Appendix 3 A3-2
d. 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 hertz position source can introduce 1 second of total latency. This 1 second must be included in the total latency calculation.
3. Equipment latency budget. There are no individual component latency requirements;
however we recommend the following guidelines:
a. Position source. We recommend using position sources where the latency of the position, velocity, and position accuracy metrics are less than or equal to 500ms between the position TOM and the position time of applicability, and that the position is output in less than 200ms after the position time of applicability.
Note: All revisions of TSO-C145, TSO-C146, and TSO-C196 equipment meet these recommendations.
b. 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 100ms. (Reference RTCA/DO-260B, appendix U).
c. 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 100ms of uncompensated latency. TSO-C166b or TSO-C154c are required by 14 CFR 91.225.
4. General Latency Issues.
a. Recommendations for reducing latency.
(1) Directly connect the position source to the ADS-B equipment.
(2) Use a TSO-C145, TSO-C146, or TSO-C196 position source (any revision).
(3) Use a position source that provides position updates at greater than 1Hz.
(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).
Appendix 3 A3-3
b. 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. 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 to 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.
d. Compensating for interface latency in unsynchronized systems. It is acceptable to install ADS-B equipment which 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 which 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 for compensation is still limited by the requirement to limit total latency to within 2.0 seconds.
e. Overcompensating. It is possible for compensation algorithms to “over compensate” 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 200ms. (Reference RTCA/DO-260B, appendix U).
f. Extrapolation during loss of position data. TSO-C166b equipment compliant with RTCA/DO-260B, paragraphs 2.2.3.2.3.7.4 and 2.2.3.2.3.8.4, allows extrapolation of the position for up to two 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 per TSO-C166b.
g. 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. In order 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.
Appendix 3 A3-4
h. 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.
(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 area navigation (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 14 CFR 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 DO-260B with Corrigendum 1 and DO-282B with corrigendum 1.
(2) Transmit time-of-applicability. The transmit time is the time which 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.
i. 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.
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 15 is considered a compliant architecture.
Appendix 3 A3-5 Table 15. Latency Analysis Example
Uncompensated Latency Compensated Latency Total Latency Notes Position Source ≤ 200 ms ≤ 500ms ≤ 700 ms Position Source to ADS-B Interface 0 0 0 Directly connected ADS-B Equipment ≤ 100 Note 1 ≤ 100 ms Asynchronous Delay 0 ≤ 1.0 sec ≤ 1.0 sec 1 Hz position source Total ≤ 300 ms ≤ 1.5 sec ≤ 1.8 sec
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 timeof-applicability is required to be compensated by all revisions of TSO-C145, TSO-C146, and TSO-C196.
Appendix 4 A4-1
Appendix 4. Definitions
1. Definitions. The following definitions are specific to this AC and may differ from definitions contained in other references.
a. Automatic Dependent Surveillance Broadcast (ADS-B). ADS-B is an advanced surveillance technology where ADS-B OUT equipped aircraft share position, altitude, velocity, and other information with ATC and other appropriately equipped aircraft.
b. ADS-B IN. Receipt, processing, and display of other aircraft’s ADS-B transmissions.
ADS-B IN is necessary to utilize airborne applications.
c. ADS-B OUT. Transmission of an aircraft’s position, altitude, velocity, and other information to other aircraft and ATC ground based surveillance systems.
d. 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.
e. 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.
f. Flight Information System - Broadcast (FIS-B). FIS-B is a ground broadcast service provided over the UAT data link. The FAA FIS-B system provides pilots and flight crews of properly equipped aircraft with a cockpit display of certain aviation weather and aeronautical information.
g. Flight manual. A generic term used throughout this AC to represent the AFM, RFM, AFM supplement, or RFM supplement.
h. Galileo. Galileo is a European satellite-based radio navigation system being developed that will provide a global positioning service.
i. Global Navigation Satellite System (GNSS). The generic term for a satellite navigation system, such as the Global Positioning System (GPS), that provides autonomous worldwide geo-spatial positioning and may include local or regional augmentations.
j. Global Positioning System (GPS). GPS is 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 GPS Standard Positioning System Performance Standard, 4th edition available at http://pnt.gov/public/docs/2008/spsps2008.pdf.
Appendix 4 A4-2
k. GNSS time of applicability. The time that the position output from the GNSS sensor is applicable.
l. GNSS Time of Measurement (TOM). TOM for GNSS sources is the time when the last GNSS signal used to determine the position arrives at the aircraft GNSS antenna.
m. 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% probability under fault-free conditions at the time of applicability.
n. 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. It 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.
o. Horizontal Protection Level Fault Free (HPLFF). Fault Free horizontal protection level. See RTCA/DO-229D appendix R.
p. 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.
q. 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 (e.g., 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.
r. Satellite-Based Augmentation System (SBAS). A wide coverage augmentation system in which the user receives augmentation information from a satellite-based transmitter.
In the U.S., this is referred to as Wide Area Augmentation System (WAAS).
s. Selective Availability (SA). A protection technique employed by the Department of Defense which degraded GPS accuracy. Selective availability was discontinued on May 1, 2000.
t. 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 which provides position, velocity, and other information on traffic which is detected by a secondary surveillance radar, but is not transmitting an ADS-B position.
Appendix 4 A4-3
u. 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).
v. Uncompensated latency. Any latency in the ADS-B system which 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.
w. Wide Area Augmentation System (WAAS). The U.S. implementation of SBAS.
Appendix 5 A5-1 Appendix 5. Acronyms
1. Acronyms.
Acronym Definition 14 CFR Title 14 of the Code of Federal Regulations AC Advisory Circular ACO Aircraft Certification Office ADIRS Air Data And Inertial Reference System ADS-B Automatic Dependent Surveillance - Broadcast ADS-R Automatic Dependent Surveillance - Rebroadcast A/FD Airport/Facility Directory AFM Airplane Flight Manual AFMS Airplane Flight Manual Supplement ANSP Air Navigation Service Provider ARP Aerospace Recommended Practice ASDE-X Airport Surface Detection Equipment, Model X ATC Air Traffic Control ATCRBS Air Traffic Control Radar Beacon System dB Decibel DME Distance Measuring Equipment EMC Electro Magnetic Compatibility EMI Electromagnetic Interference EPU Estimated Position Uncertainty FAA Federal Aviation Administration FHA Functional Hazard Assessment FIS-B Flight Information Services - Broadcast FMS Flight Management System GBAS Ground Based Augmentation System GNSS Global Navigation Satellite System GNSS/IRS Global Navigation Satellite System / Inertial Reference System GPS Global Positioning System GVA Geometric Vertical Accuracy HAE Height Above Ellipsoid HAG Height Above Geoid HFOM Horizontal Figure of Merit HIL Horizontal Integrity Level HPL Horizontal Protection Level HUL Horizontal Uncertainty Level Hz Hertz ICAO International Civil Aviation Organization ICA Instructions for Continued Airworthiness IFR Instrument Flight Rules INS Inertial Navigation System IRS Inertial Reference System IRU Inertial Reference Unit
Appendix 5 A5-2 Acronym Definition kts Knots LPV Localizer Performance with Vertical Guidance m/s Meters per second MHz Megahertz MOPS Minimum Operational Performance Standards MSL Mean Sea Level NACP Navigational Accuracy Category for Position NACV Navigational Accuracy Category for Velocity NAS National Aerospace System NIC Navigational Integrity Category NICBARO Barometric Altitude Integrity Code NM Nautical Mile POA Position Offset Applied RA Resolution Advisory Rc Radius of containment RAIM Receiver Autonomous Integrity Monitoring RFM Rotorcraft Flight Manual RFMS Rotorcraft Flight Manual Supplement RVSM Reduced Vertical Separation Minimum SA Selective Availability SBAS Satellite Based Augmentation System SDA System Design Assurance SIL Source Integrity Level SILSUPP SIL Supplement SIS Signal-in-Space SSR Secondary Surveillance Radar STC Supplemental Type Certificate TC Type Certificate TCAS II Traffic Alert and Collision Avoidance System TIS-B Traffic Information Service - Broadcast TOM Time of Measurement TSO Technical Standard Order TSOA Technical Standard Order Authorization UAT Universal Access Transceiver UTC Coordinated Universal Time VFR Visual Flight Rules VFOM Vertical Figure of Merit VOR VHF Omnidirectional Range VAPP Target approach airspeed VFE Maximum flap extended speed VH Maximum speed in level flight at maximum continuous power VMO Maximum operating limit speed VNE Never-exceed speed VS Stalling speed or the minimum steady flight speed at which the airplane is controllable VY Speed for best rate of climb
Appendix 5 A5-3 Acronym Definition V2 Takeoff safety speed WAAS Wide Area Augmentation System WGS-84 World Geodetic System 1984
Appendix 6 A6-1 Appendix 6 Related Documents
1. 14 CFR. Title 14 of the Code of Federal Regulations (14CFR) can be obtained at www. access.gpo.gov. Select “Access,” then “Online bookstore.” Then select “Aviation,” then “Code of Federal Regulations.”
2. Advisory Circulars. Order copies of Advisory Circulars from the U.S. Department of Transportation, Subsequent Distribution Office, M-30, Ardmore East Business Center, 3341 Q 75th Avenue, Landover, MD 20785. You can also get copies from our website at www.faa.gov/regulations_policies/advisory_circulars/.
a. AC 20-131(), Airworthiness Approval of Traffic Alert and Collision Avoidance Systems (TCAS II) and Mode S Transponders
b. AC 20-138(), Airworthiness Approval of Global Navigation Satellite System (GNSS) Equipment
c. AC 20-151(), Airworthiness Approval of Traffic Alert and Collision Avoidance Systems (TCAS II) Version 7.0 & 7.1 and Associated Mode S Transponders
d. AC 20-172(), Airworthiness Approval for ADS-B IN Systems and Applications
e. AC 21-40(), Guide for Obtaining a Supplemental Type Certificate
f. AC 23.1309-1(), System Safety Analysis and Assessment for Part 23 Airplanes
g. AC 25.1309-1(), System Design and Analysis
h. AC 27-1(), Certification of Normal Category Rotorcraft
i. AC 29-2(), Certification of Transport Category Rotorcraft
j. AC 43-6(), Altitude Reporting Equipment and Transponder System Maintenance and Inspection Practices
3. TSO. You can find a current list of technical standard orders on the FAA Internet website at www.airweb.faa.gov/rgl. You will also find the TSO Index of Articles at the same site.
a. TSO-C5, Direction Instrument, Non-Magnetic (Gyroscopically Stabilized)
b. TSO-C6, Direction Instrument, Magnetic (Gyroscopically Stabilized)
c. TSO-C8(), Vertical Velocity Instruments
d. TSO-C10(), Altimeter, Pressure Actuated, Sensitive Type
Appendix 6 A6-2
e. TSO-C66(), Distance Measuring Equipment (DME) Operating Within the Radio Frequency Range of 960-1215 Megahertz
f. TSO-88(), Automatic Pressure Altitude Reporting Code-Generating Equipment
g. TSO-C106(), Air Data Computer
h. TSO-C112(), Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/Mode S) Airborne Equipment
i. TSO-C119(), Traffic Alert and Collision Avoidance System (TCAS) Airborne Equipment
j. TSO-C129(), Airborne Supplemental Navigation Equipment Using the Global Positioning System (GPS)
k. TSO-C145(), Airborne Navigation Sensors Using the Global Positioning System (GPS) Augmented by the Wide Area Augmentation System (WAAS)
l. TSO-C146(), Stand-Alone Airborne Navigation Equipment Using the Global Positioning System (GPS) Augmented by the Wide Area Augmentation System (WAAS)
m. TSO-C154c, Universal Access Transceiver (UAT) Automatic Dependent Surveillance Broadcast (ADS-B) Equipment Operating on the Frequency of 978 MHz
n. TSO-C166b, Extended Squitter Automatic Dependent Surveillance - Broadcast (ADS-B) and Traffic Information Service - Broadcast (TIS-B) Equipment Operating on the Radio Frequency of 1090 Megahertz (MHz)
o. TSO-C196(), Airborne Supplemental Navigation Sensors for Global Positioning System Equipment Using Aircraft-Based Augmentation
4. RTCA, Inc. You can order copies of RTCA documents from RTCA, Inc., 1150 18th Street, NW, Suite 910, Washington, DC 20036; telephone: (202) 833-9339; website:
http://www.rtca.org.
a. RTCA DO-160D, E, F or G Environmental Conditions and Test Procedures For Airborne Equipment
b. RTCA/DO-178B, Software Considerations in Airborne Systems and Equipment Certification
c. RTCA/DO-208 change 1, Minimum Operational Performance Standards for Airborne Supplemental Navigation Equipment Using Global Positioning System (GPS)
d. RTCA/DO-229D, Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System Airborne Equipment