Appendix B B-31 B.6.3 Inertial Navigation System/Inertial Reference Unit (INS/IRU) Loosely Coupled With DME or GNSS.
B.6.3.1 The GNSS equipment or DME equipment must meet the requirements in this appendix.
B.6.3.2 Loosely coupled INS/IRU equipment must meet 14 CFR part 121, appendix G.
B.6.3.3 The loosely coupled INS/IRU position source must provide all of the required position source outputs listed in this appendix. Qualify the outputs during installation approval of the ADS-B system; refer to section B.3 of this appendix. Velocity accuracy may be qualified and set statically. Update the position accuracy and position integrity metrics dynamically.
B.6.3.4 § 91.227 requires a SIL = 3, which means the probability of exceeding the NIC containment radius should be less than 1x10-7 per hour or per sample.
A GNSS/IRS that continues to provide the integrity containment radius based on a 1x10-7 probability after loss of GNSS or GNSS RAIM is preferred. Potential errors, caused by GNSS updating before the loss of GNSS, must continue to be bounded.
B.6.3.4.1 If the integrity containment probability output of a loosely coupled GNSS/IRS position source changes from 1x10-7 to 1x10-5 following a loss of GNSS or a loss of GNSS RAIM, the position source must relay this change to the ADS-B equipment. The overall system time to transmit a change in SIL must be 10 seconds or less.
B.6.3.4.2 If the integrity containment probability output of a loosely coupled GNSS/IRS position source changes from per-hour to per-sample following a loss of GNSS or a loss of GNSS RAIM, the position source must relay this change to the ADS-B equipment.
B.7 Future Position Sources.
It is expected that future position sources such as dual frequency GPS and GPS/Galileo sources will be acceptable position sources for ADS-B and meet the performance requirements of § 91.227. Future revisions of this AC will address new position source technology when it becomes available.
Appendix C C-1 APPENDIX C. LATENCY ANALYSIS C.1 Purpose.
The purpose of this appendix is to provide guidelines for accomplishing a latency analysis on your ADS-B system.
C.2 Analysis.
Accomplish the analysis by determining the applicable latencies for each component and totaling all of the individual component latencies. You must include all sources of position latency, including but not limited to: the position source, intermediary devices between the position source and ADS-B equipment, and ADS-B equipment. Use the following guidelines to determine latency for each component:
C.2.1 Position Source Latency Considerations.
In general, the latency information should be generated by the position source manufacturer and presented as part of the latency analysis. The latency measurement should begin at the TOM and end when the position is output from the position source.
C.2.1.1 TSO-C145, TSO-C146, and TSO-C196 GNSS.
Use the TSO latency standards in the latency analysis or use actual latency information generated by the GNSS manufacturer to determine the position source maximum total latency and uncompensated latency. If the GNSS equipment is classified as Class 3 pursuant to any revision of TSO-C145, there are tighter latency standards for the LPV modes. If the Class 3 standard is implemented across all modes, the tighter latency numbers may be used; however, if the tighter latency standards are only met when in approach mode, use the worst-case latency across all modes.
C.2.1.2 TSO-C129 GNSS.
There are no latency standards for any revision of TSO-C129 GNSS equipment. Latency information must be generated by the GNSS manufacturer and included as part of the latency analysis.
C.2.1.3 Tightly-Coupled GNSS/Inertial.
There are no latency standards for tightly-coupled GNSS/Inertial equipment. Total and uncompensated latency information should be generated by the position source manufacturer and presented as part of the latency analysis. Base the latency analysis on the update rate of the inertial sensor, as 10-second or 20-second GNSS updates to the inertial sensor are not impacting the latency of the position output. However, the GNSS update latency does affect the position accuracy and should be appropriately reflected in the position source accuracy output.
C.2.1.4 Other Position Sources.
Appendix C C-2 Total and uncompensated latency information should be generated by the position source manufacturer and included as part of the latency analysis.
C.2.2 Intermediary Device.
Intermediary devices are typically data concentrators. The latency information should be generated by the intermediary device manufacturer and presented as part of the latency analysis. If the intermediary device latency is variable, use the worst-case latency.
C.2.3 ADS-B Equipment.
Use the TSO-C166b and TSO-C154c latency standards for the latency analysis or use the actual latency information generated by the ADS-B equipment manufacturer.
TSO-C166b and TSO-C154c require the uncompensated latency of the ADS-B equipment to be less than 100 ms.
C.2.4 Asynchronous Delay.
Total latency analysis must include the maximum asynchronous delay caused by position updates arriving at the ADS-B equipment out-of-synch with when the ADS-B system transmits the position. This delay is a factor of the position source update rate rather than the ADS-B equipment transmission rate. For example, a 1 Hz position source could provide a position update immediately after an ADS-B position transmission. This position would be extrapolated, up to 1 second, until the next position update arrives from the position source. Thus, a 1 Hz position source can introduce 1 second of total latency. This 1 second must be included in the total latency calculation.
C.3 Equipment Latency Budget.
C.3.1 Position Source.
We recommend using position sources where the latency of the position, velocity, and position accuracy metrics are less than or equal to 500 ms between the position TOM and the position time of applicability, and that the position is output in less than 200 ms after the position time of applicability.
Note: All revisions of TSO-C145, TSO-C146, and TSO-C196 equipment meet these recommendations.
C.3.2 Position Source to ADS-B Interface.
Directly connecting the position source to the ADS-B equipment is the preferred method of installation. Alternately, if this architecture is not used, we recommend that any latency introduced between the position source output and the ADS-B equipment input be less than 100 ms (refer to RTCA/DO-260B, appendix U).
C.3.3 ADS-B Equipment.
The latency requirements for the ADS-B equipment are included in TSO-C166b and TSO-C154c and allow for the ADS-B equipment to introduce no more than 100 ms of
Appendix C C-3 uncompensated latency. TSO-C166b or TSO-C154c are required by § 91.225 and § 91.227.
C.4 General Latency Issues.
C.4.1 Recommendations for Reducing Latency.
C.4.1.1 Directly connect the position source to the ADS-B equipment.
C.4.1.2 Use a TSO-C145, TSO-C146, or TSO-C196 position source (any revision).
C.4.1.3 Use a position source that provides position updates at greater than 1 Hz.
C.4.1.4 Use the GNSS time mark in TSO-C166b systems to reduce position source and intermediary device uncompensated latency. (Use of the GNSS time mark is required by TSO-C154c).
C.4.2 Latency Applicability.
The 2.0 second total latency requirement applies to the aircraft position (latitude and longitude), velocity, and the velocity accuracy metric (NACV). The 0.6 second uncompensated latency requirement only applies to the aircraft position (latitude and longitude).
C.4.3 Mean Latency Versus Maximum Latency.
In instances where the latency is variable, use the worst-case latency under fault-free conditions in the analysis. Variable latency, for example, can occur due to variance in loading of a data concentrator or the asynchronous nature of a GNSS to ADS-B interface. As the applicant, you must propose to the FAA how to deal with variable latencies introduced by intermediary devices such as data concentrators.
C.4.4 Compensating for Interface Latency in Unsynchronized Systems.
It is acceptable to install ADS-B equipment that compensates for latency that occurs outside of the ADS-B equipment, even if the position source and ADS-B equipment are not time synchronized. Establishing the proper corrections for external latency is problematic because the TSO-C166b equipment may be interfaced to numerous different aircraft architectures. These architectures could include different position sources, with different latencies, as well as different data concentrators with different delays. To interface unsynchronized ADS-B equipment that compensates for external latencies, the ADS-B equipment manufacturer must provide a list of the acceptable equipment and the acceptable architectures. Typically this type of ADS-B equipment will only be installed in closely-integrated architectures. You may not attempt to integrate ADS-B equipment that compensates for external latencies unless the ADS-B equipment manufacturer has expressly documented the installation architecture and design data is available for each component. The total amount of time that can be used
Appendix C C-4 for compensation is still limited by the requirement to limit total latency to within
2.0 seconds.
C.4.5 Overcompensating.
It is possible for compensation algorithms to “overcompensate” for the effects of latency, essentially transmitting a position that is out in front of the actual aircraft position rather than behind the actual aircraft position. This type of system is acceptable as long as the transmitted position is no further ahead than 200 ms, (refer to RTCA/DO-260B, appendix U).
C.4.6 Extrapolation During Loss of Position Data.
TSO-C166b equipment compliant with RTCA/DO-260B, sections 2.2.3.2.3.7.4 and 2.2.3.2.3.8.4, allows extrapolation of the position for up to 2 seconds when the position data is not available from the position source. This allowance is in case position data is lost for a single sample, and it does not have to be considered in the total latency calculation, provided it is a non-normal condition. If the position data is lost, several position updates could exceed the latency requirement, but the position would then be invalidated within 2 seconds, pursuant to TSO-C166b.
C.4.7 UTC Epoch Synchronization.
The position transmitted from the ADS-B equipment may be aligned with a UTC epoch.
TSO-C154c requires UAT systems to extrapolate the position to the 1.0 second or
0.2 second UTC epoch. TSO-C166b allows 1090ES systems to extrapolate to the
0.2 second UTC epoch or transmit asynchronously. To synchronize the position output with the UTC epoch, the position source needs to provide a time mark. The ADS-B equipment uses this time mark to extrapolate the position to the UTC epoch. Typically the time mark will be from a GNSS position source. Implementation of the time synchronization in the 1090ES systems will help minimize uncompensated latency.
C.4.8 Latency Points of Measurement.
Latency is defined as the time between when the position is measured by the position source to when it is transmitted by the ADS-B equipment.
C.4.8.1 Time of Measurement (TOM).
The latency analysis starts at the position source TOM. The position source TOM for GNSS sources is the time when the last GNSS signal used to determine the position arrives at the aircraft GNSS antenna.
TOM for an inertial position source or a GNSS-aided inertial position source is the time of the last accelerometer measurement. TOM for an RNAV system using multiple DME signals would be the time the last DME signal arrives at the aircraft’s DME antenna.
Note: To demonstrate compliance with § 91.227, you must calculate latency from the position source TOM. Do not calculate latency from the position source time of applicability, as defined in RTCA/DO-260B with corrigendum 1 and RTCA/DO-282B with corrigendum 1.
Appendix C C-5 C.4.8.2 Transmit Time of Applicability.
The transmit time is the time when the ADS-B system broadcasts the position. The transmitted position’s time of applicability for synchronized systems is the appropriate UTC epoch. The transmitted position’s time of applicability for unsynchronized systems is the actual time the ADS-B equipment begins transmission of the message that contains the position.
Note: Synchronized ADS-B systems randomly vary the position transmission around the UTC epoch to avoid interference with other ADS-B transmitters. This randomization should not be included in the latency analysis.
C.4.9 Minor Changes to Position Source Type Design.
If the ADS-B installation relies on position source latency performance, versus a TSO latency standard, the ADS-B system installer must update the ICA for the position source with a process that ensures continued airworthiness of the ADS-B system following design changes to the position source.
C.5 Latency Analysis Example.
This example uses a GNSS meeting the minimum performance requirements of TSO-C145 (any revision) directly connected to TSO-C166b ADS-B equipment. This installation is a T = 0 installation; thus it is unsynchronized. The example in Table C-1 is considered a compliant architecture.
Table C-1. Latency Analysis Example Item Uncompensated Latency Compensated Latency Total Latency Notes Position Source ≤ 200 ms ≤ 500 ms ≤ 700 ms Position Source to ADS-B Interface 0 0 0 Directly connected ADS-B Equipment ≤ 100 ms Note 1 ≤ 100 ms Asynchronous Delay 0 ≤ 1.0 second ≤ 1.0 second 1 Hz position source Total ≤ 300 ms ≤ 1.5 second ≤ 1.8 second Note 1: ADS-B equipment compensated latency is bounded by the asynchronous nature of the position source delivery and ADS-B system transmission. Thus ADS-B equipment compensated latency is included in the asynchronous delay row.
Note 2: The latency between the position source TOM and the position source time of applicability is required to be compensated by all revisions of TSO-C145, TSO-C146, and TSO-C196.
Appendix D D-1 APPENDIX D. DEFINITIONS AND ACRONYMS D.1 Definitions.
The following definitions are specific to this AC and may differ from definitions contained in other references.
D.1.1 Automatic Dependent Surveillance Broadcast (ADS-B).
An advanced surveillance technology where ADS-B OUT equipped aircraft share position, altitude, velocity, and other information with ATC and other appropriately equipped aircraft.
D.1.2 ADS-B IN.
Receipt, processing, and display of other aircraft’s ADS-B transmissions. ADS-B IN is necessary to use airborne applications.
D.1.3 ADS-B OUT.
Transmission of an aircraft’s position, altitude, velocity, and other information to other aircraft and ATC ground-based surveillance systems.
D.1.4 Automatic Dependent Surveillance - Rebroadcast (ADS-R).
Retransmission of UAT ADS-B messages from aircraft on the 1090ES link and 1090ES messages on the UAT link. ADS-R ensures aircraft equipped with different links can receive messages from one another when equipped with ADS-B IN.
D.1.5 Area Navigation (RNAV).
A method of navigation that permits aircraft operation on any desired flight path within the coverage of station-referenced navigation aids or within the limits of the capability of self-contained aids, or a combination of these.
D.1.6 Barometric Altitude Integrity Code (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.
D.1.7 Flight Information System - Broadcast (FIS-B).
A ground broadcast service provided over the UAT data link. The FAA FIS-B system provides pilots and flightcrews of properly equipped aircraft with a cockpit display of certain aviation weather and aeronautical information.
D.1.8 Flight Manual.
A generic term used throughout this AC to represent the AFM, RFM, AFM supplement, or RFM supplement.
Appendix D D-2 D.1.9 Galileo.
A European satellite-based radio navigation system being developed that will provide a global positioning service.
D.1.10 Global Navigation Satellite System (GNSS).
The generic term for a satellite navigation system, such as GPS, that provides autonomous worldwide geo-spatial positioning and may include local or regional augmentations.
D.1.11 Global Positioning System (GPS).
A U.S. satellite-based radio navigation system that provides a global positioning service. The service provided by GPS for civil use is defined in the Global Positioning System Standard Positioning Service Performance Standard, 4th edition, dated September 2008, available at http://www.gps.gov/technical/ps/2008-SPS-performancestandard.pdf.
D.1.12 GNSS Time of Applicability.
The time when the position output from the GNSS sensor is applicable.
D.1.13 GNSS Time of Measurement (TOM).
The time when the last GNSS signal used to determine the position arrives at the aircraft GNSS antenna.
D.1.14 Horizontal Figure of Merit (HFOM ).
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 at the time of applicability.
D.1.15 Horizontal Protection Level Fault Detection (HPLFD).
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. HPLFD is a horizontal region where the missed alert and false alert requirements are met for the chosen set of satellites when autonomous fault detection is used. It is a function of the satellite and user geometry and the expected error characteristics; it is not affected by actual measurements. Its value is predictable given reasonable assumptions regarding the expected error characteristics.
D.1.16 Horizontal Protection Level Fault Free (HPLFF).
Fault-free horizontal protection level. Refer to RTCA/DO-229D, appendix R.
D.1.17 Mode Control Panel The Mode Control Panel, (MCP) contains controls that allow aircrew to interface with the autopilot system. The MCP can be used to instruct the autopilot to perform tasks such as; hold a specific altitude, change altitudes at a specific rate, hold a specific heading, turn to a new heading, and or follow the directions of a flight management
Appendix D D-3 computer. The MCP is not the autopilot, it just controls the mode in which the autopilot operates.
D.1.18 Navigation Accuracy Category for Position (NACP).
Used to indicate, with 95 percent certainty, the accuracy of the aircraft reported horizontal position. Table D-1 provides a list of possible NACP values. A NACP of 8 or greater is required by § 91.227.
Table D-1. NACP Encoding Value Horizontal Accuracy Bound (Estimated Position Uncertainty) 0 EPU ≥ 18.52 km (10.0 nm) 1 EPU < 18.52 km (10.0 nm) 2 EPU < 7.408 km (4.0 nm) 3 EPU < 3.704 km (2.0 nm) 4 EPU < 1.852 m (1.0 nm) 5 EPU < 926 m (0.5 nm) 6 EPU < 555.6 m (0.3 nm) 7 EPU < 185.2 m (0.1 nm) 8 EPU < 92.6 m (0.05 nm) 9 EPU < 30 m 10 EPU < 10 m 11 EPU < 3 m D.1.19 Navigational Accuracy Category for Velocity (NACV).
Used to indicate, with 95 percent certainty, the accuracy of the aircraft reported horizontal velocity. Table D-2 provides a list of possible NACV values. A NACV of 1 or greater is required by § 91.227.
Appendix D D-4 Table D-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 D.1.20 Navigation Integrity Category (NIC).
A parameter that specifies an integrity containment radius. Table D-3 provides a list of possible NIC values. A NIC of 7 or greater is required by § 91.227.
Table D-3. NIC Encoding Value Radius of Containment 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 RC < 1.111 km (0.6 nm) 6 RC < 926 m (0.5 nm) 6 RC < 555.6m (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 D.1.21 Position Source.
The on-board avionics equipment that provides the latitude, longitude, geometric altitude, velocity, position and velocity accuracy metrics, and position integrity metric.
Additionally, the position source may provide the vertical rate parameters.
Appendix D D-5 D.1.22 Receiver Autonomous Integrity Monitoring (RAIM).
Any algorithm that verifies the integrity of the position output using GPS measurements, or GPS measurements and barometric aiding, is considered a RAIM algorithm. An algorithm that uses additional information (such as a multi-sensor system with inertial reference system) to verify the integrity of the position output may be acceptable as a RAIM equivalent. Within this AC, the term RAIM is a synonym for aircraft-based augmentation system (ABAS) and is used to refer to both RAIM and RAIM-equivalent algorithms.
D.1.23 Satellite-Based Augmentation System (SBAS).
A wide coverage augmentation system in which the user receives augmentation information from a satellite-based transmitter. In the United States, this is referred to as Wide Area Augmentation System (WAAS).
D.1.24 Selective Availability (SA).
A protection technique employed by the Department of Defense that degraded GPS accuracy. Selective availability was discontinued on May 1, 2000.
D.1.25 Source Integrity Level (SIL).
The probability of the reported horizontal position exceeding the radius of containment defined by the NIC without alerting, assuming the avionics has no faults. Table D-4 provides a list of possible SIL values. A SIL of 3 is required by § 91.227.
Table D-4. SIL Encoding Value Probability 0 > 1x10-3 or unknown 1 ≤ 1x10-3 2 ≤ 1x10-5 3 ≤ 1x10-7 D.1.26 System Design Assurance (SDA).
The failure condition that the position transmission chain is designed to support.
Table D-5 provides a list of possible SDA values. An SDA of 2 or greater is required by § 91.227. Refer to A.2.29 for more information.
Appendix D D-6 Table D-5. SDA Encoding Value Probability of Undetected Fault Causing the Transmission of False or Misleading Information 0 > 1x10-3 or unknown 1 ≤ 1x10-3 2 ≤ 1x10-5 3 ≤ 1x10-7 D.1.27 Traffic Collision Avoidance System.
Collision Avoidance systems which rely on transponder interrogations and replies of other airborne aircraft.
D.1.28 Traffic Collision Avoidance System I.
TCAS I is the first generation of collision avoidance technology. TCAS I systems are able to monitor the traffic situation around an aircraft and offer information on the approximate bearing and altitude of other aircraft. It can also generate collision warnings in the form of a "Traffic Advisory" (TA). The TA warns the pilot that another aircraft is in near vicinity, announcing "Traffic, traffic", but does not offer any suggested remedy D.1.29 Traffic Collision Avoidance System II.
TCAS II is the second and current generation of instrument warning TCAS. It offers all the benefits of TCAS I, but also offers the pilot direct, vocalized instructions to avoid danger, known as a "Resolution Advisory" (RA). TCAS II systems coordinate their resolution advisories before issuing commands to the pilots, so that if one aircraft is instructed to descend, the other will typically be told to climb — maximizing the separation between the two aircraft.
D.1.30 Traffic Information Service - Broadcast (TIS-B).
TIS-B is a ground broadcast service provided from an ADS-B ground system network over the UAT and 1090ES links that provides position, velocity, and other information on traffic detected by a secondary surveillance radar, but is not transmitting an ADS-B position.
D.1.31 Total Latency.
The total time between when the position is measured by the position source (GNSS TOM for GNSS systems) and when the position is transmitted from the aircraft (ADS-B time of transmission).
D.1.32 Uncompensated Latency.
Any latency in the ADS-B system that is not compensated through extrapolation.
Uncompensated latency can be represented as the difference between the time of applicability of the broadcast position and the actual time of transmission.