20 design data to determine if all error sources are taken into consideration, or if the position source limits the HPL output, when computing an unaugmented Receiver Autonomous Integrity Monitoring (RAIM) based HPL. This applies to all TSO-C129() and TSO-C196() position sources, and to TSO-C145() and TSO-C146() position sources when operating in unaugmented modes where the HPL is based on RAIM. This may apply to some position sources even when operating in an augmented mode.
If the position source does not account for all errors or accomplish the appropriate HPL limiting, you must ensure you interface the position source to ADS-B equipment that limits the NIC ≤ 8. Refer to section
4.5.6Appendix B, of this AC for additional information regarding HPL considerations.
3.3.3.6 Navigation Accuracy Category for Position (NACP).
The ADS-B equipment must set the NACP based on the real-time 95-percent accuracy metric provided by the position source. When interfacing GNSS sources, the NACP should be based on a qualified Horizontal Figure of Merit (HFOM).
3.3.3.7 Navigation Accuracy Category for Velocity (NACV).
Set the NACV based on design data provided by the position source manufacturer. The NACV may be updated dynamically from the position source, or set statically based on qualification of the position source.
3.3.3.7.1 A NACV = 1 (< 10 m/s) may be permanently set at installation for GNSS equipment passing the tests identified in 4.5.6Appendix B of this AC, or may be set dynamically from velocity accuracy output of a position source qualified in accordance with the guidance in 4.5.6Appendix B.
3.3.3.7.2 A NACV = 2 (< 3 m/s) mustbe set dynamically from velocity accuracy output of a position source qualified in accordance with the
4.5.6Appendix B guidance. Do not permanently pre-set a NACV = 2 at installation, even if the position source has passed the tests identified in
4.5.6Appendix B.
3.3.3.7.3 A NACV = 3 or NACV = 4 should not be set based on GNSS velocity accuracy unless you can demonstrate to the FAA that the velocity accuracy actually meets the requirement.
3.3.3.8 Geometric Altitude.
Ensure the geometric altitude provided by the position source is based on Height-Above-Ellipsoid (HAE) instead of Height-Above-Geoid (HAG).
Do not interface a position source that provides HAG or Mean Sea Level (MSL) altitude to the ADS-B equipment unless the ADS-B equipment has the ability to determine the difference between an HAG and HAE input, and the ADS-B equipment has demonstrated during design approval that it can properly convert HAG to HAE using the same model as the position
21 source. It would also be acceptable to demonstrate that the error due to conversion of HAG to HAE does not cause the GVA to be exceeded.
3.3.3.9 Geometric Vertical Accuracy (GVA).
Set the GVA based on design data provided by the position source manufacturer. GNSS position sources may provide the geometric altitude accuracy through the Vertical Figure of Merit (VFOM). If the position source does not output a qualified vertical accuracy metric, the GVA parameter should be set to “0”.
3.3.3.10 Ground Track Angle.
For installations that do not have heading information available, ground track from the position source must be transmitted while on the surface.
Many position sources will provide accurate ground track information, but the ground track may only be accurate above certain ground speeds. If the position source ground track is inaccurate below a certain ground speed and the position source does not inhibit output of the ground track at these slower speeds, the installer should ensure the ADS-B equipment has the capability to invalidate the ground track when the GNSS ground speed falls below 7 knots. Erroneous ground track readings could be misleading for ATC surface operations and ADS-B IN applications. If the position source itself inhibits output of ground track at slower speeds where the ground track would be inaccurate, the installer may interface the position source ground track to the ADS-B equipment without any restrictions.
3.4 Barometric Altitude Source.
3.4.1 Equipment Eligibility.
3.4.1.1 Use barometric altitude from a barometric altimeter that meets the minimum performance requirements of 14 CFR 91.217. The following three subparagraphs are each an acceptable means of compliance.
3.4.1.1.1 TSO-C10, Altimeter, Pressure Actuated, Sensitive Type (any revision)
3.4.1.1.2 TSO-C106, Air Data Computer (any revision)
3.4.1.1.3 Ensure the equipment was tested and calibrated to transmit altitude data corresponding within 125 feet (on a 95-percent probability basis) of the indicated or calibrated datum of the altimeter normally used to maintain flight altitude, with that altimeter referenced to 29.92 inches of mercury for altitudes from sea level to the maximum operating altitude of the aircraft.
3.4.1.2 If appropriate, use a digitizer meeting the minimum performance requirements of any revision of TSO-C88, Automatic Pressure Altitude Reporting Code-Generating Equipment.
3.4.2 Installation Guidance.
3.4.2.1 The barometric altitude used for the ADS-B broadcast must be from the same altitude source as the barometric altitude used for the ATC transponder Mode C reply, if an altitude-encoding transponder is installed in the aircraft.
3.4.2.2 § 91.225 and § 91.227 do not alter any existing regulatory guidance regarding the barometric altitude accuracy or resolution. For example, if an operation requires a 25-foot altitude resolution or a part 91, appendix G (Operations in Reduced Vertical Separation Minimum (RVSM) Airspace) accuracy, that resolution and accuracy should be reflected in the ADS-B message.
3.4.2.3 If a secondary altitude source is used when a secondary transponder is selected or a secondary altitude source is selected for a single transponder, the altitude source for ADS-B must also be changed so the altitude source remains the same for both the transponder and ADS-B system.
3.4.3 Configuration of Associated Parameters.
This section provides additional guidance on setting key ADS-B OUT parameters.
Definitions for each of the following associated parameters are included in
4.5.6Appendix A:
3.4.3.1 Barometric Altitude.
The ADS-B equipment must update the barometric altitude based on the real-time barometric altitude provided by the barometric altitude source.
3.4.3.2 Barometric Altitude Integrity Code (NICBARO).
You should verify the type of altitude source installed in the aircraft and interface the altitude system per the ADS-B equipment manufacturer’s instructions. For aircraft with an approved, non-Gillham altitude source, NICBARO should be preset at installation to “ONE”. For aircraft with a Gillham altitude source without an automatic cross-check, NICBARO must be preset at installation to “ZERO”. For aircraft that dynamically crosscheck a Gillham altitude source with a second altitude source, the NICBARO must be set based on the result of this cross-check. We recommend that ADS-B installations use non-Gillham altitude encoders to reduce the potential for altitude errors.
3.5 Heading Source.
3.5.1 Equipment Eligibility.
For installations that integrate heading on the airport surface, the heading source must meet the minimum performance requirements of any revision of TSO-C5, Direction Instrument, Non-Magnetic (Gyroscopically Stabilized), or any revision of TSO-C6, Direction Instrument, Magnetic (Gyroscopically Stabilized). The equipment must have the appropriate installation and airworthiness approval.
3.5.2 Installation Guidance.
3.5.2.1 The heading does not need to come from the same source as the position and velocity.
3.5.2.2 Interfacing heading is not required, but is highly encouraged if the aircraft has an approved heading source.
3.5.3 Configuration of Associated Parameters.
When the aircraft is on the surface, the ADS-B system is required to transmit either heading or ground track; however, we recommend transmitting heading if a source of heading information is available and valid. True heading is preferred, but magnetic heading is acceptable. Ensure the heading type (true or magnetic) interfaced to the ADS-B equipment matches the heading type transmitted from the ADS-B equipment.
3.6 TCAS Source.
3.6.1 Equipment Eligibility.
TCAS II systems should comply with TSO-C119a, Traffic Alert and Collision Avoidance System (TCAS) Airborne Equipment, TCAS II, or subsequent version, and be installed in accordance with AC 20-131A, Airworthiness Approval of Traffic Alert and Collision Avoidance Systems (TCAS II) and Mode S Transponders, or any revision of AC 20-151, Airworthiness Approval of Traffic Alert and Collision Avoidance Systems (TCAS II), Versions 7.0 and 7.1 and Associated Mode S Transponders, as applicable.
No ADS-B interface is available or required for TCAS I systems.
Note: Many aircraft will be equipped with a Mode S transponder with ADS-B functionality and a TCAS II. The Mode S transponder is considered to be a component of the TCAS II system and also a component of the ADS-B system.
3.6.2 Installation Guidance.
3.6.2.1 TCAS II Interface.
TCAS II is not a required part of the ADS-B system; however, if TCAS II is installed on your aircraft, the equipment must be integrated so the “TCAS installed and operational” and the “TCAS traffic status” parameters indicate the real-time status of the TCAS II.
3.6.2.2 TCAS II Hybrid Surveillance.
If an ADS-B IN system is installed in an aircraft equipped with a TCAS II hybrid surveillance system compliant with RTCA/DO-300(), Minimum Operational Performance Standards (MOPS) for Traffic Alert and Collision Avoidance System II (TCAS II) Hybrid Surveillance, the TCAS II will use ADS-B IN position data to reduce the interrogation rates of low-threat intruders. The information transmitted by ADS-B OUT systems installed in accordance with the guidance in this AC is suitable for use by TCAS II hybrid surveillance. Refer to AC 20-151() for more information on hybrid surveillance.
3.6.2.3 TCAS Messages.
The ADS-B transmission of the “TCAS operational” or “TCAS Resolution Advisory (RA) active” messages does not increase the hazard level of the ADS-B equipment defined in TSO-C166b or TSO-C154c.
3.6.3 Configuration of Associated Parameters.
This section provides additional guidance on setting key ADS-B OUT parameters.
Definitions for each of the following associated parameters are included in
4.5.6Appendix A of this AC.
3.6.3.1 TCAS Installed and Operational.
This parameter must interface with the TCAS II system if a TCAS II system is installed on your aircraft. This parameter should be preset to “ZERO” if a TCAS II is not installed in your aircraft or if a TCAS I is installed in your aircraft. Typically, this parameter will already be provided to the Mode S transponder from the TCAS II. TCAS II systems compliant with TSO-C119() indicate they are operational and able to issue an RA when they transmit Reply Information (RI) = 3 or 4 to the transponder.
3.6.3.2 TCAS Traffic Status.
This parameter must be interfaced with the TCAS II system if a TCAS II system is installed on your aircraft. The TCAS traffic status parameter can be preset to “ZERO” in accordance with the ADS-B equipment manufacturer’s instructions if a TCAS II is not installed.
3.7 Pilot Interface.
3.7.1 Equipment Eligibility.
There are no unique equipment requirements.
3.7.2 Installation Guidance.
3.7.2.1 System Status.
The installation must have a method to display system operational status to the flightcrew, and should be consistent with the overall flightdeck design philosophy. The system must display flightcrew inputs such as Mode 3/A code, emergency codes, IDENT, and call sign. If an existing transponder is used to input Mode 3/A codes, emergency codes, and IDENT into the ADS-B system, the current transponder control interface is sufficient. The following two failure annunciations must be included in the initial airworthiness certification (that is, STC or TC) type design data for the ADS-B OUT equipment, and should be consistent with the overall flightdeck design philosophy for surveillance equipment. These failure conditions are advisory only and do not constitute a caution or warning condition. For legacy Mode C installations that are adding a UAT device, the following two failure annunciations are optional.
3.7.2.1.1 ADS-B Device Failure.
If the ADS-B equipment is unable to transmit ADS-B messages, the system should provide an appropriate annunciation to the flightcrew.
3.7.2.1.2 ADS-B Function Failure.
The ADS-B system depends on a position source to provide the data to populate the ADS-B messages and reports. If the position source or its interface with the ADS-B equipment fails, the ADS-B system will not be able to broadcast the required ADS-B data. In this case, the ADS-B equipment has not failed, but it cannot perform its function due to a failure to receive the position source data. The ADS-B system should distinguish between a position source or interface failure and an ADS-B equipment failure. The installer must provide documentation, in the applicable flight manual, or flight manual supplement, that explains how to differentiate between annunciation of an equipment failure and a function failure if the failure annunciations are not independent. The ADS-B function failure must not cause a TCAS II system failure.
Note: Certain advanced ADS-B IN applications may require flightcrew knowledge of own-ship ADS-B OUT operational status. Refer to AC 20-172() for guidance regarding ADS-B IN installations.
3.7.2.2 Turning Off ADS-B.
14 CFR 91.225 and § 91.227 requires that all aircraft equipped with ADS-B OUT operate with the equipment turned on at all times. There are no requirements to disable ADS-B broadcasts at the request of ATC.
When ADS-B functionality resides in the Mode S transponder, it is acceptable to disable the ADS-B transmissions by disabling the transponder (that is, “Standby” or “Off”). If this architecture is used, specify the impact in the flight manual or pilot’s guide (for example, loss of ADS-B, transponder, and TCAS functionality). Locate the ADS-B on/off controls to prevent inadvertent actuation.
3.7.2.3 Anonymity Feature.
§ 91.227 contains specific provisions allowing operators with TSO-C154c equipment to transmit a self-assigned (randomized) temporary 24-bit address and no call sign. No such provision is provided for TSO-C166b equipment. After January 1, 2020, and in the airspace identified in § 91.225, the UAT anonymous 24-bit address feature may only be used when the operator has not filed a flight plan and is not requesting ATC services. The UAT call sign may also be omitted, but only when the anonymous 24-bit address is chosen. We do not recommend integrating the anonymity features, as the operator will not be eligible to receive ATC services, may not be able to benefit from enhanced ADS-B search and rescue capabilities, and may impact ADS-B IN situational awareness benefits. The following considerations must be included in the ADS-B system design when installing equipment capable of using the anonymity feature:
3.7.2.3.1 When the ADS-B equipment is initially powered-on, the 24-bit address must default to the aircraft’s assigned ICAO 24-bit address.
3.7.2.3.2 When the ADS-B equipment is initially powered-on, the call sign may not be blank (Not Available per RTCA/DO-282()). At initial power-on, it is acceptable for the call sign to revert to a non-blank call sign that existed before the ADS-B equipment being powered off, or to the aircraft registration number.
3.7.2.3.3 The ADS-B equipment can only allow an anonymous 24-bit address selection if the Mode 3/A code is set to “1200”.
3.7.2.3.4 The ADS-B equipment may only allow selection of the anonymous 24-bit address via a dedicated pilot interface. The ADS-B OUT equipment may not automatically set an anonymous 24-bit address or set a blank (Not Available per RTCA/DO-282()) call sign based solely on pilot selection of the 1200 Mode 3/A code.
3.7.2.3.5 The ADS-B OUT equipment must automatically disable the anonymity feature if any Mode 3/A code other than 1200 is selected. The 24-bit address must automatically revert to the aircraft’s assigned ICAO 24-bit address. If the call sign was blank, the call sign must automatically revert to the aircraft registration number.
3.7.2.3.6 Describe the effects of selecting the anonymity features in the flight manual or pilot’s guide. Effects include the inability to receive Instrument Flight Rule (IFR) or Visual Flight Rule (VFR) separation services, potential loss of enhanced search and rescue benefits, and potential negative impacts to ADS-B IN applications.
3.7.3 Configuration of Associated Parameters.
27 This section provides additional guidance on setting key ADS-B OUT parameters.
Definitions for each of the following associated parameters are included in
4.5.6Appendix A of this AC.
3.7.3.1 Call Sign/Flight ID.
The assigned aircraft registration number must be set as the call sign/flight ID during installation. Procedures for dynamically selecting a call sign must be included in the flight manual or pilot’s guide if the ADS-B equipment provides a means to input a radio telephony call sign. If pilot-selectable, the call sign/flight ID should be readily apparent to the flightcrew. When the aircraft system is powered on, the call sign/flight ID must be filled. At initial power-on it is acceptable for the call sign/flight ID to revert to a previously set call sign that existed before the system being powered off, or to the aircraft registration number. Refer to section
3.7.2.3 of this AC for information on use of the anonymity feature.
Note: The preset call sign/flight ID will have to be updated if the aircraft’s registration number changes.
3.7.3.2 Emergency Status.
The installation must provide a means for the pilot to enter the emergency status of the aircraft. Although TSO-C166b and TSO-C154c identify multiple emergency codes, only the codes for general emergency, no communications, and unlawful interference are required to be available for broadcast. It is acceptable to base the ADS-B emergency status on the emergency status code input into the transponder (that is, Mode 3/A codes 7500, 7600, and 7700). Refer to section 3.7.3.5 of this AC for information on single point of entry of the emergency status.
3.7.3.3 IDENT.
The installation must provide a means for the pilot to enter the IDENT feature. Refer to section 3.7.3.5 of this AC for information on single point of entry of the IDENT.
3.7.3.4 Mode 3/A Code.
The installation must provide a means for the pilot to enter the Mode 3/A code. Refer to section 3.7.3.5 of this AC for information on single point of entry of the Mode 3/A code.
3.7.3.5 Single Point of Entry.
Aircraft equipped with a separate transponder and ADS-B system should provide the pilot a single point of entry into both systems for the Mode 3/A code, IDENT, and emergency status. If ADS-B equipment sets the emergency status, IDENT, or Mode 3/A code based on entry of these parameters into a separate transponder, the STC/TC needs to identify the appropriate transponder interfaces. Experience in the FAA’s Alaska CAPSTONE program demonstrated that operator mitigations to prevent
28 differing codes from being entered in the transponder and ADS-B system were ineffective and resulted in numerous false and misleading proximity alerts for ATC. Additionally, there are workload and safety concerns of requiring the pilot to enter the Mode 3/A code, IDENT, and emergency codes multiple times. Thus, if you do not provide a single point of entry for the mode 3/A code, IDENT, and emergency code, you must accomplish a human factors evaluation and an additional system safety assessment as follows:
3.7.3.5.1 Human Factors Evaluation.
Installations not providing a single point of entry must accomplish an evaluation of the pilot interface controls to ensure the design minimizes the potential for entry errors by the flightcrew, and enables the flightcrew to detect and correct errors that do occur. Evaluate the system interface design to ensure dual entry of the emergency status, IDENT, and Mode 3/A code does not introduce significant additional workload, particularly when communicating an aircraft emergency. Refer to section 4.1.5.4 of this AC for additional information on the human factors evaluation.
3.7.3.5.2 System Safety Assessment.
Transmission of false or misleading information is considered to be a major failure effect and may not occur at a rate greater than 1x10-5 per flight hour for ADS-B systems. Installations not providing a single point of entry must accomplish a safety assessment that demonstrates that the probability of the transponder and ADS-B system ever transmitting differing Mode 3/A codes is less than 1x10-5 per flight hour. The analysis must consider the potential of all pilot errors.
3.8 ADS-B Antenna Interface.
3.8.1 Antenna Location and Number Required.
The aircraft ADS-B antenna is an important part of the overall ADS-B OUT system because antenna systems are major contributors to the system link performance. The location and number of antennas required for the airborne ADS-B OUT system is a function of the equipment class of the selected broadcast link (UAT or 1090ES). Single bottom-mounted antenna (TSO-C166b and TSO-C154c A1S and B1S classes) installations are allowed. For the UAT link, 16 watts minimum transmit power at the antenna output is required. For the 1090ES link, 125 watts minimum transmit power at the antenna output is required.
3.8.2 Equipment Eligibility.
ADS-B antennas must meet requirements defined in the ADS-B equipment manufacturer’s installation manual.
3.8.3 Installation Guidance.
3.8.3.1 Using an Existing Antenna.
When using an existing antenna system, if the installation does not modify the existing antenna(s), cabling, or output specifications, the antenna installation does not have to be reevaluated.
3.8.3.2 Installing a New Shared Transponder/ADS-B Antenna.
Follow the transponder antenna installation guidance in AC 20-151().
3.8.3.3 Installing a New Stand-Alone UAT ADS-B Antenna.
If the UAT system is installed in an aircraft without a transponder or the installation will not use the existing transponder antenna, use the following guidance:
3.8.3.3.1 Antenna Location.
Mount antennas as near as practical to the centerline of the fuselage and locate them in a position to minimize obstruction in the horizontal plane.
3.8.3.3.2 Antenna Distance From Other Antennas.
The spacing between the UAT antenna and any transponder (Mode S or Air Traffic Control Radar Beacon System (ATCRBS)) antenna must provide a minimum of 20 dB of isolation between the two antennas. If both antennas are conventional omni-directional matched quarter-wave stubs, 20 dB of isolation is obtained by providing a spacing of at least 20 inches between the centers of the two antennas. If either antenna is other than a conventional stub, the minimum spacing must be determined such that 20 dB or more of isolation is achieved.
3.8.3.3.3 Transmit Power.
Transmit power will be verified during ground test.
3.8.3.3.4 Structural Analysis.
You may need to submit a structural analysis of new antenna installation to show compliance with the applicable regulations.
3.8.3.4 Antenna Diplexers.
Diplexers manufactured in accordance with TSO-C154b or TSO-C154c may be installed so UAT ADS-B equipment and a transponder may share the same antenna. The TSO-C154b and TSO-C154c diplexer installation instructions are required to have a limitation that ensures insertion of the diplexer does not exceed the maximum cable attenuation allowance between the transponder and antenna.
3.8.3.5 Single Antenna.
Single antenna systems must use a bottom-mounted antenna.
3.8.4 Configuration of Associated Parameters.
This section provides additional guidance on setting key ADS-B OUT parameters.
Definitions for each of the following associated parameters are included in
4.5.6Appendix A to this AC.
3.8.4.1 GNSS Antenna Offset and Position Offset Applied (POA).
Although not required to comply with § 91.227, it is highly encouraged for ADS-B equipment manufacturers to provide instructions to installers for setting this parameter and for installers to configure the offset during installation. The GNSS antenna offset information will be extremely valuable for surface ATC surveillance and future ADS-B IN surface situational awareness and surface collision alerting applications.
3.8.4.2 If the ADS-B equipment is interfaced to multiple GNSS position sources that use GNSS antennas in different locations on the aircraft, the installation must have provisions to ensure the appropriate GNSS antenna offset is being transmitted when the ADS-B equipment switches from one position source to another.
3.8.4.3 The POA setting of the GNSS antenna indicates if the broadcast position of the vehicle is referenced to either a) the aircraft’s ADS-B position reference point, or b) the lateral distance from centerline and longitudinal distance from the most forward part of the aircraft, (reference 4.5.6B.4.1).
Note: Either the transmitted position should be adjusted to the reference point described in paragraph 3.8.4.4 OR the GNSS antenna offsets should be provided. It is not required to do both.
3.8.4.4 The ADS-B position reference point is the center of the rectangle used to describe the length and width of the aircraft in the length and width code.
Refer to section 3.2.3.2 and figure 2 of this AC. For a more detailed description of POA, refer to RTCA/DO-338, Minimum Aviation System Performance Standards (MASPS) for ADS-B Traffic Surveillance Systems and Applications (ATSSA), section 3.2.4.1.
31 Figure 3. Position Offset Longitudinal Axis Reported Length (using next higher value) Reported Width (using next higher value) Position of GNSS antenna ADS-B reference point Center of defining rectangle (center of solid box) ● Center of leading edge of Length Width code “box” Antenna longitudinal offset Antenna lateral offset Lateral Axis
3.8.4.5 Single Antenna Bit.
For aircraft using a single antenna, this parameter should be set to one, “True”.
3.8.5 Mutual Suppression.
Follow the ADS-B equipment manufacturer’s guidance on interfacing the ADS-B OUT equipment to the mutual suppression bus.
3.9 Vertical Rate Source.
We recommend that the ADS-B system output the vertical rate field when available.
The vertical rate may come from a barometric air data computer, a GNSS source, or a system that filters barometric and geometric vertical rates. Vertical rate will typically come from a position source or an air-data computer. This section addresses this unique parameter, and augments section 3.3 and 3.4 of this AC, as applicable.
3.9.1 Equipment Eligibility.
Unlike position accuracy, vertical velocity accuracy is not transmitted in ADS-B messages. Thus it is important that vertical velocity sources integrated into the ADS-B