AC 20-138C

U.S. Department of Transportation Federal Aviation Administration

Advisory Circular

Subject: Airworthiness Approval of Positioning and Navigation Systems Date: 5/8/12 Initiated by: AIR-130 AC No: 20-138C

This advisory circular (AC) provides guidance material for the airworthiness approval of installed positioning and navigation equipment. This revision corrects minor errors, clarifies guidance, adds a frequently asked question section, and updates technical standard order (TSO) information. Major changes in this AC reflect TSO-C129a cancellation; updated guidance related to TSO-C115c; clarifying information in paragraphs 14-6.8 and 14-8.13 for the ground proximity warning system/terrain awareness and warning system (GPWS/TAWS) glideslope alerting function, and adding additional testing procedures in Appendix 4 for global navigation satellite systems (GNSS) that support automatic dependent surveillance - broadcast (ADS-B).

Positioning and navigation equipment may be used for a variety of functions such as navigation, automatic dependent surveillance, and/or terrain awareness and warning systems. This AC addresses the following avionics:

 Global positioning system (GPS) equipment including those using GPS augmentations.

 Area navigation (RNAV) equipment integrating data from multiple navigation sensors.

 RNAV equipment intended for required navigation performance (RNP) operations.

 Barometric vertical navigation (baro-VNAV) equipment.

This AC does not address new satellite constellations that are planned or currently under construction. This AC will be updated when sufficient documentation is available from the GNSS provider countries and RTCA to support service definition, service performance commitments, and minimum operational performance standards for multi-constellation equipment.

Susan J. M. Cabler Assistant Manager, Aircraft Engineering Division

Table of Contents

Chapter 1. General Inform

ation1

1-1. Purpose1

1-2. Audience2

1-3. Cancellation2

1-4. Frequently Asked Questions3

Chapter 2. Technical Standard Order Authorization (TSOA) and Airworthiness Approval

Considerations - General5

2-1. Guidance for TSOA/Letter of Desi gn Approval (LODA) and Airworthiness

Approvals5

2-2. TSOA5

2-3. STC-Approved Model List (AML)6

2-4. Type Certificate6

Chapter 3. TSO Authorization7

3-1. TSO-C196(AR)7

3-2. TSO-C145(AR)/C146(AR)8

3-3. TSO-C161a/C162a9

3-4. TSO-C115(AR)10

3-5. Multiple TSO Authorizations13

Chapter 4. Equipment Performance - Advisory Vertical Guidance14

4-1. Introduction14

4-2. Implementations14

4-3. Limitations16

Chapter 5. Equipment Performance - GNSS18

5-1. Introduction18

5-2. GPS (TSO-C129(AR) and TSO-C196(AR) receivers)19

5-2.1 Antennas19

5-2.2 Sensors20

5-2.3 Prediction Program21

5-3. GPS/SBAS (TSO-C145/C146(AR) receivers)22

5-3.1 Antenna22

5-3.2 Sensor and Sensor/Navigation Computer Configuration23

5-3.3 Navigation Database - Data Conversion for GPS/SBAS LNAV/VNAV

Approaches24

5-4. GPS/GBAS (TSO-C161a/C162a receivers)25

5-4.1 Antennas25

5-4.2 VHF Data Broadcast Recei ving Equipment (TSO-C162a)25

5-4.3 Sensor/Navigation Computer (TSO-C161a)25

5-5. GNSS Upgrades26

5-5.1 Adding Additional GNSS Equipment26

ii

Table of Contents (Continued)

5-5.2

Replacing GNSS Equipment27

5-5.3 GPS/SBAS Upgrade27

5-5.4 GNSS Antenna Re-Use/Replacement28

5-5.5 Marking29

5-6. Navigation Database29

5-6.1 Database Installation Instructions29

5-6.2 Database Process30

5-7. GNSS Geometric Altitude30

5-8. Terrain Awareness and Warning System (TAWS)30

Chapter 6. Equipment Performance - RNAV Multi-Sensor Equipment31

6-1. Introduction31

6-2. Navigation Database31

6-2.1 Database Installation Instructions31

6-2.2 Database Process31

6-3. GNSS31

6-4. DME/DME32

6-4.1 Reasonableness Checks35

6-4.2 Performance Confirmation36

6-4.3 Multi-Sensor System FMS37

6-5. VOR/DME37

6-6. INS/IRU40

6-7. INS/IRU – GNSS Integration40

6-8. INS/IRU - DME/DME Integration42

Chapter 7. Equipment Performance - RNP (General)44

7-1. Introduction44

7-2. Special Characteristics of RNP Instrument Approaches45

7-3. Special Characteristics of RNP 1 Operations46

Chapter 8. Equipment Performance - RNP Approach47

8-1. Introduction47

8-2. Aircraft and System Requirements47

8-3. Performance and Functional Requirements for RNP Systems48

8-4. System Eligibility and Approval for RNP Operations53

Chapter 9. Equipment Performance - RNP Terminal56

9-1. Introduction56

9-2. Aircraft and System Requirements for RNP Terminal Operations56

9-3. System Performance Monitoring and Alerting57

9-4. System Eligibility and Approval for RNP 1 Operations61

Chapter 10. Equipment Performance - Baro-VNAV63

10-1. General Limitations for Operations under IFR63

10-2. Baro-VNAV Equipment Performa nce for Operations under IFR63

iii

Table of Contents (Continued)

Chapter 11. General Installation Considerations.

.68

11-1. Introduction68

11-2. Failure Classification68

11-3. Software Considerations69

11-4. Hardware Considerations69

11-5. Installed Equipment Compatibility.......................................................................................6 9

11-6. Installation Instructions70

11-7. Environmental Considerations70

11-8. General Human Factors Considerations71

11-9. Magnetic Variation Data and Onboard Database73

Chapter 12. General Installation Considerations - Antennas74

12-1. Antenna placement74

12-2. Anti-Ice Protection75

12-3. Antenna Cables75

12-4. Structure75

Chapter 13. General Installation Considerations - Sensors76

13-1. Introduction76

13-2. Positioning and Navigation Configuration Strapping76

13-3. Navigation Source Selection76

13-4. Dual Positioning and Na vigation Installations77

13-5. Intermixing RNAV equipment77

13-6. Interface to FGS78

13-7. Interface to Magnetic/True Switch. ......................................................................................7 9

13-8. Interface to Air Data and Inertial Reference Systems80

13-9. Antenna/Sensor Compatibility80

13-10. Interference - Electromagne tic Compatibility (EMC)81

13-11. Interface Requirements81

Chapter 14. Installation Considerations - GNSS83

14-1. Introduction83

14-2. Navigation Displays83

14-3. Failure Indications84

14-4. Documenting Effects of GNSS Outage85

14-5. GPS Equipment85

14-6. GPS/SBAS Equipment86

14-6.1 Limitations86

14-6.2 LP Approach87

14-6.3 Update Rate87

14-6.4 Performance Monitoring87

14-6.5 System Status88

14-6.6 Annunciations88

14-6.7 Accessibility89

iv

Table of Contents (Continued)

14-6.8 GPS/SBAS

Glideslope Low Deviation Alerting Function89

14-7. GPS/GBAS VDB Receiver...................................................................................................8 9

14-7.1 Interference – EMC90

14-7.2 Compatibility with Other Systems90

14-7.3 VDB Aircraft Im plementation Loss90

14-8. GPS/GBAS Navigation Computer Receiver91

14-8.1 GPS/GBAS Equipment Intended for Category I91

14-8.2 GPS/GBAS Stand-Alone or MMR E quipment Intended for Category I92

14-8.3 Performance and Performance Monitoring92

14-8.4 Integrity92

14-8.5 Continuity of Navigation93

14-8.6 Fault Detection and Annunciation93

14-8.7 System Status93

14-8.8 Annunciation93

14-8.9 Accessibility94

14-8.10 Glidepath Capture94

14-8.11 Equipment Qualification94

14-8.12 Interoperability95

14-8.13 GPS/GBAS Glideslope Low Deviation Alerting Function95

14-9. Special Use Mission Equipm ent - Not for Navigation95

Chapter 15. Installation Considerations - RNAV Multi-Sensor Equipment96

15-1. Introduction96

15-2. Interface to Primary Navigation Display96

15-2.1 Non-numeric Deviation Displays97

15-2.2 Map Displays97

15-3. Interface to Remote Annunciator97

15-4. Unique Software Considerations98

Chapter 16. Installation Considerations - RNP99

16-1. Introduction99

16-2. Flight Technical Error99

16-3. Radius to Fix (RF) Legs99

16-4. Interface to FGS100

Chapter 17. Installation Considerations - Baro-VNAV101

17-1. Failure Protection101

17-2. Manufacturer’s Instructions101

17-3. Annunciations101

17-4. Software Changes101

17-5. Integration with GNSS-Pr ovided Vertical Guidance102

17-6. Glidepath Low Deviation Alerting Function103

Chapter 18. Installed Performance - Data Submittal105

18-1. Introduction105

v

Table of Contents (Continued)

18-2. Project

Specific Certification Plan105

18-3. TSO Technical Data105

18-4. Review the installation manual106

18-5. Structural Analysis106

18-6. Power Supply107

18-7. Environment107

18-8. Database107

18-9. Operations manual or AFMS/RFMS107

18-10. Stand-Alone Navigation Equipment107

18-11. Ground and Flight Test Plans (f unctional and EMI/EMC testing)108

Chapter 19. Installed Performance - Test (General)109

19-1. Intended Function109

19-2. Power Supply109

19-3. Accuracy109

19-4. Human Factors109

Chapter 20. Installed Performance - Test (GNSS)110

20-1. Ground Test - GPS, GPS/SBAS, and GPS/GBAS110

20-1.1 Interference110

20-1.2 Antenna to Aircraft Navi gation Reference Offset110

20-1.3 Sensor Installation111

20-1.4 Stand-Alone Navigation Equipment Installation111

20-2. Flight Test - GPS, G PS/SBAS, and GPS/GBAS111

20-2.1 Equipment Installation111

20-2.2 LPV and Precision Approaches112

20-2.3 GPS/GBAS VDB Receiver Installation113

Chapter 21. Installed Performance - Test (RNAV Multi-Sensor Equipment)114

21-1. Ground Test114

21-2. Flight Test114

21-2.1 Interface to FGS115

21-2.2 GNSS Precision Approach116

Chapter 22. Installed Performance - Test (Baro-VNAV)117

22-1. Introduction117

22-2. Ground Test117

22-2.1 Lab/Bench Tests and Equipment Data Evaluation117

22-2.2 Aircraft Installati on Data Evaluation117

22-3. Flight Test118

22-3.1 Functional Flight Test118

22-3.2 Navigation Error Flight Test. .........................................................................................11 9

22-4. Follow-on Approval120

vi

Table of Contents (Continued)

Appendix 1. GPS Oce

anic/Remote NavigationA1-1

A1-1. GPS Oceanic/Remote NavigationA1-1

A1-2. Approval ProcessA1-1

A1-3. Equipment Performance. ................................................................................................. A1 -2

Appendix 2. RNP AR OperationsA2-1

A2-1. IntroductionA2-1

A2-2. Performance Requirements.............................................................................................. A2- 2

A2-3. RNP AR General RequirementsA2-4

A2-4. Requirements for RNP AR Approaches with RF LegsA2-15

A2-5. Requirements for RNP AR Approach values less than 0.3A2-15

A2-6. Requirements for Approaches with a Missed Approach Less Than RNP 1A2-15

A2-7. Navigation Data Va lidation ProgramA2-17

A2-8. Installation Considerations for RNP ARA2-20

Appendix 3. RNP Advanced FeaturesA3-1

A3-1. IntroductionA3-1

A3-2. Radius to Fix Leg Capability........................................................................................... A3 -1

Appendix 4. GNSS Tests to Support ADS-BA4-1

A4-1. IntroductionA4-1

A4-2. Velocity Accuracy and Veloc ity Figure of Merit TestsA4-1

A4-3. Horizontal Velocity Accuracy Test Conditions Commensurate with NACV = 1A4-2

A4-4. Horizontal Accuracy Pa ss/Fail DeterminationA4-4

A4-5. Horizontal Velocity Figure of Merit Pass/Fail DeterminationA4-4

A4-6. Vertical Velocity Accuracy Test Conditions Commensurate with NACV = 1A4-5

A4-7. Vertical Velocity Accuracy Pass/Fail DeterminationA4-7

A4-8. Vertical Velocity Figure of Merit Pass/Fail DeterminationA4-7

A4-9. Additional Tests to Demonstr ate Accuracy Commensurate with NACV = 2A4-8

A4-10. Test to Demonstrate HAE Output and Vertical Position Accuracy MetricA4-9

A4-11. Test to Demonstrate Horizontal Position Accuracy MetricA4-10

A4-12. Demonstrating True Track Angl e/True Track Angle ValidityA4-11

Appendix 5. Sample Airplane/Rotorcraft Flight Manual SupplementA5-1

A5-1. IntroductionA5-1

Appendix 6. Installation of GNSS Equipment for VFR Use OnlyA6-1

A6-1. IntroductionA6-1

A6-2. GeneralA6-1

A6-3. ApplicabilityA6-1

A6-4. Technical InstructionsA6-1

Appendix 7. Definitions and AcronymsA7-1

A7-1. DefinitionsA7-1

vii

viii Table of Contents (Continued)

A7-2. AcronymsA7-3

Appendix 8. Related DocumentsA8-1

A8-1. FAA Advisory Circulars.................................................................................................. A 8-1

A8-2. FAA Technical Standard OrdersA8-3

A8-3. RTCA, Inc. Documents (RTCA/DO) documentsA8-4

A8-4. Other DocumentsA8-5

A8-5. How to Get Related DocumentsA8-6

List of Tables

Table 1. Effective Noise Density for All GNSS Sources18

Table 2. Acceptable Standard Antennas29

Table 3. DME Usable Range33

Table 4. 2D Accuracy Requirements for Equipment Usin g a Single Co-located VOR/DME39

Table 5. Error Elements40

Table 6. Vertical Path Performanc e Limit for Baro-VNAV64

Table 7. Vertical Path Deviations65

Table 8. Typical Hazard Classifications68

Table 9. RNP FTE Performance99

Table 10. INS/IRU Accuracy After Loss of Radio Updating115

Table 11. Tailwind Component (VKTW) for Turn CalculationsA2-8

Table 12. Platform Dynamics for Horiz ontal Velocity Accuracy TestA4-3

Table 13. Platform Dynamics for Vertical Velocity Accuracy TestA4-6

List of Figures

Figure 1. Minimum Straight Path Before DAA2-17

Chapter 1. General In formation.

1-1. Purpose.

This advisory circular (AC) provides guidance m aterial for the airworthiness approval of installed positioning and navigation equipment. Positioning and navigation equipment may be used for a variety of functions such as navigation, automatic dependent surveillance, and/or terrain awareness and warning systems. This AC addresses the following equipment:

a. Global positioning system (GPS) sensors or stand-alone navigation equipment, including those incorporating aircraft-based augmentation system (ABAS), satellite-based augmentation system (SBAS), or ground-based augmentation system (GBAS).

Note: For standardization within this document, the acronyms GPS/SBAS and GPS/GBAS are used to indicate SBAS augmenting GPS and GBAS augmenting GPS respectively.

b. Area navigation (RNAV) integrating data from multiple navigation sensors such as global navigation satellite system (GNSS), inertial reference units (IRU), and distance measuring equipment (DME).

c. RNAV intended for required navigation performance (RNP) operations, including advanced functions and RNP authorization required (AR) formerly referred to as special aircraft and aircrew authorization required (SAAAR).

Note: RNP AR was previously named RNP SAAAR. The name has been changed to RNP AR for international harmonization, but might not yet be standardized throughout all documents.

d. Barometric vertical navigation (baro-VNAV) equipment.

e. This AC does not address new satellite constellations that are planned or currently under construction. This AC will be updated when sufficient documentation exists to support multi-constellation equipment.

f. This AC incorporates the airworthiness considerations contained in AC 90-101A, Approval Guidance for RNP Procedures with AR, and AC 90-105, Approval Guidance for RNP Operations and Barometric Vertical Navigation in the U.S. National Airspace System, as a necessary first step to consolidating all the positioning and navigation equipment and RNP airworthiness guidance into one AC. Future AC 90-101 and AC 90-105 revisions will remove the airworthiness guidance. This AC will not replace the operational guidance found in the RNP 90-series ACs. However, this AC does replace the ACs listed in paragraph 1-3 by incorporating and updating the information they contained.

g. This AC is not m andatory and is not a regulation. This AC describes an acceptable means, but not the only means, to comply with applicable regulations.

h. This AC provides guidance information intended for new approvals. This AC is not intended to modify, change or cancel existing equipment design or airworthiness approvals.

Equipment with existing approvals can continue to be installed within the provisions of their original design and airworthiness certification.

i. The guidance in this AC cannot ensure compliance with Title 14 of the Code of Federal Regulations (CFR) parts 23, 25, 27, and 29 airworthiness regulations for every possible installation configuration. The applicant remains responsible for regulatory compliance and should work closely with their geographic aircraft certification office (ACO) to ensure regulatory compliance. Mandatory terms such as “must” used in this AC are only applicable when following the compliance method outlined.

j. In lieu of following this method without deviation, the applicant may elect to follow an alternate method, provided the alternate method is also found by the Federal Aviation Administration (FAA) to be an acceptable means of complying with the requirements of the federal aviation regulations 14 CFR.

1-2. Audience.

This AC is for aircraft m anufacturers, avionics manufacturers, installation shops, or other applicants seeking design and/or airworthiness approval for the positioning and navigation systems described in this document.

1-3. Cancellation.

a. This AC supersedes the ACs listed b elow. Equipment previously approved per the guidance in the superseded ACs is still valid for the operations and conditions stated in their approvals. The following documents are cancelled:

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

(2) AC 20-130A, Airworthiness Approval of Navigation or Flight Management Systems Integrating Multiple Navigation Sensors.

(3) AC 20-138B, Airworthiness Approval of Positioning and Navigation Systems.

(4) AC 25-4, Inertial Navigation Systems.

b. This AC supersedes the airworthiness considerations in AC 90-45A, Approval of Area Navigation Systems for use in the U.S. National Airspace System. Equipment previously approved per the airworthiness guidance in AC 90-45A is still valid for the operations and conditions stated in their approvals.

1-4. Frequently Asked Questions.

This paragraph contains a list of questions that have been raised since AC 20-138B was first published. The responses to these questions are intended to help applicants interpret the guidance contained in this AC.

a. Ground Proxim ity Warning System/Terrain Awareness and Warning System (GPWS/TAWS) Mode 5 alert.

(1) Question: Is a GPWS or Class A TAWS Mode 5 alert a hard requirement to add localizer performance with vertical guidance (LPV) or GBAS landing system (GLS) capability?

(2) Answer: No. It is not the FAA’s intent to burden installers by changing the GPWS or TAWS functions; impacting the GPWS or TAWS technical standard order authorization (TSOA); or, requiring that either GPWS or TAWS become the only means to provide the function. The intent is to promote the GPWS/TAWS safety benefits by including a comparable alerting function during LPV or GLS approaches for deviations below glideslope.

Using the GPWS or TAWS Mode 5 alerting to provide the function is certainly acceptable, but is not required as the only acceptable method. See paragraphs 14-6.8 and 14-8.13 for clarification and additional guidance.

b. Using the term vertical navigation (VNAV).

(1) Question: Doesn’t RTCA/DO-229D assign a very specific meaning to the term ‘VNAV’ and dictate the term’s specific uses for SBAS-based vertical navigation?

(2) Answer: No. RTCA/DO-229D, section 2.2.1.1.7 provides a recommended usage for the ‘VNAV’ label, but only requires that the label be used in a consistent manner.

‘VNAV’ can be used for a lateral navigation/vertical navigation (LNAV/VNAV) approach procedure generated by GPS/SBAS-based or barometric-based vertical navigation capability (GPS/GBAS does not provide an LNAV/VNAV approach capability). The ‘VNAV’ label can also be used for an advisory vertical navigation function that provides vertical path deviation guidance indications on a non-essential, not-required basis. Refer to the following AC 20-138C chapters for additional information:

 Chapter 4, Advisory Vertical Guidance;

 Chapter 10, Baro-VNAV;

 Chapter 11, section 11-8, General Human Factors Considerations;

 Chapter 14, section 14-6.6, GPS/SBAS Equipment Annunciations; and,  Chapter 17, section 17-5, Baro-VNAV integration with GNSS-Provided Vertical Guidance.

c. Tightly-coupled inertial navigation system/global positioning system (INS/GPS).

(1) Question: AC 20-138B, section 6-7 c. includes the following statement: "The FAA assumes that there is a limitation on the use of the GPS/inertial position data output to support enroute through lateral navigation (LNAV) approach only." Can you please explain the basis of this assumption? It appears to exclude the use of tightly coupled INS/GPS for oceanic use. Is this the intent? Or, is the intent to exclude it for use for LPV?

(2) Answer: Paragraph 6-6 addresses loosely-coupled systems and paragraph 6-7 addresses tightly-coupled systems. There is no intent to exclude either loosely-coupled or tightly-coupled INS/GPS from oceanic use. However, un-augmented GPS, even with tightlycoupled INS, is not adequate for vertical approach capability, and hybrid inertial vertical approach capability is not addressed in RTCA/DO-229D, appendix R. As stated in this AC, a GPS-based vertical approach capability requires GPS/SBAS or GPS/GBAS augmentation.

d. Flight management system (FMS) limitations.

(1) Question: AC 20-130A, section 5.d.(2) contained a limitation on FMS use and required an alternate means of navigation for the route flown. AC 20-138B does not include this section or make a mention of this limitation. What was the reasoning behind not including a similar section in AC 20-138B? Is there some higher level guidance that addresses this or is it left up to the safety analysis to address the need for an alternate means of navigation?

(2) Answer: It is important to differentiate between operational limitations and equipment performance limitations. AC 20-138C addresses equipment performance limitations while the 90-series ACs produced by Flight Standards address operational limitations (for example, 90-100, 90-101A, 90-105 for RNAV, RNP AR, and RNP respectively) as well as the Airman's Information Manual and operational rules (parts 91, 121, 135).

(a) AC 20-130A was written prior to implementing performance-based navigation concepts and the evolving RNAV/RNP-based National Airspace System. An FMS incorporating multiple navigation sensor inputs by its very nature has multiple sources of navigation, but the individual sensors have performance limitations on their navigation capabilities. Accordingly, the FMS functions will meet different requirements depending upon the sensors involved and the intended navigation capability. AC 20-138C addresses the performance limitations of the individual sensors contained in the document. These performance limitations must be considered for the overall intended function at the aircraft level during the airworthiness approval (i.e., during the type certificate (TC), amended type certificate (ATC), or supplemental type certificate (STC) approval) to determine any limitations on the aircraft.

(b) In summary, operational rules dictate the navigation equipage requirements.

It is up to the operator to determine how to comply based on their specific operating rules and mission needs.