AC 20-138A

AC-20-138A

AIRWORTHINESS APPROVAL OF GLOBAL

NAVIGATION SATELLITE SYSTEM (GNSS)

EQUIPMENT

December 22, 2003

TABLE OF CONTENTS

Paragraph Page

1. Purpose1

2. Cancellation1

3. Related Federal Aviation Regulations1

4. How to Use this Document1

5. Definitions1

6. Related Reading Materials3

7. Background6

8. Approval Process9

9. GNSS Equipment Limited to VFR Use12

10. TSO Authorizations13

11. Equipment Performance – GPS Antenna15

12. Equipment Performance – GNSS Sensor17

13. Equipment Performance – Stand Alone Navigation Equipment20

14. Equipment Performance – LAAS VDB Receiver24

15. Installation Issues – General24

16. Installation Issues – GPS Antenna25

17. Installation Issues – GNSS Sensor26

18. Installation Issues – Stand Alone Navigation Equipment28

19. Installation Issues – LPV and GLS Approaches33

20. Installation Issues – LAAS VDB Receiver34

21. Data Submittal35

22. Ground Test36

23. Flight Test38

APPENDIX 1: GPS Oceanic/Remote Navigation (3 pages) APPENDIX 2: Integration of GLONASS with GPS or GPS/WAAS (4 pages) APPENDIX 3: Example Format for a Quick Reference Guide (8 pages) APPENDIX 4: Sample Airplane/Rotorcraft Flight Manual Supplement (5 pages) APPENDIX 5: Installation of En Route GPS Equipment For VFR Use Only (2 pages) Page i (and ii)

1. PURPOSE. This ad visory circular (AC) pr ovides guidance m aterial for t he airworthiness approval of Global Na vigation Satellite System (GNSS) equipm ent. Like all AC m aterial, th is AC is not m andatory and does not constitu te a regulation. It is issued for g uidance purposes and to outline a method of com pliance with the ru les. In lieu of following this method without deviation, the applicant m ay elect to follow an alternate m ethod, provided the alternate m ethod is also found by the Federal Aviation Administration (FAA) to be an accep table means of complying with the requirem ents of the federal aviation regulations (Title 14 of the Code of Fe deral Regulations, 14 C FR).

This AC addresses the following equipment:

a. GNSS sensors, including those incorpor ating Wide Area Augm entation System (WAAS), Local Area Aug mentation System (LAAS), or the Russian G lobal Navigation Satellite System (GLONASS).

b. GNSS stand-alone navigation equipm ent that provides deviations (steering commands) for en route, terminal, or approach operations (including Category I precision approach).

2. CANCELLATION.

a. AC 20-138, Airworthiness Approval of Global Positioning System (GPS) Navigation Equipm ent for Use as a VFR and IFR Supplem ental Navigation System , dated May 25, 1994, is canceled.

3. RELATED FEDERAL AVIATION REGULATIONS . 14 CFR Parts 21, 23, 25, 27, 29, 43, 91, 121, and 135.

4. HOW TO USE THIS DOCUME NT. This docum ent is organized into general categories of equipment and installations. Paragraph 8 summarizes the approval process and applies to all projects. Paragraph 9 applie s to GNSS as an aid to visual f light rules (VFR) navigation, while paragraphs 10 through 23 address instrum ent flight rules (IFR) navigation. Guidance concerning the equipm ent performance and functions is provided in paragraphs 11 through 14, and installati on guidance is provided in paragraphs 15 through 20. Paragraphs 21 through 23 provi de additional inform ation on the data submittal, ground test and flight test as they apply to an installation approval.

5. DEFINITIONS.

a. Aircraft-Based Augm entation Syste m (ABAS) . An augmentation sys tem that augments and/or integrates the information obtained from the other GNSS elem ents with information available on board the aircraft.

b. Ground-Based Augm entation System (GBAS). An augmentation sys tem in which the user receives augm entation in formation directly from a ground-based transmitter.

c. GLONASS. GLONASS is a Russian satellit e based radio navigation system, which provides a positioning se rvice anywhere in th e worl d. Until th e FAA declares GLONASS fully operational for U.S. aviation, it s use is lim ited to a supplem ent to GPS navigation. The FAA will declare GLONA SS fully operational after the sys tem Par 1 Page 1

management and maintenance are mature. The following capabilities are expected before the FAA will declare GLONASS operational for U.S. aviation:

(1) Mature maintenance practices;

(2) Commitment to maintain a complete constellation of satellites; and

(3) Issuance of international NOTAMs prior to any scheduled maintenance, and after the onset of any unscheduled outages.

d. Global Navigation Satellite System (GNSS). The GNSS is a generic term for satellite-based navigation, including GPS, SBAS, GBAS , GLONASS, and any other satellite navigation system.

e. GNSS Landing System (GLS). A satellite-based navigation system that provides precision approach (including SBAS/WAAS and GBAS/LAAS).

f. Global Positioning System (GPS). GPS is a U.S. sate llite based radio navigation system that provides a precise positioning se rvice anywhere in the world. The service provided by GPS for ci vil use is defined in the GPS Standard Positioning System Signal Specification.

g. Local Area Augmentation System (LAAS). LAAS is the G BAS provided by the FAA.

h. Navigation. Within the context of this AC, navigation refers to any function used to direct the course of an aircraft. A navi gation system as addressed in this AC provides the functio ns of position estim ation, path definition, p ath steering , and situ ation indications and alerting to the flight crew.

i. Receiver Autonom ous Integrity Monitoring (R AIM). According to T SO-C129 (all rev isions (AR)), an y al gorithm which ve rifies the inte grity of the position output using GPS m easurements, or GPS m easurements and barom etric aiding, is considered a RAIM algor ithm. An algorithm which us es ad ditional inf ormation (e.g ., m ulti-sensor system) to verify the integrity of the position output m ay be acceptable as a RAIM- equivalent. Within this AC, the term RAIM is a synonym for ABAS and is used to refer to both RAIM or RAIM-equivalent algorithms.

j. Satellite-Based Augmentation System (SBAS). A wide coverage augm entation system in which the user rec eives augm entation inf ormation f rom a satellite-ba sed transmitter.

k. Special Category I (SCAT-I). In order to expedite the im plementation of satellite-based navigation, the aeronautical industry and the FAA in 1993 developed RTCA/DO-217, Minimum Aviation System Perfor mance Standards (MASPS) DGNSS Instrument Approach System: Special Category I . The F AA adopted this standard in Order 8400.11, IFR Approval for Differen tial Gl obal Position ing System Special Category I Instrument Approaches Using Private Ground Facilities . The inten t of SCAT-I is the sam e as LAAS, except it is li mited to Ca tegory I p recision approach and does not ensure interoperability between di fferent vendors. SCAT- I allows private facilities to be f ielded bef ore all of the issues associated with a public sys tem are Page 2 Par 5

resolved. With the adoption of ICAO standards for LAAS in 2001, the SCAT-I standards became obsolete and should no longer be used.

l. Stand-alone. Stand-alone as addressed in this AC refers to navigation equipm ent incorporating the GPS or GPS/ WAAS position se nsor and a navigation function, so that the equipment provides path deviations relative to a selected path.

m. Wide Area Augmentation System (WAAS). WAAS is the SBAS provided by the FAA.

6. RELATED READING MATERIALS.

a. FAA Orders and Technical Standard Or ders (TSO). The following docum ents may be obtained from the Departm ent of Tr ansportation, FAA, Aircraf t Certif ication Service, Aircraft Engineering Divi sion, AIR-130, 800 Independence Avenue, SW ., Washington, D.C. 20591, or on the FAA Internet at http://av-info.faa.gov/tso/.

(1) TSO-C115b, Airborne Ar ea Navigation Equipm ent Using Multi-Sensor Inputs;

(2) TSO-C129a, Airborne Supplem ental Navigation Equipment Using the Global Positioning System (GPS);

(3) TSO-C144, Airborne Global Positioning System Antenna;

(4) TSO-C145a, Airborne Navigation Sens ors Using the Global Positioning System (GPS) Augmented by the Wide Area Augmentation System (WAAS);

(5) TSO-C146a, Stand-Alone Airborne Navigation Equipment Using the Global Positioning System (GPS) Augm ented by the W ide Area Augm entation System (WAAS);

(6) TSO-C161, Local Area Augm entation Sy stem Position ing and Navigation Equipment;

(7) TSO-C162, Local Area Augm entation Sy stem Very High Frequency Data Broadcast Equipment;

(8) Order 8400.11, IFR Approval of Differential GPS Spec ial Category I Instrument Approaches Using Private Ground Facilities; and

(9) Order 8400.12A, Re quired Navigation Perform ance 10 (RNP-10) Operational Approval.

b. FAA Advisory Circulars. The following documents m ay be obtained from the Department of Transportation, Subseque nt Distribution Office, SVC-121.23, Ardmore East Business Center, 3341 Q 75 th Ave, Landover, MD 20785. The A dvisory Circular Checklist (AC 00-2) is available at http:// www.faa.gov. T he advisory circular checklist contains status and order information for the FAA advisory circulars.

(1) AC 20-115B, Radio Technical Comm ission for Aeronautics, Inc. Document RTCA/DO-178B;

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(2) AC 20-129, Airworthiness Approval of Vertical Navigation (VNAV) Systems for Use in the U.S. National Airspace System (NAS) and Alaska;

(3) AC 20-130A, Airworthiness Approval of Navigation or Flight Managem ent Systems Integrating Multiple Navigation Sensors;

(4) AC 21-40, Application Guide for Obtaining a Supplem ental Type Certificate;

(5) AC 23-17A, System s and Equipm ent Gu ide f or Certif ication of Part 23 Airplanes;

(6) AC 23-1309-1C, Equipm ent, System s, and Installations in Part 23 Airplanes;

(7) AC 23.1419-2A, Certification of Part 23 Aircraft for Flight in Icing Conditions;

(8) AC 25-1309-1A, System Design and Analysis;

(9) AC 25.1329-1A, Automatic Pilot Systems Approval;

(10) AC 25.1419-1, Certification of Part 25 Aircraft for Flight in Icing Conditions;

(11) AC 43.13-1B, Acceptable Method s, Tec hniques and Practices – Aircraft Inspection and Repair;

(12) AC 43-13-2A, Acceptable Method s, Tec hniques and Practices – Aircraft Alterations;

(13) AC 90-79, Recomm ended Practices and Procedures for the Use of Electronic Long-Range Navigation Equipment;

(14) AC 90-94, Guidelines for Using GPS Equipm ent for IFR En Route and Terminal Operations & for Nonprecision Instrument Approaches;

(15) AC 90-96, Approval of U.S. Ope rators and Aircraft to Operate under Instrument Flight Rule s (IFR) in Europ ean Airspace Designa ted f or Basic Area Navigation (BRNAV/RNP-5);

(16) AC 91-49, General Aviation Procedures for Flight in North Atlantic Minimum Navigation Performance Specification Airspace;

(17) AC 120-29A, Criteria for Approving Ca tegory I and Category II Landing Minima for FAR 121 Operators. This AC supplements Appendix 2 of AC 120-29A with respect to the installation of GPS/WAAS Category I equipment;

(18) AC 120-33, Operational Approval of Airborne Long-Range Navigation Systems f or Flight within the North A tlantic Minimu m Navi gation Perfor mance Specification Airspace.

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c. RTCA, Inc. docum ents. The following docum ents m ay be purchased from RTCA, Inc., 1828 L Street, NW, Suite 805, Washington, DC 20036, or purchased on-line at http://www.rtca.org/.

(1) RTCA/DO-160D, Environm ental Conditions and Test Procedures for Airborne Equipment;

(2) RTCA/DO-178B, Soft ware Considerat ions in Airborne System s and Equipment Certification;

(3) RTCA/DO-200A, Standards for Processing Aeronautical Data;

(4) RTCA/DO-201A, Standards for Aeronautical Information;

(5) RTCA/DO-208, Minimum Operational Performance Standards for Airborne Supplemental Navigation Equipment Using Global Positioning System (GPS);

(6) RTCA/DO-228 with Change 1, Mi nimum Operational Performance Standards For Global Navigation Satellite System (GNSS) Airborne Antenna Equipment;

(7) RTCA/DO-229C, Minim um Ope rational Perform ance Standards for Airborne GPS/WAAS Equipment;

(8) RTCA/DO-236A, Minim um Avia tion Sy stem Perfor mance Standards:

Required Navigation Performance for Area Navigation;

(9) RTCA/DO-246B, GNSS Based Precisi on Approach Local Area Augmentation System (LAAS) Signal-in-Space Interface Control Document (ICD);

(10) RTCA/DO-253A, Minimum Operational Performance Standards for GPS Local Area Augmentation System Airborne Equipment;

(11) RTCA/DO-257A, Minimum Operational Performance Standards for the Depiction of Navigation Information on Electronic Maps.

d. The FAA and Industry Guide to Avionics Approvals, April 13, 2001. Copies of this docum ent m ay be obtained at http://www1.faa.gov/certification/aircraft/avionics_approvals_guide.htm.

e. GPS Standard Positioning Service Performance Standard, October, 2001. Copies of this docum ent and general infor mation related to GPS may be requested at http://www.navcen.uscg.gov/.

f. Department of Defens e Interface C ontrol Docum ent (ICD) ICD-GPS-200C, Navstar GPS Space Segment/Navigation User Interface. Copies of this document may be requested from the GPS Joint Program O ffice, SSD/CZ, Los A ngeles AFB, CA 90006.

Alternatively, copies of this document may be requested at http://www.navcen.uscg.gov/.

g. National Imagery and Mapping A gency (NIMA) Technical Report NIMA TR 8350.2, W orld Geodetic System 1984, Its Defin ition and Relationships with Lo cal Geodetic Systems. Copies of this document may be requested at http://www.nima.mil/.

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h. FAA National Airspace System Architecture Version 4.0. An electronic version of this docum ent and architecture upd ates is availab le at http://www.faa.gov/nasarchitecture/.

i. FAA Specification W ide Area Augm entation System ( WAAS), FAA- E-2892B.

This docum ent is available on-line at http://www2.faa.gov/asd/international/GUIDANCE_MATL/2892b_c1.pdf.

j. DOD/DOT 2001Federal Radionavigation P lan (FRP). The FRP is published every two years in which copies m ay be ordered through the National Technical Information Service, Springfield, VA 22161; internet: http://www.ntis.gov/.

k. DOD/DOT 2001 Federal Radionavigation Systems (FRS). The FRS is published every two years in which copies m ay be ordered through the National Technical Information Service, Springfield, VA 22161; internet: http://www.ntis.gov/.

7. BACKGROUND.

a. Global Positioning System (GPS).

(1) System Descrip tion. The Global Position ing System is a satellite based radio navigation system, which broadcasts a sign al that is used by receivers to determine precise position anywhere in th e world. Th e rec eiver tracks m ultiple sa tellites and determines a pseudorange m easurement that is then used to determ ine the user location.

A m inimum of four satellites are n ecessary to estab lish an accurate th ree-dimensional position. The Department of Defense (DOD) is responsible fo r operating the GPS satellite constellation and m onitors the GPS satellites to ensu re proper operation. Every satellite's orbital parameters (ephem eris da ta) are sent to each sate llite f or broadcas t as part of the data message embedded in the GPS signal. The GPS coordinate system is the Cartesian earth-centered earth-fixed coordi nates as specified in the W orld Geodetic System 1984 (WGS-84).

(2) System Availability and Reliability.

(i) The status of GPS satellites is bro adcast as part of the data m essage transmitted by the GPS sate llites. GPS status information is also available by means of the U.S. Coast Guard navigation infor mation service: (703) 313-5907, Internet:

http://www.navcen.uscg.mil/gpsnotices/. Add itionally, s atellite s tatus is available through the Notice to Airmen (NOTAM) system.

(ii) The operational status of GNSS opera tions depends upon the type of equipment being used. For GPS-only equipm ent TSO-C129(AR), the operational status of nonprecision approach capability for flight planning purposes is provided through a prediction program that is embedded in the receiver or provided separately.

(3) Receiver Autonom ous Integ rity Mon itoring (RAIM). W hen GNSS equipment is not using integrity inform ation from WAAS or LAAS, the GPS navigation receiver using RAIM provides GPS signal integrity monitoring. RAIM is necessary since delays of up to two hours ca n occur before an erroneous satellite transm ission can be detected an d corrected by the satellite con trol segm ent. The RAIM function is also referred to as fault detection. Another capabili ty, fault exclusion, refers to the ability of Page 6 Par 6

the receiver to exclude a failed satellite from the position s olution. Fault detection and exclusion (FDE) is provided in ocea nic/remote GPS e quipment (Appendix 1, paragraph 2b of this AC) and in all GPS/WAAS equipment.

(4) Selective Availability. Selective Availability (SA) is a method by which the accuracy of GPS is intentionally degraded. This feature is designed to deny hostile use of precise GPS positioning data. SA wa s discontinued on May 1, 2000, but m any GPS receivers are designed to assum e that SA is still active. New receivers m ay take advantage of the discontinuance of SA base d on the performance values in ICAO Annex 10, and do not need to be designed to operate outside of that performance.

(5) GPS Modernization. Initiatives to modernize the Global Positioning System include two new civilian signals. One of the new signals will be cen tered at 117 6.45 MHz, and will prov ide an im proved serv ice that supports safety-of-life applications, including aeronautical app lications. The other new si gnal will be centered at 122 7.60 MHz, and will be available for general use in non-safety cr itical applications. Depending on the rate of GPS satellite launches, initial operating capability will be available between the years 2012 and 2015.

(6) System Performance. The GPS perform ance specifications contained in this AC are based upon unaugm ented perform ance ach ieved w ith the Standard Positioning Service (SPS). GPS perform ance for a repr esentative single-frequency receiver, as defined in the GPS SPS Performance Standard, is summarized in the following table.

Table 1. Summary of GPS Performance Availability Unspecified Horizontal Navigation System Accuracy (95%) 33 m Vertical Navigation System Accuracy (95%) 73 m SIS Integrity Unspecified Service Volume Global

(7) General Operational Limitations.

(i) TSO-C129(AR) equipm ent: the requirements defined in TSO-C129(AR) were developed for GPS to be used in addition to other navigation equipment.

(ii) GPS Oceanic/Remote operation: the requirements specified in appendix 1 of this AC were developed for GPS to be used for lim ited operations (e.g.

oceanic/remote operations as discussed in AC 90-94) without th e need for other navigation equipment appropriate to the route to be flown.

(8) Equipment Classes. TSO-C129(AR) defi nes different classes of equipment to support different equipm ent configurations . These classes are defined in paragraph

(a)(2) of TSO-C129(AR).

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b. Satellite-Based Augm entation Syste m (SBAS) . To i mprove the accuracy, integrity and availability of GPS signals, ICAO has defined Standards and Recommended Practices (SARPs) for satellite-based augmentation system s (SBAS). The FAA developed the W ide Area Augm entation Syst em ( WAAS) to provide SBAS service throughout U.S. airspace. In addition, the FA A is working with civil aviation authorities in Europe and Japan to coordinate the de velopment of their SB AS, (the European Geostationary Navigation Ov erlay System , or EGNOS, a nd the Japan Multifunctional Transport Satellite (MTSAT) Satellite-based Augmentation System, or MSAS).

(1) WAAS Description.

(i) The WAAS uses measurements from a wide area network of reference stations to determ ine satellite cloc k a nd ephem eris corrections and to m odel the propagation effects of the ionosphere. A m onitoring system ensures that the SBAS i s operating c orrectly and that the co rrection inf ormation is corre ct. This inf ormation is broadcast to users via a geosta tionary satellite, using a signal that is simila r to the GPS signal. Sinc e the SBAS satellite sig nal is similar to GPS, it also provide s an additio nal pseudorange measurement to the receiver. The objectives of WAAS are:

(A) to improve the integrity of GPS through real-time monitoring;

(B) to im prove the av ailability of GPS by providing an ad ditional satellite signal; and (C) to im prove the accuracy of GPS by providing differential corrections.

(ii) WAAS provides a level of service th at supports all pha ses of flight including LNAV/VNAV and L PV appro aches. Both LNAV/VNAV and LP V approaches are approach procedures with ve rtical guidance as defined in ICAO Annex 6.

These approaches provide vertical guidance, but do not meet the more stringent standards of a precis ion approach . In th e lo ng term , W AAS will provide Ca tegory I p recision approach services in conjunction with m odernized GPS. The FAA pla ns to decommission a significant percentage of its existing VOR and ILS inf rastructure after WAAS services are av ailable. Although re quirements for surface op erations are not mature, GPS/WAAS is expected to m eet those requirements for a variety of surveillance and navigation applications.

(2) WAAS Availab ility and Reliability. The FAA will provid e NOTAMs to advise pilots of the status of the WAAS. This will also include NOTAMs advising if the operational capability of GPS/WAAS equipment is degraded. WAAS monitors both GPS and WAAS satellites and provides integrity.

(3) WAAS Per formance Requirem ents (w ithin Service Vo lume). W AAS performance defined in this AC is based on a single frequency user as defined in FAA-E2892B. The performance of WAAS is summarized in the following table.

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