Appendix A A-1 APPENDIX A. MESSAGE ELEMENT DESCRIPTIONS A.1 Purpose.
This appendix provides a description of the message elements that may be contained in an ADS-B OUT message.
A.2 Message Elements.
A.2.1 ADS-B IN Capability.
Two messages indicate the ADS-B IN status of the aircraft. The 1090 ADS-B IN message indicates if the aircraft has the ability to receive 1090ES ADS-B messages installed. The UAT ADS-B IN message indicates if the aircraft has the ability to receive UAT ADS-B messages installed. An indication of ADS-B IN capability is important because TIS-B and ADS-R services are provided specific to an aircraft’s position relative to other aircraft. The FAA may only provide complete TIS-B and ADS-R services to aircraft that indicate they are ADS-B IN capable. ADS-B IN capability is required to be transmitted by § 91.227.
A.2.2 Airspeed.
Optionally, true airspeed or indicated airspeed may be transmitted. The airspeed source should be approved to output airspeed data. An air data computer meeting the minimum performance requirements of TSO-C106 is one acceptable source. Do not interface an airspeed source to the ADS-B that has not been approved for cockpit display.
A.2.3 Barometric Pressure Altitude.
This parameter indicates the aircraft’s barometric pressure altitude referenced to standard sea level pressure of 29.92 inches of mercury or 1013.2 hectopascals. The barometric pressure altitude is required to be transmitted by § 91.227.
A.2.4 Call Sign/Flight ID.
The term “aircraft call sign” is the radiotelephony call sign assigned to an aircraft for voice communications purposes. (This term is sometimes used interchangeably with “flight identification” or “flight ID”). For general aviation aircraft, the aircraft call sign is normally the national registration number; for airline and commuter aircraft, the call sign is usually comprised of the company identification and flight number (and therefore not linked to a particular airframe) and, for the military, it usually consists of numbers and code words with special significance for the operation conducted. The call sign or aircraft registration number is required to be transmitted by § 91.227 except when using the TSO-C154c anonymity feature.
A.2.5 Emergency Status.
This parameter alerts ATC that the aircraft is experiencing emergency conditions and indicates the type of emergency. Applicable emergency codes are found in ICAO
Appendix A A-2 Annex 10 Volume 4, Surveillance Radar and Collision Avoidance Systems. This information alerts ATC to potential danger to the aircraft so it can take appropriate action. Emergency status is required to be transmitted by § 91.227.
A.2.6 Emitter Category.
The emitter category provides an indication of the aircraft’s size and performance capabilities. Emitter categories are defined in TSO-C166b and TSO-C154c. Emitter category is designed primarily to provide information on the wake turbulence that an aircraft produces. Emitter category is required to be transmitted by § 91.227.
A.2.7 Geometric Altitude.
The geometric altitude is a measure of altitude provided by a satellite-based position service and is not affected by atmospheric pressure. Geometric altitude is only available with a GNSS position source. Geometric altitude for ADS-B purposes is the height above the World Geodetic System 1984 (WGS-84) ellipsoid (HAE). Geometric altitude is required to be transmitted by § 91.227.
A.2.8 Geometric Vertical Accuracy (GVA).
The GVA indicates the 95-percent accuracy of the reported vertical position (geometric altitude) within an associated allowance.
A.2.9 GNSS Antenna Offset and Position Offset Applied (POA).
A.2.9.1 The GNSS antenna offset indicates the longitudinal distance between the most forward part of the aircraft and the GNSS antenna, and the lateral distance between the longitudinal center line of the aircraft and the GNSS antenna. Also, refer to section 3.8.4.1 – 3.8.4.3 and Figure 3 of this AC.
A.2.9.2 The POA setting of the GNSS antenna offset indicates that the broadcast position is referenced to the aircraft’s ADS-B position reference point versus the GNSS antenna location. Also, refer to section 3.8.4.1 -
3.8.4.3and Figure 3 of this AC. For further details about POA, refer to RTCA/DO-338, section 3.2.4.1.
A.2.10 Ground Speed.
This parameter is also derived from the position sensor and provides ATC with the aircraft’s speed over the ground. This parameter is reported in the surface position message.
A.2.11 Ground Track Angle.
The ground track angle is the direction of the horizontal velocity vector over the ground. Ground track or heading is required to be transmitted while on the ground to transmit complete velocity information.
Appendix A A-3 A.2.12 Heading.
Heading indicates the direction in which the nose of the aircraft is pointing. There is no heading accuracy metric. Heading or ground track is required to be transmitted while on the ground to transmit complete velocity information.
A.2.13 Horizontal Velocity.
The horizontal velocity provides the rate at which an aircraft changes its horizontal position with a clearly stated direction. Horizontal velocity is provided with the north/south velocity and the east/west velocity parameters while airborne. Horizontal velocity is provided by a combination of the ground speed and heading or ground track while on the surface. TSO-C166b and TSO-C154c require that the north/south velocity, east/west velocity, ground speed, and ground track come from the same source as the position. Heading information may come from a separate source. Horizontal velocity is required to be transmitted by § 91.227.
A.2.14 ICAO 24-bit Address.
The ICAO 24-bit address is a unique address assigned to an aircraft during the registration process. ICAO 24-bit addresses are defined blocks of addresses assigned for participating countries or states worldwide. In the United States, civil aircraft are assigned an address from an encoding scheme based on the aircraft registration number (“N” number). Additional information regarding the 24-bit address can be found in ICAO Annex 10, Part I, Volume III, appendix to Chapter 9, A World-Wide Scheme for the Allocation, Assignment and Application of Aircraft Addresses. The ICAO 24-bit address is required to be transmitted by § 91.227 except when using the TSO-C154c anonymity feature.
A.2.15 IFR Capability.
This parameter existed in TSO-C166a and TSO-C154b compliant equipment, but was removed from TSO-C166b and TSO-C154c equipment.
A.2.16 IDENT.
IDENT is a flag manually set by the pilot at the request of ATC in ATCRBS, Mode S, and ADS-B messages. The pilot manually enables the IDENT state, which highlights their aircraft on the controller’s screen. IDENT is required to be transmitted by § 91.227.
A.2.17 Latitude and Longitude.
These parameters are derived from the position source and provide a geometric based position. Reference all geometric position elements broadcast from the ADS-B unit to the WGS-84 ellipsoid. Latitude and longitude are required to be transmitted by §
91.227.
A.2.18 Length and Width of Aircraft.
This parameter provides ATC and other aircraft with quick reference to the aircraft’s dimensions while on the surface. Aircraft length and width is required to be transmitted by § 91.227.
Appendix A A-4 A.2.19 Mode 3/A Code.
Currently ATC automation relies on the Mode 3/A code to identify aircraft under radar surveillance and correlate the target to a flight plan. The mode 3/A code is a four digit number ranging from 0000 to 7777. Secondary Surveillance Radars (SSR) and ADS-B will concurrently provide surveillance, so the Mode 3/A code is included in the ADS-B OUT message and is required to be transmitted by § 91.227.
Note: ADS-B systems will not transmit the Mode 3/A code if the Mode 3/A code is set to 1000.
A.2.20 Navigation Accuracy Category for Position (NACP).
The NACP specifies the accuracy of the aircraft’s horizontal position information (latitude and longitude) transmitted from the aircraft’s avionics. The ADS-B equipment derives a NACP value from the position source’s accuracy output, such as the HFOM from the GNSS. The NACP specifies with 95 percent probability that the reported information is correct within an associated allowance. A minimum NACP value of “8” must be transmitted to operate in airspace defined in § 91.227. Table A-1 provides the applicable NACP values.
Table A-1. NACP Values NACP Horizontal Accuracy Bound 0 EPU ≥ 18.52 km (10nm) 1 EPU < 18.52 km (10nm) 2 EPU < 7.408 km (4nm) 3 EPU < 3.704 km (2nm) 4 EPU < 1852 m (1nm) 5 EPU < 926 m (0.5nm) 6 EPU < 555.6 m (0.3nm) 7 EPU < 185.2 m (0.1nm) 8 EPU < 92.6 m (0.05nm) 9 EPU < 30 m 10 EPU < 10 m 11 EPU < 3 m
A.2.21 Navigation Accuracy Category for Velocity (NACV).
The NACV is an estimate of the accuracy of the horizontal geometric velocity output.
The coding of “ZERO,” indicating that the accuracy is unknown or either equal to or worse than 10 meters per second (m/s), is of little value to ADS-B applications. There
Appendix A A-5 is no vertical rate accuracy metric. A NACV of greater than or equal to “1” is required by § 91.227. Table A-2 provides the applicable NIC values Table A-2. NACV Value Velocity Accuracy Bound (Estimated Velocity Uncertainty) 0 ≥ 10 m/s or unknown 1 < 10 m/s 2 < 3 m/s 3 < 1 m/s 4 < 0.3 m/s
A.2.22 Navigation Integrity Category (NIC).
The NIC parameter specifies a position integrity containment radius. NIC is reported so surveillance applications, such as ATC or other aircraft, may determine whether the reported geometric position has an acceptable level of integrity for the intended use.
The NIC parameter is closely associated with the SIL. While NIC specifies the integrity containment radius, SIL specifies the probability of the actual position lying outside that containment radius without indication. ADS-B systems should derive the NIC from an approved position source’s integrity output, such as the HPL from the GNSS. A minimum NIC value of “7” must be transmitted to operate in airspace defined in § 91.225. Table A-3 provides the applicable NIC values.
Appendix A A-6 Table A-3. NIC Values NIC Containment Radius 0 Unknown 1 RC < 37.04 km (20.0 nm) 2 RC < 14.816 km ( 8.0 nm) 3 RC < 7.408 km ( 4.0 nm) 4 RC < 3.704 km ( 2.0 nm) 5 RC < 1.852 km ( 1.0 nm) 6 Sup A=1 Sup B=1 RC < 1111.2 m ( 0.6 nm) 6 Sup A=0 Sup B=0 RC < 926 m ( 0.5 nm) 6 Sup A=0 Sup B=1 RC < 555.6 m ( 0.3 nm) 7 RC 370.4 m ( 0.2 nm) 8 RC < 185.2 m ( 0.1 nm) 9 RC < 75 m 10 RC < 25 m 11 RC < 7.5 m A.2.23 NICBARO.
NICBARO indicates if pressure altitude is provided by a single Gillham encoder or another more robust altitude source. Because of the potential for an undetected error in a Gillham encoding, many Gillham installations are cross-checked against a second altitude source. NICBARO annotates the status of this cross-check.
A.2.24 Position.
These parameters are derived from the position source and provide a geometric based position. Reference all geometric position elements broadcast from the ADS-B unit to the WGS-84 ellipsoid. Latitude and longitude is required to be transmitted by § 91.227.
A.2.25 Receiving ATC Services.
This parameter is a bit set in the ADS-B system of an aircraft indicating that the Mode A code is not set to “1200”. This parameter existed in TSO-C166a and TSO-C154b compliant equipment, but was removed from TSO-C166b and TSO-C154c equipment.
A.2.26 Single Antenna Bit.
This parameter indicates if the ADS-B equipment is transmitting through a single antenna.
Appendix A A-7 A.2.27 Source Integrity Level (SIL).
The SIL field defines the probability of the reported horizontal position exceeding the radius of containment defined by the NIC, without alerting, assuming no avionics faults.
Although the SIL assumes there are no unannunciated faults in the avionics system, the SIL must consider the effects of a faulted Signal-In-Space (SIS), if a SIS is used by the position source. A SIL value of “3” must be transmitted to operate in airspace defined in § 91.225. Table A-4 outlines the SIL values.
Note 1: The probability of an avionics fault causing the reported horizontal position to exceed the radius of containment defined by the NIC, without alerting, is covered by the SDA parameter.
Note 2: The SIL probability can be defined as either per sample or per-hour as defined in the SIL supplement (SILSUPP).
Table A-4. SIL Values, Probability of Exceeding the NIC Containment Radius SIL Value Probability of exceeding the NIC containment radius 0 > 1x10-3 Per-hour or Sample or Unknown 1 ≤ 1x10-3 Per-hour or Sample 2 ≤ 1x10-5 Per-hour or Sample 3 ≤ 1x10-7 Per-hour or Sample A.2.28 Source Integrity Level Supplement (SILSUPP).
The SILSUPP defines whether the reported SIL probability is based on a per-hour probability or a per-sample probability as defined in Table A-5.
Table A-5. Source Integrity Level Supplement SIL Supplement Basis for SIL Probability 0 Probability of exceeding NIC containment radius is based on per-hour.
1 Probability of exceeding NIC containment radius is based on per-sample.
A.2.29 System Design Assurance (SDA).
The SDA parameter defines the failure condition that the ADS-B system is designed to support as defined in Table A-5. The supported failure condition will indicate the probability of an ADS-B system malfunction causing false or misleading position information or position quality metrics to be transmitted. This should include the probability of exceeding the containment radius without annunciation. Because the installer of ADS-B OUT equipment does not know how the broadcast data will be used, the installer cannot complete a Functional Hazard Assessment (FHA) evaluating the use of the broadcast data. The SDA provides a surrogate for such a FHA by identifying the
Appendix A A-8 potential impact of an erroneous position report caused by an equipment malfunction.
The definitions and probabilities associated with the supported failure effect are defined in AC 25.1309-1, AC 23.1309-1(),and AC 29-2 (Changes 1-3 incorporated). The SDA includes the position source, ADS-B equipment, and any intermediary devices that process the position data. § 91.227 requires an SDA of 2 or 3 as defined in Table A-6.
Table A-6. System Design Assurance SDA Value Supported Failure Condition Note 2 Probability of Failure Causing Transmission of False or Misleading Information Note 3,4 Software & Hardware Design Assurance Level Note 1,3 0 Unknown/ No safety effect > 1x10-3 Per-hour or Unknown N/A 1 Minor ≤ 1x10-3 Per-hour D 2 Major ≤ 1x10-5 Per-hour C 3 Hazardous ≤ 1x10-7 Per-hour B Note 1: Software design assurance pursuant to RTCA/DO-178C, Software Considerations in Airborne Systems and Equipment Certification, or equivalent.
Airborne electronic hardware design assurance pursuant to RTCA/DO-254, Design Assurance Guidance for Airborne Electronic Hardware, or equivalent.
Note 2: Supported failure classification defined in AC 25.1309-1(), AC 23.1309-1(), and AC 29-2().
Note 3: Because the broadcast position can be used by any ADS-B IN equipped aircraft or by ATC, the provisions in AC 23.1309-1() that allow reduction in failure probabilities and design assurance level for aircraft under 6,000 pounds do not apply for the ADS-B OUT system.
Note 4: Includes probability of transmitting false or misleading latitude, longitude, or associated position accuracy and integrity metrics.
A.2.30 TCAS Installed and Operational.
This parameter indicates whether the aircraft is fitted with a TCAS II and if the TCAS II is turned on and operating in a mode that can generate resolution advisory alerts. The TCAS installed and operational parameter is required to be transmitted by § 91.227.
A.2.31 TCAS Traffic Status.
This parameter indicates if a TCAS II equipped aircraft is currently generating a TCAS resolution advisory. The TCAS traffic status parameter is required to be transmitted by § 91.227 if the aircraft is TCAS II equipped.
A.2.32 Trajectory Change Report Capability.
Appendix A A-9 This information is permanently set to “zero” in TSO-C166b or TSO-C154c equipment.
No installation interface is required. Trajectory change reports are reserved for future use.
A.2.33 Vertical Rate.
The vertical rate is the barometric or geometric rate at which the aircraft is climbing or descending, measured in feet per minute. The vertical rate is typically generated by an air data computer or GNSS position source, or equipment that blends barometric vertical rate with inertial vertical rate and/or GNSS vertical rate.
A.2.34 Version Number.
The applicable TSO Minimum Operational Performance Standard (MOPS) level is communicated through the version number, which is fixed at the time the ADS-B equipment is manufactured. Version 2 applies to ADS-B equipment that meets MOPS documents RTCA/DO-260B with corrigendum 1 or RTCA/DO-282B with corrigendum
1. ADS-B equipment outputting version 2 or higher is required by § 91.227.
Appendix B B-1 APPENDIX B. IDENTIFYING AND QUALIFYING ADS-B POSITION SOURCES B.1 Purpose.
This appendix defines the minimum requirements for position sources interfaced to ADS-B systems. The appendix also defines appropriate position source qualification methods when the existing GNSS TSOs do not contain specific requirements or test procedures. The position source manufacturer should provide design data where appropriate, preferably in the GNSS equipment installation manual, so the installer can properly interface the position source to the ADS-B system. Position source suppliers must ensure any supplied data is incorporated into the article design, and changes to any documented characteristics result in a change to the part number.
B.2 Organization.
This appendix includes general guidance that applies to all position sources, as well as GNSS-specific guidance. The appendix also provides high-level requirements for tightly-coupled GNSS/IRU position sources and non-GNSS position sources. Unless otherwise specified, all references in this AC to TSO-C129, TSO-C145, TSO-C146, and TSO-C196 refer to any revision of the TSO.
B.3 General Guidance for All Position Sources.
B.3.1 Position.
The position source must provide a latitude and longitude output. Requirements and test procedures in TSO-C129/145/146/196 are sufficient and GNSS equipment with Technical Standard Order Authorization (TSOA) for the aforementioned TSOs require no additional qualification for the position output.
B.3.2 Horizontal Velocity.
The position source must output north/south and east/west velocities. We recommend the position source also output the velocity in a ground speed and track angle format.
B.3.3 Position Accuracy (Vertical).
The position source should output a vertical position accuracy metric. The vertical position accuracy metric must have been qualified during the system’s TSOA or design approval. This output must describe the vertical position accuracy with 95 percent probability under fault-free conditions.
B.3.4 Position Accuracy (Horizontal).
The position source must have a horizontal position accuracy output, and the output must have been qualified during the system’s TSOA or design approval. This output must describe the radius of a circle in the horizontal plane, with its center being at the true position that describes the region assured to contain the indicated horizontal position with at least 95 percent probability under fault-free conditions.
Appendix B B-2 B.3.5 Position Integrity (Horizontal).
The position source must have a horizontal position integrity output qualified during the system’s TSOA or design approval. This integrity output should describe the radius of a circle in the horizontal plane, with its center being at the true position that describes the region assured to contain the indicated horizontal position with at least
99.99999 percent probability under fault-free avionics conditions. Position sources that degrade from a 99.99999 percent probability to a 99.999 percent probability (such as a tightly-coupled inertial/GNSS system after the loss of GNSS) can still be installed;
however, they will not meet § 91.227 following the degradation. In this case, the position source must have a way of indicating the change to the ADS-B equipment.
Additionally, if the change of probability is due to a change in position source, the new position source must meet all of the requirements in this appendix.
B.3.5.1 Mode.
If interpretation of the integrity output of the position source can change due to a change in the position source mode, the position source must have a way of communicating that change of mode to the ADS-B equipment.
Additionally, the position source manufacturer should provide a description of the modes and a description of how the position source outputs the mode indication.
B.3.5.2 Validity Limit.
If the integrity value of the output cannot be trusted beyond a certain limit, indicate this limitation in the design documentation.
B.3.5.3 Integrity Fault.
The position source must be able to identify, and output, an indication of an integrity fault. This indication should occur within 8 seconds of output of an erroneous position. The position source manufacturer must provide information on how this integrity fault is output.
B.3.6 Position Integrity (Probability).
The position source manufacturer must provide information describing the basis for the probability of exceeding the horizontal integrity containment radius. This basis must indicate the probability of exceeding the integrity containment radius as well as the sampling duration (per-hour or per-sample).
B.3.7 SIS Error Detection.
The position source should provide a means to detect a SIS error when the system uses a SIS. The probability of missed detection for a faulty SIS should be less than 1x10-3.
GNSS equipment provides the appropriate SIS error detection.
B.3.8 Velocity Accuracy.
The position source should have a velocity accuracy output that was qualified in conjunction with the system’s TSOA or design approval. Instead of a dynamic output, the position source manufacturer may demonstrate a worst case velocity accuracy that
Appendix B B-3 can be assumed based on testing. A test for GNSS position sources is contained in the latest revision of AC 20-138, appendix 4. The position source manufacturer may propose a test method for non-GNSS sources or an alternate test for GNSS sources during the TSOA or design approval.
B.3.9 Design Assurance.
The position source must support a major or greater failure effect. This includes software compliant with RTCA/DO-178C, Level C, and airborne electronic hardware (AEH) compliant with RTCA/DO-254, Level C. For airborne electronic hardware determined to be simple, RTCA/DO-254, section 1.6 applies. Because the broadcast position can be used by any ADS-B IN equipped aircraft or by ATC, the provisions in AC 23.1309-1() that allow reduction in failure probabilities and design assurance level for aircraft under 6,000 pounds do not apply for the ADS-B OUT system. The overall probability of a position source malfunction causing a position to be output that exceeds the output integrity radius must be less than 1x10-5 per-hour.
B.3.10 Geometric Altitude.
The position source must have a geometric altitude output. The geometric altitude must be referenced to the WGS-84 ellipsoid.
B.3.11 Update Rate.
The position source must output a new position at least once per second. Faster position update rates reduce latency of the transmitted position and are encouraged.
B.3.12 Position Source Latency.
The position source manufacturer must provide position source latency information.
Specifically, the manufacturer must provide the amount of position source total latency and uncompensated latency. Because the latency requirements are based on the entire ADS-B OUT system, and not just the position source, the following position source latency targets are only guidelines. Position source uncompensated latency should be less than 200 ms, compensated latency should be less than 500 ms, and total latency should be less than 700 ms.
Note 1: System latency requirements are described in section 3.1.3 and Appendix C of this AC.
Note 2: This section addresses position latency only.
B.3.13 Position, Velocity, and Accuracy Time of Applicability.
For each position output by the source, a velocity, horizontal position accuracy metric, and horizontal velocity accuracy metric must also be output. All measurements and metrics must have the same time of applicability. A horizontal position integrity metric must also be output, but its time of applicability may lag the position. Refer to TSO-C145, TSO-C146, or TSO-C196 for additional information on the integrity time to alert.
B.3.14 Time Mark.