards or flight operations. It is intended to provide a repository of information that may be
relevant to current changes to improve WAAS operations or to expected improvements in the
WAAS performance standards.
The first topic in this appendix deals with the difference between the observed Navigation System
Error (NSE) and the WAAS Protection Level. The WAAS protects the user against navigation
errors by broadcasting a protection level that is greater than the expected error with a probability
that meets the integrity requirement.
The second topic in this appendix deals with the effect of the DoD plans to no longer support usersof codeless or semi-codeless receivers starting December 31, 2020. WAAS uses semi-codelessreceivers to measure the ionosphere induced propagation delay. WAAS plans to transition toprocessing of the ‘new’ L5 civil signal in order to avoid a disruption in service upon loss of the L2
P(Y) capability.

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SECTION A.2 Integrity Protection Levels
This section discusses the WAAS integrity protection levels. Historical performance for thehorizontal and vertical protection levels derived from the WAAS service are examined. Their effecton the availability of service is examined. Actual statistical error is compared to the protectionlevels. The effect on flight operations is examined.

A.2.1 Background – Protection Levels
WAAS provides integrity by transmitting the information necessary for a user to correct the GPS
signal and to calculate an integrity protection level in the horizontal and vertical plane. This iscompared against the HALs and VALs required to an operation to determine whether this operation
is available to the pilot.
Statistical performance of the WAAS service in providing VPL and HPLs is available in the WAAS
Performance Analysis Reports posted on the FAA Technical Center WAAS Test Bed web site at
http://www.nstb.tc.faa.gov/. The Report lists the 95% average VPL and HPL values for eachreference site in the WAAS reference network. An example of these values is included in below inTable B.2-1.
Table A.2-1 Example Protection Levels from WAAS Performance Analysis ReportParameter
CONUS
AlaskaMaximum
Maximum
HPL 95%
BostonBarrow
28.615 meters49.90 meters
VPL 95%
BostonBarrow
46.743 meters
93.816 meters

A.2.2 Actual Error – Compared to Protection Levels
This does not reflect, however, what the actual necessary protection level should have been. The
WAAS calculated protection level is more conservative than the observed error, but flight safety isevaluated based upon the presumption that the flight user could be exposed to an error of the
maximum protection level – that is equivalent to the alert limit. Unfortunately, although this is
accepted as a conservative assumption, the WAAS is not able to assure a protection level closer tothe actual error with the requisite integrity.
The relationship between position error, protection level and the alert limit designated for aparticular flight operation is depicted in the Figure A.2.2-1, Vertical Triangle Chart for Kansas City,
below. The VPL is on the Y-axis and the vertical position error along the X-axis. The diagonal lineis the limit where vertical position error would exceed the VPL, in which case HMI would bepresent. The horizontal lines designated at 12m, 20m, and 50m show the alert limits for GNSS
Landing System (GLS), APV-II, and APV-I (LNAV/VNAV) respectively. Colors are used to depict
the number of data samples at a particular point on the chart. If any position error points appear on
the right side of the diagonal line, the HMI exists and the cause for this must be investigated,possibly requiring a design change to the WAAS to ensure the broadcast messages bound theusers errors at all times. Position error points that appear above the alert limit lines (horizontal)
indicate that the intended flight operation is not available.

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Figure A.2-1 Vertical Triangle Chart for Kansas City

Actual error observed at the reference sites was examined by the FAA Technical Center. The
results were used to support the WAAS LPV-200 Safety Risk analysis. These results arepresented below in two forms. The first is Table A.2-2 which presents the maximum observed
vertical error when VPL was reported to be less than 50 m over the entire period of WAAS
performance at the time of the study. The second is Figure A.2-1 which presents a histogram of
the full data set from all sites. These show that when LPV was available, the maximum observed
error was less than 10.0 m based on observed system performance. This is substantially less than
the 16 – 28 m VPL calculated for these sites.

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Table A.2-2 Maximum Vertical Position Error

SITE
Washington, DC
Anderson, SC
Grand Forks, ND
Great Falls, MT
Oklahoma City, OK
Albuquerque, NM
Atlanta, GA
Billings, MT
Boston, MA
Chicago, IL
Cleveland, OH
Dallas, TX

Max. Vert. SITE
Error (m)7.47
Denver, CO
6.89
Houston, TX
11.60
Jacksonville, FL
7.32
Kansas City, MO
7.17
Los Angeles, CA
6.62
Memphis, TN
6.26
Miami, FL
6.95
Minneapolis, MN
9.49
New York, NY
6.46
Oakland, CA
6.95
Salt Lake City, UT
9.46
Seattle, WA

Max. Vert.
Error (m)
7.59
6.85
7.64
6.86
8.02
5.41
10.50
8.03
9.82
7.63
8.58
6.89

A histogram of the total data set is shown below. The figure shows data points larger than 12m
that were later determined to be invalid.

Figure A.2-2 Histogram of WAAS WRS Vertical Navigation Signal Error (VNSE)

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A.2.3 Effect on Flight Operations
The largest effect on flight operations is caused by the VPL. A VAL of 35 m is required to support
LPV-200. The availability of LPV-200 depends on the WAAS service at the user location obtaining
a VPL of 35 m or less. Current availability of this procedure is approximately 90% of CONUS, as
can be readily seen when consulting the FAA Technical Center web site above and pulling up thecurrent LPV 200 availability map.
Actual vertical error for WAAS, however, has not been observed to exceed 12 m in the history ofoperational service, as shown in Figure A.2-1. This means that if the predicted VPL based on
historical statistics for observed error would be 12 m – always less than 35 m – and LPV-200 would
always be available. The real-time VPL predicted by the WAAS safety processor algorithms,
however, is substantially larger than the actual vertical error measured at the WAAS Reference
Stations. The difference between this potential service and the actual service that is provided issubstantial.
Availability of LPV-200 is not the only substantial flight restriction resulting from the difficulty togenerate tighter bounds for vertical error. Autoland operations require the VPL to be 12 m or less.
Because of the 35 m VAL and the inability of the WAAS to guarantee actual vertical error
performance, auto-coupling is restricted to altitudes above 200 feet under the LPV-200 rules. This
restriction is applied even though actual statistics suggest that full autoland operations could besafely conducted, because past vertical error statistics are not a sufficient basis for approval.

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SECTION A.3 Semi-Codeless GPS Receiver Transition
This section discusses the transition of WAAS from semi-codeless receivers to modernized civil
code receivers and algorithms. On September 23, 2008, the DoD announced its intent to transition
from semi-codeless and codeless receiver support by December 31, 2020. WAAS plans totransition its architecture from one dependent upon semi-codeless receivers to measureionosphere induced propagation delay to one that uses the civil codes on L1 and L5. Thistransition will require updates to the safety-critical algorithms in addition to replacement of thereceivers. Transition of the WAAS will also support a two frequency service (L1, L5) with a muchlarger service volume.

A.3.1 Background – Semi-Codeless Receivers
GPS transmits PPS signals known as P(Y)-code at the L1 and L2 frequencies. The P(Y) signals
are intended primarily for U.S. and allied military use, but the civilian community has developed
techniques that exploit P(Y) at L1 and L2 to achieve significant accuracy gains. Such techniques,collectively known as codeless or semi-codeless GPS access, have been integrated into a range ofGPS receivers sold commercially around the world. WAAS uses semi-codeless receivers, primarily
to measure ionospheric delays, at each WAAS Reference Site.
The U. S. Government has historically supported codeless and semi-codeless users by
documenting a time-limited commitment to the P(Y) signal in the GPS Standard Positioning Service(SPS) Performance Standard and the Federal Radionavigation Plan. For example, the 2001
version of the GPS SPS Performance Standard included the statement that, "Until such time as asecond coded civil GPS signal is operational, the U.S. Government has agreed to not intentionallyreduce the current received minimum Radio Frequency signal strength of the P(Y)-coded signal on
the L2 link, as specified in ICD-GPS-200C or to intentionally alter the P(Y)-coded signal on the L2
link." As a result, manufacturers have continued to develop codeless/semi-codeless GPS
equipment.
The U.S. Government acknowledges the global use of GPS codeless and semi-codelesstechniques and has committed to maintain the existing GPS L1 P(Y) and L2 P(Y) signal
characteristics until such time that an alternative capability exists to replace it. Since 1999, theDepartment of Defense has worked closely with the civilian agencies on the National Executive
Committee for Space-Based PNT (and its predecessor, the Interagency GPS Executive Board) to
add new capabilities to GPS that supplant the need for codeless/semi-codeless access. In 2005,
the U.S. Air Force began launching modernized GPS satellites featuring a new civil signal at L2
called L2C. L2C is designed to work in combination with the legacy C/A signal at L1 to enable highaccuracy without codeless/semi-codeless techniques. In 2009, the Air Force will begin adding athird civil signal called L5 to all new GPS satellites. L5 will also work in combination with L1 C/A
and/or L2C to enable high accuracy without codeless/semi-codeless techniques.
The National Executive Committee for Space-Based PNT seeks to encourage the development
and adoption of next-generation GPS receivers that achieve high accuracy via use of L2C and/or
L5 instead of codeless/semi-codeless techniques. To facilitate business decisions and stable
planning for equipment developers and end users, the National Executive Committee has
established a fixed date of December 31, 2020, for the equipment transition.
The date is based upon the current launch schedule for the GPS program, which will have 24 GPS
satellites transmitting the L2C signal to users by 2016, and 24 GPS satellites transmitting L5 by
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interviews and through public comments submitted by GPS user equipment manufacturers, service
providers, and users in response to a Federal Register notice dated May 16, 2008. The
manufacturers and service providers indicated that a 12-year transition period should be sufficient
to allow the installed base of codeless/semi-codeless GPS users to re-equip with next-generation
receivers as part of their normal equipment amortization, obsolescence, and upgrade cycle.
Should there be unforeseen delays in the GPS modernization program, the transition date willreassessed.
After 2020, the characteristics of the P(Y) signals transmitted by modernized GPS satellites maychange without further notice and may preclude codeless/semi-codeless use of the P(Y) signals.
However, for those legacy satellites that have no modernized capabilities, codeless/semi-codelessaccess to P(Y) at L1 and L2 will continue until those satellites are decommissioned.

A.3.2 Semi-Codeless Receiver Requirements
The civil community assumptions associated with codeless/semi-codeless processing of the L2 SIS
are as follows:•

Maintain a stable 90 degree phase offset between L1 C/A and L1 P(Y) codes (C/A mustlag L1-P(Y)). The offset is expected to be within +/- 100 milliradians.

•

Code offset between P(Y) on L1 and L2 less than 15 ns (as specified in IS-GPS-200D).
The offset is expected to be within +/- 15 ns and corrected with TGD in the navigation
message.

•

Maintain a stable 90 degree offset between L2C and L2 P(Y) codes (L2C must lag L2
P(Y)). The offset is expected to be within 100 milliradians.

•

L1 modulated by C/A and P(Y) codes in phase quadrature, i.e., P-code nominal -90
degrees from C/A (as defined in IS-GPS-200D).

•

Y-code is an encrypted version of P-Code.

•L2 modulated with same P(Y) as L1.

•

Same navigational data broadcast on L1 P(Y) and L2 P(Y) (note this is stated as optionalin IS-GPS-200D).

•

L1-L2 differential bias stability less than 3 ns, 2-sigma (as stated in IS-GPS-200D to
support clock accuracy, based on L1 combined with L2) over any 5 minute interval.

A.3.3 Effect on WAAS Users
When WAAS transitions to use L1 and L5, it will no longer monitor GPS satellites that don’ttransmit L5. At the transition date, 24 GPS satellites will be broadcasting the L5 signal. There may
be some Block IIR or IIR-M satellites operational within the constellation that cannot broadcast theL5 signal. WAAS will no longer monitor these satellites after transition.
WAAS transition will enable a new two frequency service. This new service will be available tousers with receivers that rely on L1 and L5 to correct for the ionosphere induced error instead ofusing the ionosphere error correction grid provided for single frequency users. .

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A.3.4 Semi-Codeless Federal Register Notice
On September 23, 2008, the DoD published a Notice in the Federal Register titled “Preservation ofContinuity for Semi-Codeless GPS Applications”. The Notice was published under the Office of theSecretary, Assistant Secretary of Defense for Networks and Information Integration/DoD Chief
Information Officer, Department of Defense.
It may be obtained electronically at
http://edocket.access.gpo.gov/2008/pdf/E8-22197.pdf. The complete Notice, FR Doc. E8-22197, is
as follows:
Federal Register: September 23, 2008 (Volume 73, Number 185)
Notices
Page 54792-54793
From the Federal Register Online via GPO Access (wais.access.gpo.gov)
DOCID:fr23se08-49

DEPARTMENT OF DEFENSE
Office of the Secretary
Preservation of Continuity for Semi-Codeless GPS Applications
AGENCY: Assistant Secretary of Defense for Networks and Information Integration/DoD
Chief Information Offices, Department of Defense.
ACTION: Notice
SUMMARY: To enable an orderly and systematic transition, the U.S. Government hasestablished December 31, 2020 as the date by which users of semi-codeless/codelessreceiving equipment are expected to transition to using GPS civil-coded signals. Based on
the current launch schedule and projected budget, the December 31, 2020 transition daterepresents the planned availability of the second and third coded civil GPS signal being
broadcast from a minimum of 24 GPS satellites. Department of Defense will reassess thetransition date should significant GPS program delays arise.
FOR FURTHER INFORMATION CONTACT: Mr. Raymond Swider, 703-607-1122.
SUPPLEMENTARY INFORMATION:
The Department of Defense (DoD) provides the GPS Standard Positioning Service(SPS) for peaceful civil, commercial, and scientific uses on a continuous worldwide basis
free of direct user fees. The SPS is a single-frequency GPS service which is presently
limited to the coarse acquisition (C/A) code on the L1 frequency.
Access to two or more civil signals are needed to enable high accuracy civil
applications. Civil users are currently employing codeless or semi-codeless techniques togain access to encrypted GPS signals, L1 P(Y) and L2 P(Y). To facilitate expansion of civil
GPS applications, the DoD has planned and begun to broadcast additional civil signals thatwill obviate the further need for use of codeless and semi-codeless techniques. The
second coded civil GPS signal (L2C) and the third coded civil GPS signal (L5) are planned
to be broadcast from 24 GPS satellites in 2016 and 2018, respectively. Full operational
capability of the L2C and L5 GPS signals in combination with the existing L1 C/A signal will
enable the full spectrum of dual frequency applications without using the P(Y) signals.
The U.S. Government acknowledges global use of GPS codeless and semicodeless techniques and commits to maintaining the existing GPS L1 C/A, L1 P(Y), L2C
and L2 P(Y) signal characteristics until December 31, 2020 when the second and third civil
signals (L2C and L5) are planned to be broadcast from a minimum of 24 GPS satellites.

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After the planned transition date, the characteristics of the L1 P(Y) and L2 P(Y) signals
transmitted by any or all GPs satellites broadcasting two or more civil-coded signals maychange without further notice and may preclude the use of P(Y) coded signals for highaccuracy applications.
The U.S. Government is committed to support civil PNT services based on GPS
civil signals: L1 C/A, L2C, L5, and L1C. To this end, GPS civil signal characteristics are
specified in the relevant Interface Specifications (ISs) and will be included in PerformanceStandards (PSs) subject to the operating descriptions contained in the Federal
Radionavigation Plan (FRP). The U.S. Government has met or exceeded the GPS service
performance commitments in the Standard Positions Service Performance Standard since
1993 and is committed to continually improving GPS services as codeless and semicodeless users complete a timely transition to dual-coded civil GPS equipment.
Dated: September 16, 2008
Patricia L. Toppings
OSD Federal Register Liaison Officer, Department of Defense
FR Doc. 8-22197 Filed 9-22-08: 845 am

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GLOBAL POSITIONING SYSTEM
WIDE AREA AUGMENTATIONSYSTEM (WAAS)PERFORMANCE STANDARD
APPENDIX B
KEY TERMS, DEFINITIONS,ABBREVIATIONS AND ACRONYMS

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TABLE OF CONTENTSSECTION B.1 Key Terms .............................................................................. B-1
SECTION B.2 Definitions .............................................................................. B-2
SECTION B.3 Abbreviations and Acronyms............................................... B-3

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SECTION B.1 Key Terms
Alert. An indication provided to other aircraft systems or annunciation to the pilot to identity that an
operating parameter of a navigation system is out of tolerance. [ICAO definition]
Alert limit. For a given parameter measurement, the error tolerance not to be exceeded without
issuing an alert. [ICAO definition]
Baseline 24-Slot Constellation. Constellation of operational satellites deployed in the 24 definedbaseline orbital slots. Each orbital slot is characterized by a near one-half sidereal day period suchthat 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 groundtrace. In the baseline 24-slot constellation, there are six orbit planes, each with four slots.
Block II Satellites. The current deployed constellation consists entirely of Block II series satellites.
For the purposes of these performance standards, the Block II series of satellites (the IIA, IIR, IIRM, and IIF) provide an identical service.
Global navigation satellite system (GNSS). A worldwide position and time determination systemthat includes one or more satellite constellations, aircraft receivers and system integrity monitoring,augmented as necessary to support the required navigation performance for the intended
operation. [ICAO definition]
Hazardously Misleading Information (HMI). Information that persists beyond the allowable TTA
causing the errors in the position solution output by a WAAS receiver to exceed the user’s
particular tolerance for error in the current application.
Integrity. A measure of the trust that can be placed in the correctness of the information suppliedby the total system. Integrity includes the ability of a system to provide timely and valid warnings to
the user (alerts). [ICAO definition]
Precise Positioning Service (PPS). The GPS broadcast signals based on the L1 P(Y)-codes, L1
C/A-codes, and L2 P(Y)-codes, as defined in the GPS ISs/ICDs, providing constellation
performance to authorized users, as established in the PPS Performance Standard (PPS PS).
Receiver Autonomous Integrity Monitoring (RAIM). RAIM is an algorithm used in a GPS
receiver to autonomously monitor the integrity of the output position/time solution data. There aremany different RAIM algorithms. All RAIM algorithms operate by evaluating the consistency of
redundant measurements.
Satellite-based augmentation system (SBAS). A wide coverage augmentation system in which
the user receives augmentation information from a satellite-based transmitter. [ICAO definition]
Standard Positioning Service (SPS). The GPS broadcast signals based on the L1 C/A-codes, as
defined in IS-GPS-200, providing constellation performance to civil, commercial, and scientificusers, as established in the SPS Performance Standard (SPS PS), in accordance with U.S.
Government (USG) policy.
Time-to-alert. The maximum allowable time elapsed from the onset of the navigation systembeing out of tolerance until the equipment enunciates the alert. [ICAO definition]

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SECTION B.2 Definitions
Accuracy. Accuracy is defined to be the statistical difference between the estimate ormeasurement of a quantity and the true value of that quantity. For the purposes of this WAAS PS,
accuracy is expressed as either as 95th percentile (95%) differences or as rms differences.
Availability. Availability is defined as the percentage of time that a particular WAAS service isavailable to the WAAS user. Real-time availability maps are published by the FAA TechnicalCenter at www.nstb.tc.faa.gov, and dispatch availability predictions are available to support flight
operations.
Continuity. Continuity is defined to be the probability that a WAAS service will continue to beavailable over an hour time interval for en route through LNAV operations, over a 15 second
interval for LNAV/VNAV and LPV operations, given that it was available at the beginning of theinterval and that an outage was not announced in a prior notice to airmen (NOTAM).
Integrity. Integrity is a measure of the trust which can be placed in the correctness of theinformation supplied by the total system. Integrity includes the ability of the WAAS SIS to provide
timely alerts (alarms or warnings) to receivers when the WAAS service HPL or VPL no longer
bound the horizontal position error (HPE) or vertical position error (VPE) or a GPS satellite should
not be used as part of the WAAS augmentation solution.

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SECTION B.3 Abbreviations and Acronyms-A-B-CC/A
C&V
CAT
CONUS
CS

Coarse Acquisition
Corrections & Verification
Category
CONtiguous U.S.
Control Segment

dBm
DCP
DGPS
DoD
DOT

Decibels with respect to one milliWatt
Data Collection Processor
Differential GPS
Department of Defense
Department of TransportationER

En Route

FAA
FCs
FDE
FRP

Federal Aviation Administration
Fast Corrections
Fault Detection and Exclusion
Federal Radionavigation Plan

GEO
GES
GIVE
GNSS
GPS
GUS

Geostationary Earth Orbit
Ground Earth Station
Grid Ionospheric Vertical Error
Global Navigation Satellite System
Global Positioning System (or Navstar Global Positioning System)GEO Uplink Subsystem

HAT

Height Above Touchdown

-D-

-E-

-F-

-G-

-H-

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HAL
HMI
HPL

Horizontal Alert Limit
Hazardously Misleading Information
Horizontal Protection Level

ICs
ICAO
ICD
IFOR
IFR
IGP
ISs

IGP Corrections
International Civil Aviation Organization
Interface Control Document
Interagency Forum on Operational Requirements
Instrument Flight Rules
Ionospheric Grid Points
Interface Specifications

L1
L1C
L2
L2C
L5
LNAV
LPV
LTC

The SIS centered at the 1575.42 MHz frequency
A future civil signal centered at L1
The SIS centered at the 1227.60 MHz frequency
A future civil signal centered at L2
A future civil SIS centered at the 1176.45 MHz frequency
Lateral Navigation
Localizer Performance with VerticalLong Term Corrections

MCS
MHz
MOPS
MS
MSL

GPS Master Control Station (part of the OCS)
MegaHertz
Minimum Operational Performance Standard
GPS Monitor Station (part of the OCS)
Mean Sea Level

NAS
NOCC
NOTAM
NSE
NSTB

National Airspace System
National Operations and Control Center
Notice to Airmen
Navigation System Error
National Satellite Test Bed-I--J-K-L--M-- N-

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-OOCS
OCS
O&M

GPS Operational Control System
Obstacle Clearance Surface (defined within TERPS)
Operations and MaintenanceP-code
PDOP
POCC
PPS
PRN
PS
P(Y)-code

Unencrypted Precise PRN ranging code
Position Dilution of Precision
Pacific Operations and Control Center
Precise Positioning Service
Pseudorandom Noise (a characteristic of the SIS ranging codes)
Performance Standard (as in PPS PS or SPS PS)
Precise PRN ranging code (unencrypted or encrypted)

RAIM
RAIM/FDE
RF
RFU
RHCP
rms
RNAV
RNP
RTCA Inc.

Receiver Autonomous Integrity Monitoring
RAIM with FDERadio Frequency
RF Uplink
Right Hand Circularly Polarized
Root-Mean-Square
Area Navigation
Required Navigation Performance
Formerly Radio Technical Commission for Aeronautics

SARPs
SBAS
SC
SGS
SIS
SP
SPS

Standards and Recommended Practices
Satellite Based Augmentation System
Safety Computer
Signal Generation Subsystem
Signal-In-Space
Safety Processor
Standard Positioning Service

TERPS
TCN
TSO
TTA
TT&C

Terminal and En Route Instrument Procedures
Terrestrial Communication Network
Technical Standard Order
Time to Alert
Telemetry, Track, and Command

-P-

-Q-R-

-S-

-T-

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User Differential Range Error

VPL
VAL

Vertical Protection Level
Vertical Alert Limit

WAAS
WMP
WMS
WRE
WRS

Wide Area Augmentation System
WAAS Message Processor
Wide-area Master Station
Wide-area Reference Equipment
Wide-area Reference Station

-V-

-W-

-X-Y-Z-

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