15 TSO-C196.
(6) GPS / UAT time mark synchronization. When integrating a UAT with an external GPS, the design of the hardware time mark must be interoperable. Some GPS synchronize the leading edge of the time mark to the UTC second. Other GPS let the time mark pulse be asynchronous to the UTC second and then record the time of the leading edge in the digital data along with the position solution. The UAT equipment must support the GPS time mark design. If the UAT equipment and GPS do not share a common time mark design, the UAT equipment will not be properly synchronized with the ground system and other aircraft.
c. Configuration of associated parameters. The following paragraphs provide additional guidance on setting key ADS-B OUT parameters. Definitions for each of the following associated parameters are included in appendix 1.
(1) Latitude and longitude. The ADS-B equipment must set the latitude and longitude based on the real-time position information provided by the position source.
(2) Horizontal velocity. The ADS-B equipment must set the horizontal velocity based on the real-time velocity information provided by the position source. The ADS-B equipment must transmit a north/south east/west velocity while airborne and a combination of ground speed and ground track or heading while on the surface. Ensure the position source provides horizontal velocity in both formats or ensure the ADS-B equipment can properly convert between formats.
We recommend transmitting heading instead of ground track while on the surface, see paragraph 3-5c for additional information on interfacing heading.
(3) Source Integrity Level (SIL). SIL is typically a static (unchanging) value and may be set at the time of installation if a single type of position source is integrated with the ADS-B system.
SIL is based solely on the position source’s probability of exceeding the reported integrity value and should be set based on design data from the position source equipment manufacturer. Installations which derive SIL from GNSS position sources compliant with any revision of TSO-C129, TSO-C145, TSO-C146, or TSO-C196 which output Horizontal Protection Level (HPL) or Horizontal Integrity Level (HIL) should set the SIL = 3 because HPL and HIL are based on a probability of 1x10-7 per hour. Do not base NIC or SIL on Horizontal Uncertainty Level (HUL) information. If integrating with a non-compliant GPS, SIL must be set to 0.
(4) Source Integrity Level Supplement (SILSUPP). SILSUPP is based on whether the position source probability of exceeding the reported integrity value is calculated on a per hour or per sample basis and should be set based on design data from the position source equipment manufacturer. ADS-B systems interfaced with a GNSS position source compliant with any revision of TSO-C129, TSO-C145, TSO-C146, or TSO-C196 may preset SILSUPP to ZERO, as GNSS position sources use a per hour basis for integrity.
(5) Navigation Integrity Category (NIC). The ADS-B equipment must set the NIC based on the real-time integrity metric provided by the position source. When interfacing GNSS position sources, the NIC should be based on the HPL or HIL. However, while HPL values significantly smaller than 0.1 nm can be output from single frequency GNSS sources, the HPL
16 may not actually achieve the reported level of protection as there are error contributions that are no longer considered negligible. You should review the position source design data to determine if all error sources are taken into consideration, or if the position source limits the HPL output, when computing an un-augmented 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 un-augmented 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 which limits the NIC ≤ 8. See appendix 2, paragraph 4.f for additional information on HPL considerations.
(6) Navigation Accuracy Category for Position (NACP). The ADS-B equipment must set the NACP based on the real time 95% accuracy metric provided by the position source.
When interfacing GNSS sources, the NACP should be based on a qualified Horizontal Figure of Merit (HFOM).
(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.
(a) A NACV = 1 (< 10 m/s) may be permanently set at installation for GNSS equipment passing the tests identified in appendix 2, or may be set dynamically from velocity accuracy output of a position source qualified in accordance with the appendix 2 guidance.
(b) A NACV = 2 (< 3 m/s) may be set dynamically from velocity accuracy output of a position source qualified in accordance with the appendix 2 guidance. Do not permanently preset a NACV = 2 at installation, even if the position source has passed the tests identified in appendix 2.
(c) 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.
(8) Geometric Altitude. Ensure that 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 that the ADS-B equipment has demonstrated during design approval that it can properly convert HAG to HAE using the same model as the position source. It would also be acceptable to demonstrate that the error due to conversion of HAG to HAE does not cause the reported geometric accuracy to be exceeded.
(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
17 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.”
(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 that 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.
a. Equipment eligibility.
(1) Utilize barometric altitude from a barometric altimeter meeting the minimum performance requirements of:
(a) TSO-C10, Altimeter, Pressure Actuated, Sensitive Type, (any revision) or
(b) TSO-C106, Air Data Computer (any revision).
(2) If appropriate, utilize a digitizer meeting the minimum performance requirements of any revision of TSO-C88, Automatic Pressure Altitude Reporting Code-Generating Equipment.
b. Installation guidance.
(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.
(2) 14 CFR 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 14 CFR part 91 appendix G, Operations in Reduced Vertical Separation Minimum (RVSM), accuracy, that resolution and accuracy should be reflected in the ADS-B message.
(3) If a secondary altitude source is utilized 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 that the altitude source remains the same for both the transponder and ADS-B.
c. Configuration of associated parameters. The following paragraphs provide additional guidance on setting key ADS-B OUT parameters. Definitions for each of the following associated parameters are included in appendix 1.
(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.
(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 which dynamically cross-check 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.
a. 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.
b. Installation guidance.
(1) The heading does not need to come from the same source as the position and velocity.
(2) Interfacing heading is not required, but highly encouraged if the aircraft has an approved heading source.
c. 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 Status Source.
a. Equipment eligibility. TCAS II systems should comply with TSO-C119a, Traffic Alert and Collision Avoidance System (TCAS) Airborne Equipment, or subsequent, 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
19 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.
b. Installation guidance.
(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 that the “TCAS installed and operational” and the “TCAS traffic status” parameters indicate the real-time status of the TCAS II.
(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 utilize 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. See AC 20-151A for more information on hybrid surveillance.
(3) 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.
c. Configuration of associated parameters. The following paragraphs provide additional guidance on setting key ADS-B OUT parameters. Definitions for each of the following associated parameters are included in appendix 1.
(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.
(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.
a. Equipment eligibility. There are no unique equipment requirements.
b. Installation guidance.
(1) System status. The installation must have a method to display system operational status to the flight crew, which is consistent with the overall flight deck design philosophy. The system must display flight crew 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 available to the flight crew.
(a) ADS-B device failure. If the ADS-B equipment is unable to transmit ADS-B messages, the system provides an appropriate annunciation to the flight crew.
(b) 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 then 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. TSO-C166b and TSO-C154c require this condition to be annunciated. 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 either an AFM or Pilot’s Guide, which 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.
(2) Turning off ADS-B. 14 CFR 91.225 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 (i.e. Standby or Off). If this architecture is used, specify the impact in the flight manual or pilot’s guide, (e.g., loss of ADS-B, transponder and TCAS functionality).
Locate the ADS-B on/off controls to prevent inadvertent actuation.
(3) Anonymity feature. 14 CFR 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 14 CFR 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 utilizing the anonymity feature:
(a) When the ADS-B equipment is initially powered-on, the 24-bit address must default to the aircraft’s assigned ICAO 24-bit address.
(b) When the ADS-B equipment is initially powered-on, the call sign may not be blank (Not Available per DO-282()). At initial power-on it is acceptable for the call sign to revert to a non-blank call sign which existed prior to the ADS-B equipment being powered off, or to the aircraft registration number.
(c) The ADS-B equipment can only allow an anonymous 24-bit address selection if the Mode 3/A code is set to 1200.
(d) 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 DO-282()) call sign based solely on pilot selection of the 1200 Mode 3/A code.
(e) 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.
(f) Describe the ramifications of selecting the anonymity features in the flight manual or Pilot’s Guide. Ramifications 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.
c. Configuration of associated parameters. The following paragraphs provide additional guidance on setting key ADS-B OUT parameters. Definitions for each of the following associated parameters are included in appendix 1.
(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 flight crew. When the ADS-B equipment is initially 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 which existed prior to the ADS-B equipment being powered off, or to the aircraft registration number. See paragraph 3-7b(3) 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.
(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
22 emergency status on the emergency status code input into the transponder (i.e. Mode 3/A codes 7500, 7600, and 7700). See paragraph 3-7c(5) for information on single point of entry of the emergency status.
(3) IDENT. The installation must provide a means for the pilot to enter the IDENT feature. See paragraph 3-7c(5) for information on single point of entry of the IDENT.
(4) Mode 3/A code. The installation must provide a means for the pilot to enter the Mode 3/A code. See paragraph 3-7c(5) for information on single point of entry of the Mode 3/A code.
(5) Single point of entry. Aircraft equipped with a 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 CAPSTONE program demonstrated that operator mitigations to prevent 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:
(a) 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 flight crew, and enables the flight crew to detect and correct errors that do occur. Evaluate the system interface design to ensure that dual entry of the emergency status, IDENT, and Mode 3/A code does not introduce significant additional workload, particularly when communicating an aircraft emergency. See paragraph 4-1e(4) for additional information on the human factors evaluation.
(b) 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. 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.
a. Equipment eligibility. ADS-B antennas must meet requirements defined in the ADS-B equipment manufacturer’s installation manual.
b. Installation guidance.
(1) Utilizing an existing antenna. When utilizing 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.
(2) Installing a new shared transponder/ADS-B antenna. Follow the transponder antenna installation guidance in AC 20-151A.
(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 utilize the existing transponder antenna, use the following guidance:
(a) 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.
(b) 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 20dB 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.
(c) Transmit power. Transmit power will be verified during ground test.
(d) Structural analysis. You must submit a structural analysis of new antenna installations to show compliance with the applicable regulations.
(4) Antenna diplexers. Diplexers manufactured in accordance with TSO-C154b or TSO-C154c may be installed so that 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.
(5) Single antenna. Single antenna systems must utilize a bottom mounted antenna.
c. Configuration of associated parameters. The following paragraphs provide additional guidance on setting key ADS-B OUT parameters. Definitions for each of the following associated parameters are included in appendix 1.
(1) GNSS antenna offset and position offset applied (POA). Although not required to comply with 14 CFR 91.227, it is highly encouraged for ADS-B equipment manufacturers to
24 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. If the ADS-B equipment is interfaced to multiple GNSS position sources that utilize 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.
(2) Single antenna bit. For aircraft using a single antenna, this parameter should be set to True.
d. 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 which 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 paragraph 3-3 and 3-4, as applicable.
a. 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 system meet minimum performance requirements at installation. Use the following guidance:
(1) Hybrid vertical rate source. Vertical rate may be taken from a hybrid system which filters barometric vertical rate with an Inertial Reference Unit (IRU) vertical rate and GNSS vertical rate, provided the hybrid system was tested and approved to provide a vertical rate output with an accuracy that is at least as good as barometric vertical rate sources (e.g., TSO-C106). Hybrid vertical rate could come from a Flight Management System (FMS), Air Data and Inertial Reference System (ADIRS), or an IRU. ADS-B equipment should transmit hybrid vertical rate solutions as barometric vertical rates.
(2) Blended vertical rate source. Vertical rate may be taken from a blended system which filters IRU vertical rate and barometric vertical rate, provided the blended system was tested and approved to provide a vertical rate output with an accuracy that is at least as good as barometric vertical rate sources (e.g., TSO-C106). Blended vertical rate could come from a FMS, ADIRS, or an IRU. ADS-B equipment should transmit blended vertical rate solutions as barometric vertical rates.
(3) Barometric vertical rate source. Barometric vertical rate may be taken from an air data computer meeting the minimum performance requirements of any revision of TSO-C106 or a vertical velocity instrument meeting the minimum performance requirements of applicable revisions of TSO-C8, Vertical Velocity Instruments. We recommend you use any revision of a TSO-C106 compliant air data computer if you interface barometric vertical rate to the ADS-B OUT equipment.
(4) GNSS vertical rate source. Geometric vertical rate may be taken from any revision of TSO-C129, TSO-C145, TSO-C146, or TSO-C196 GNSS equipment if the position source has been qualified to provide vertical rate in accordance with appendix 2. Do not interface GNSS vertical velocity if the equipment has not been qualified in accordance with appendix 2.
(5) Inertial vertical rate source. Vertical velocity from an inertial sensor that is not blended with barometric altitude should not be transmitted from the ADS-B system.
(6) Barometric altitude source. ADS-B systems should not derive a barometric altitude rate by sampling barometric altitude measurements. This could lead to misleading vertical velocity information. If barometric vertical rate is not available, use geometric vertical rate.
b. Installation guidance. The vertical rate field can be populated with either barometric vertical rate or geometric vertical rate. There is no requirement to interface multiple vertical velocity sources. We recommend that you use the following priority scheme when selecting or interfacing multiple vertical rate sources:
(1) Hybrid vertical rate or blended vertical rate.
(2) Barometric vertical rate.
(3) GNSS vertical rate.
c. Configuration of associated parameters. The following paragraphs provide additional guidance on setting key ADS-B OUT parameters.
(1) Vertical rate. Interface vertical rate from one or more of the sources listed above.
Ensure the source provides vertical rate in feet per minute, or ensure the ADS-B equipment can recognize the vertical rate basis and convert the vertical rate to feet per minute.
(2) Vertical rate source. The source bit for vertical rate should be coded as barometric when utilizing barometric rate from an air data computer, or when using a blended or hybrid vertical rate. The source bit for vertical rate should only be coded as geometric when using vertical rate from a GNSS source.
3-10. Air-Ground Considerations.
a. The length and width code is required by 14 CFR 91.227, and is only transmitted in the surface position message. Thus, to comply with the rule, the aircraft must automatically determine its air-ground status and transmit the surface position message, which includes the length and width code, when on the ground.
b. For aircraft with retractable landing gear, the air-ground status determination is typically provided through a landing gear weight-on-wheels switch. For aircraft that have fixed-gear, the
26 ADS-B system must still be able to determine the air-ground status of the aircraft. Installations that provide a means to automatically determine air-ground status based on inputs from other aircraft sensors may be acceptable if they can be demonstrated to accurately detect the status. For example, air-ground status may be derived from Weight-on-Wheels (WOW) switch and GPS velocity, OR GPS velocity, an airport database and geometric altitude, OR GPS velocity and airspeed. These algorithms should be tested and validated during the installation approval.
Note: 1: We recommend that any automatic air-ground determination be more robust than just a simple comparison of ground speed to a single threshold value. Field experience has shown that this method can lead to false air-ground status.
Note: 2: Manual selection of the air-ground status is not acceptable.
c. TSO-C112d, Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/Mode S) Airborne Equipment, requires Mode S transponders to inhibit the reply to Mode A/C/S all-call and Mode S-only all-call interrogations on the surface. Mode S transponders with ADS-B functionality will now remain “ON” during surface operations, thus it is imperative that you ensure the transponder interface to the air-ground status is installed correctly and that the transponder does not reply to Mode A/C/S all-call or Mode S-only all-call interrogations on the surface.
Note: In deploying Airport Surface Detection Equipment version X (ASDE-X) at various airports we have found transponder installations that have been improperly wired and therefore inappropriately respond to ATC and TCAS interrogations while on the airport surface.
3-11. Foreign Airspace Requirements.
a. Optional Parameters. If operations are planned in a country that requires parameters not mandated in the US, such as selected heading and selected altitude, follow the ADS-B equipment manufacturer’s installation guidance to interface those parameters. The FAA will cooperate with foreign regulators to ensure compliance with their requirements.