Appendix B B-2 B.3 ATCRBS Reply.
Interrogates with ATCRBS (Modes A and C) interrogations and verifies:
B.3.1 F1 to F2 spacing is 20.3 µs (±0.10 µs).
B.3.2 F1 and F2 pulse width between the 0.5 amplitude point on the leading and trailing edge is 0.45 µs (±0.10 µs).
B.4 SLS Level.
Interrogates with valid ATCRBS Interrogations including P2 pulse. Interrogations are conducted with P2 level at -9 dB and then again at 0 dB. Test verifies:
B.4.1 Transponder does not reply when P2 level is at 0 dB (UUT is suppressed).
B.4.2 Transponder replies when P2 level is at -9 dB (UUT is not suppressed).
B.5 ATCRBS-Only All-Call.
Interrogates with an ATCRBS-only All-Call and verifies:
B.5.1 If Mode S is valid, no reply is received from a Mode S transponder.
B.5.2 If no Mode S, reply is received from an ATCRBS transponder.
B.6 Mode S All-Call.
Interrogates with the ATCRBS (mode A) Mode S All-Call. Address received in downlink format (DF) 11 is then used in an uplink format (UF) 4 interrogation to solicit a DF4 reply. The address received is decoded and compared with the address sent.
B.7 Invalid ICAO 24-bit aircraft address.
Interrogates with Mode S interrogations using two addresses different from the address determined by the Mode S All-Call, UF11. Test verifies no reply is received.
Addresses used are one greater and 256 greater than the correct address.
B.8 SPR On/Off.
Interrogates with a Mode S interrogation with the Synchronous Phase Reversal (SPR) on, verifying correct reply is received. Then same interrogation is sent again with the SPR off, verifying no reply is received.
B.9 Mode S UF0.
Appendix B B-3 Interrogates with Mode S uplink format 0 (Short air-to-air surveillance, ACAS) verifying reply is received that has correct altitude (compared with Mode C altitude), address (compared with Mode Test address) and format.
B.10 Mode S UF4.
Interrogates with Mode S uplink format 4 (Surveillance, altitude request), verifying reply is received that has correct altitude (compared with Mode C altitude), address (compared with Mode Test address) and format.
B.11 Mode S UF5.
Interrogates with Mode S uplink format 5 (Surveillance, identity request) verifying reply is received that has correct identity (compared with Mode A identity), address (compared with Mode Test address) and format.
B.12 Mode S UF11.
Interrogates with Mode S uplink format 11, verifying reply is received that has correct address (compared with Mode Test address) and format.
B.13 Mode S UF16.
Interrogates with Mode S uplink format 16 (Long air-to-air surveillance, ACAS), verifying reply, if received, has correct altitude (compared with the Mode C altitude), address (compared with Mode Test address) and format. No reply to UF16 does not fail Mode S in Auto Test.
B.14 Mode S UF20.
Interrogates with Mode S uplink format 20 (Comm A, altitude request) verifying reply received has correct altitude (compared with Mode C altitude), address (compared with Mode Test address) and format. No reply to UF20 does not fail Mode S in Auto Test.
B.15 Mode S UF21.
Interrogates with Mode S uplink format 21 (Comm A, identity request) verifying reply received has correct identity (compared with Mode A identity), address (compared with Mode Test address) and format. No reply to UF21 does not fail Mode S in Auto Test.
B.16 Acquisition Squitter.
Verifies squitters are being received from the UUT every 0.8 to 1.2 seconds.
Appendix B B-4 B.17 Frequency.
Verifies frequency of transponder is 1090 MHz (± frequency tolerance as specified by equipment manufacturer). Frequency is displayed in the Auto Test screen.
B.18 Diversity.
Verifies diversity isolation (power level difference between UUT “On” antenna squitters and “Off” antenna squitters) is greater than or equal to 20 dB. Diversity isolation is displayed in Auto Test screen.
Note: To ensure >=20 dB dynamic range, test must be run within 50 feet (15.24 meters) of UUT antenna being tested.
B.19 MTL Difference.
Verifies Receiver Sensitivity (MTL) to Mode A interrogations equals MTL to Mode C interrogations (±1.0 dB).
B.20 Altitude Reporting.
Verifies the reported altitude in the AC field of DF0, DF4, D16, and D20 corresponds to the value provided from the on-board altitude source to the transponder and is reported with the quantization indicated by the Q bit in the AC field. For aircraft with 25 foot or better pressure altitude sources, pressure altitude information should be reported in 25-foot increments. Pressure altitude data obtained from a source with larger than 25-foot resolution must not be reported using 25-foot increments.
Note: To ensure the correct reporting of altitude with the proper quantization, the aircraft altitude must be incremented over a 300-foot range in 25 foot increments.
B.21 Selected Vertical Intention (BDS register 4016) if implemented.
Verify the system marks the data status invalid when information is either not available or invalid. Verify all reserved fields are set to zero. Verify that the system transmits the selected vertical intention data in BDS register 4016 correctly and that this data corresponds to the data from the originating avionics on the aircraft.
B.22 Target State and Status Information (BDS register 6216) if implemented.
Verify the system marks the data status invalid when information is either not available or invalid. Verify all reserved fields are set to zero. Verify that the system transmits the target state and status data in BDS register 6216 correctly and that this data corresponds to the data from the originating avionics on the aircraft
Appendix C C-1 APPENDIX C. HISTORICAL INFORMATION ON DEVELOPMENT OF TCAS.
C.1 Early TCAS.
C.1.1 The airline industry has been working with the Air Transport Association of America (ATA), now known as Airlines for America (A4A), since 1955 toward a collision avoidance system. It was not until the mid-1970s, however, that research centered on using signals from ATCRBS airborne transponders as the cooperative element of a collision avoidance system. This technical approach allows a collision avoidance capability on the flight deck, which is independent of the ground system. In 1981, the FAA announced our decision to implement an aircraft collision avoidance concept called the Traffic Alert and Collision Avoidance System (TCAS). The concept is based upon agency and industry development efforts in the areas of beacon based collision avoidance systems and air-to-air discrete address communications techniques utilizing Mode S airborne transponder message formats.
C.1.2 A short time later, prototypes of TCAS II were installed on two Piedmont Airlines Boeing 727 aircraft, and were flown on regularly scheduled flights. Although the displays were located outside the view of the flight crew and seen only by trained observers, these tests did provide valuable information on the frequency and circumstances of alerts and their potential for interaction with the ATC system. In a follow-on phase II program, a later version of TCAS II was installed on a single Piedmont Airlines Boeing 727, and the system was certified in April 1986 then subsequently approved for operational evaluation in early 1987. Since the equipment was not developed to full standards, the system was only operated in visual meteorological conditions. Although the flight crew operated the system, the evaluation was primarily for the purpose of data collection and its correlation with flight crew and observer observation and response.
C.1.3 Later versions of TCAS II manufactured by Bendix/King Air Transport Avionics Division were installed and approved on United Airlines airplanes in early 1988.
Similar units manufactured by Honeywell were installed and approved on Northwest Airlines airplanes in late 1988. This limited installation program operated TCAS II units approved for operation as a full-time system in both visual and instrument meteorological conditions on three different aircraft types. The operational evaluation programs continued through 1988 to validate the operational suitability of the systems.
C.2 Recent TCAS Developments.
C.2.1 Since the early 1990s, an operational evaluation known as the TCAS Transition Program (TTP) collected and analyzed data on the performance and use of TCAS II in both the U.S. National Airspace System (NAS) and in other airspace worldwide. As a result of these analyses, changes to TCAS II were developed, tested, and implemented in the early to mid-1990s. These changes, collectively known as TCAS II V7.0, were implemented by industry in the late 1990s. TCAS II V7.0 complies with the
Appendix C C-2 International Civil Aviation Organization (ICAO) Standards and Recommended Practices (SARP) for ACAS II. TCAS II V7.0 or V7.1 is mandated for carriage in Australia, India and certain European countries. V7.0 was mandated for carriage in 2003 by ICAO with V7.1 becoming mandatory by ICAO on January 2, 2014 for new aircraft and for existing aircraft by January 1, 2017. In addition, if you operate an aircraft that is equipped with TCAS II in RVSM airspace, it must be a TCAS II that meets TSO-C119b (V7.0) or a later version.
C.2.2 Investigation of operational problems resulted in changes to TCAS II V7.0. The changes resulting from that investigation resulted in TCAS II V7.1.
C.2.3 While developing corrective action for the operational problems, the Federal Aviation Administration (FAA), along with industry representatives, decided to develop performance standards for TCAS II Hybrid Surveillance. That work resulted in creation of RTCA/DO-300, Minimum Operational Performance Standards (MOPS) for Traffic Alert and Collision Avoidance System II (TCAS II) Hybrid Surveillance.
Investigation of operational problems reported after introduction of TCAS II units incorporating hybrid surveillance resulted in changes to TCAS II V7.1. Those changes are invoked by TSO-C119e. Change 2 to RTCA/DO-300 and Change 1 to RTCA/DO- 300A also incorporate these changes for TCAS II hybrid surveillance units designed to TSO-C119c and C119d, respectively.
C.2.4 As a result of a 2006 midair collision in Brazil, the FAA has determined that a need exists to strengthen requirements for crew alerting for loss of TCAS functionality.
Consistent with 14 CFR sections 25.1302, 25.1309(c), and 25.1322, we have incorporated those changes into this AC.
TSO-C119e classifies failure of the TCAS II to provide a reliable traffic alert and collision avoidance function as a Hazardous/Severe-Major failure condition.
However, previous revisions of this AC have only provided guidance for “incorrect RAs”, defined in the AC as an “RA occurring when a threat is present, but, because of a failure of the installed TCAS II, Mode S transponder, or associated sensors, commands a maneuver that reduces separation to the threat [emphasis added].” In some cases, this has resulted in design approval holders and ACO engineers incorrectly classifying failure to provide RA when a threat is present, or late provision of an RA, as simple loss of function and therefore minor (loss of TCAS II functionality is only minor if the loss of function is appropriately annunciated).
The FAA considers failure to provide an RA (or late provision of an RA) when a threat is present, without annunciation of loss of TCAS II functionality, to also be a Hazardous/Severe-Major failure of the function defined in paragraph 3.a of TSO- C119e (i.e., provide a reliable traffic alert and collision avoidance function).
Therefore, this AC clarifies TCAS failure classification and system safety assessment considerations by adding reference to a “Missing RA”, defined as “An RA that does not occur, or that occurs later than the CAS logic indicates is necessary, when a threat is present”, which is a Hazardous/Severe-Major failure condition. The System Safety
Appendix C C-3 Assessment section of this AC contains means of compliance for addressing missing RAs.
Since TCAS is a supplemental system to ATC separation services and the flight crew’s responsibility to see-and-avoid, the FAA considers that those two factors (ATC separation services and see-and-avoid responsibility) partially mitigate the normally assigned failure classification for TCAS, and allow the probability of incorrect RA and missing RA event(s) to be improbable (10-5) versus the 10-7 usually associated with a Hazardous/Severe-Major failure classification.
C.3 Accident history related to the development and evolution of TCAS.
C.3.1 This section provides links to online information on significant historical accidents that have played a role in the development and evolution of TCAS. This section does not provide a detailed narrative of those events, but rather enables interested readers to conduct further research into accidents that have influenced the development of TCAS by providing easy access to relevant information. The intent of providing links to this information is to capture historical lessons learned related to TCAS, in order to preserve key safety objectives for the system and facilitate its continued refinement.
C.3.2 PSA 727, San Diego and Aeromexico DC-9, Los Angeles, Midair Collisions with General Aviation Aircraft: Detailed information is available at the FAA’s Lessons Learned from Transport Airplane Accidents website at:
http://lessonslearned.faa.gov/ll_main.cfm?TabID=3&CategoryID=14&LLID=48 C.3.3 Boeing 757/Tupolev Tu-154 midair collison, Überlingen, Germany, July 1, 2002:
Further information is available at the Aviation Safety Network website at:
https://aviation-safety.net/database/record.php?id=20020701-1 (for the Boeing 757), and https://aviation-safety.net/database/record.php?id=20020701-0 (for the Tu-154).
The final report of the German Federal Bureau of Aircraft Accidents Investigation (BFU) is also available, in English, at: http://www.bfuweb.de/EN/Publications/Investigation%20Report/2002/Report_02_AX001-1- 2_Ueberlingen_Report.pdf?__blob=publicationFile . Appendices to the BFU final report are at: http://www.bfuweb.de/EN/Publications/Investigation%20Report/2002/Report_02_AX001-1- 2_Ueberlingen_Appendix_1-3.pdf?__blob=publicationFile C.3.4 Boeing 737-800/Embraer Legacy 600 midair collision, near Peixoto Azevedo, MT, Brazil, September 29, 2006: Further information is available at the Aviation Safety Network website at: https://aviation-safety.net/database/record.php?id=20060929-0 (for the Boeing 737-800), and https://aviationsafety.net/database/record.php?id=20060929-1 (for the Embraer Legacy 600).
Appendix D D-1 APPENDIX D. RELATED DOCUMENTS.
D.1 14 CFR Parts.
You can apply current portions of 14 CFR for the design, substantiation, certification and operational approval of TCAS II and Mode S transponders. Sections prescribing requirements for these types of systems are listed below. Order copies of 14 CFR Parts 21, 23, 25, 27, 29, 43, 91, 121, and 135 from the Superintendent of Documents, U.S. Government Publishing Office, P.O. Box 979050, St. Louis, MO 63197-9000, telephone (866) 512-1800, fax (202) 512-2104. You can also get copies from the Government Publishing Office (GPO), electronic CFR Internet website at http://www.ecfr.gov.
14 CFR § 25.301 Loads.
14 CFR § 25.303 Factor of safety.
14 CFR § 25.305 Strength and deformation.
14 CFR § 25.561 General.
14 CFR § 25.603 Materials.
14 CFR § 25.609 Protection of structure.
14 CFR § 25.629 Aeroelastic stability requirements.
14 CFR § 25.869 Fire protection: systems.
14 CFR § 25.1301 Function and installation.
14 CFR § 25.1302 Installed systems and equipment for use by the flightcrew.
14 CFR § 25.1303 Flight and navigation instruments.
14 CFR § 25.1307 Miscellaneous equipment.
14 CFR § 25.1309 Equipment, systems, and installations.
14 CFR § 25.1316 System Lightning Protection.
14 CFR § 25.1317 High-intensity Radiated Field (HIRF) Protection.
14 CFR § 25.1321 Arrangement and visibility.
14 CFR § 25.1322 Warning, caution, and advisory lights.
14 CFR § 25.1331 Instruments using a power supply.
14 CFR § 25.1333 Instrument systems.
14 CFR § 25.1335 Flight director systems.
14 CFR § 25.1351 Electrical Systems and Equipment: General.
14 CFR § 25.1353 Electrical equipment and installations.
14 CFR § 25.1355 Distribution system.
Appendix D D-2 14 CFR § 25.1357 Circuit protective devices.
14 CFR § 25.1381 Instrument lights.
14 CFR § 25.1431 Electronic equipment.
14 CFR § 25.1541 Markings and Placards: General.
14 CFR § 25.1581 Airplane Flight Manual: General.
14 CFR § 25.1585 Operating procedures.
Note: References to 14 CFR part 25 are appropriate when installing TCAS II on transport category airplanes. When TCAS II is to be certified for non-transport category airplanes, use the equivalents to the above 14 CFR part 25 sections in other parts of the regulations.
D.2 FAA ACs.
You will find a current list of ACs on the FAA Internet website at http://rgl.faa.gov.
AC 20-131A, Airworthiness Approval of Traffic Alert and Collision Avoidance Systems (TCAS II) and associated Mode S Transponders AC 20-115C, Airborne Software Assurance AC 20-165B, Airworthiness Approval of Automatic Dependent Surveillance - Broadcast OUT Systems AC 20-172B, Airworthiness Approval for ADS-B In Systems and Applications AC 25.1302-1, Installed Systems and Equipment for Use by the Flightcrew AC 25.1309-1A, System Design and Analysis AC 25.1322-1, Flightcrew Alerting AC 25.1329-1C, Approval of Flight Guidance Systems AC 25-11B, Electronic Flight Displays AC 120-55C, Air Carrier Operational Approval and Use of TCAS II D.3 FAA TSOs.
You will find a current list of TSOs on the FAA Internet website Regulatory and Guidance Library at http://rgl.faa.gov. You will also find the TSO Index of Articles at the same site.
TSO-C112e, Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/Mode S) Airborne Equipment TSO-C119e, Traffic Alert and Collision Avoidance System (TCAS) Airborne Equipment, TCAS II With Hybrid Surveillance Functionality
Appendix D D-3 D.4 FAA Other.
You will find this related information on the FAA Internet website Regulatory and Guidance Library at http://rgl.faa.gov. The document is posted adjacent to the links for AC 20-151 and TSO-C119.
Introduction to TCAS II V7.1, February 28, 2011 D.5 RTCA Inc. Publications.
Order RTCA documents from RTCA, Inc., 1150 18th Street N.W. Suite 910, Washington D.C. 20036 or order online at www.rtca.org.
RTCA/DO-160G, Environmental Conditions and Test Procedures for Airborne Equipment, dated December 12, 2010.
RTCA/DO-178C, Software Considerations in Airborne Systems and Equipment Certifications, dated December 13, 2011.
RTCA/DO-181E, Minimum Operational Performance Standards for Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/Mode S) Airborne Equipment, dated March 17, 2011.
RTCA/DO-185A, Minimum Operational Performance Standards for Traffic Alert and Collision Avoidance Systems II (TCAS II), dated December 16, 1997.
RTCA/DO-185B, Minimum Operational Performance Standards for Traffic Alert and Collision Avoidance Systems II (TCAS II), dated June 19, 2008; including Change 1 dated July 1, 2009, and Change 2 dated March 20, 2013.
RTCA/DO-300, Minimum Operational Performance Standards for Traffic Alert and Collision Avoidance System II (TCAS II) Hybrid Surveillance, dated December 13, 2006; including Change 1 dated July 1, 2009, and Change 2 dated December 15, 2015.
RTCA/DO-300A, Minimum Operational Performance Standards for Traffic Alert and Collision Avoidance System II (TCAS II) Hybrid Surveillance, dated March 20, 2013; including Change 1 dated December 15, 2015.
D.6 SAE International.
Order SAE documents from SAE International, 400 Commonwealth Drive, Warrendale, PA 15096, telephone (724) 776-4841, fax (724) 776-0790. Also, you may order copies online at www.sae.org.
ARP 926B, Fault/Failure Analysis Procedure, dated June 11, 1997.
ARP 1834A, Fault/Failure Analysis for Digital Systems and Equipment, dated June 1, 1997.
ARP 4102/7, Electronic Displays, dated July 1, 1988.
ARP 4102/8A, Flight Deck Head-Up Displays, dated November 1, 1998.
Appendix D D-4 ARP 4754A, Guidelines for Development of Civil Aircraft and Systems, dated December 21, 2010.
ARP 4761, Guidelines and Methods for Conducting the Safety Assessment Process on Civil Airborne Systems and Equipment, dated December 1, 1996.
D.7 AERONAUTICAL RADIO, INC. (ARINC).
Publications are available from SAE-ITC, ARINC Industry Activities, 16701 Melford Blvd., Suite 120, Bowie, MD 20715, telephone (240) 334-2578 or online at http://store.aviation-ia.com/cf/store/customer_service.cfm.
ARINC 718( ), Mark 3 Air Traffic Control Transponder (ATCRBS/MODE S).
ARINC 735( ), Mark 2 Traffic Alert and Collision Avoidance System (TCAS).
D.8 International Civil Aviation Organization (ICAO).
Publications are available from ICAO, Attention: Customer Service unit, 999 University Street, Montreal, Quebec, Canada H3C5H7; telephone +1 514-954-8022, facsimile: 514-954-6769, sitatex YULCAYA, email: sales@icao.int or on line at http://www.icao.int.
Airborne Collision Avoidance System (ACAS) Manual document 9863, First Edition dated 2006.
D.9 JAA.
JAA documents transposed to publications of the European Aviation Safety Agency (EASA) are available on the EASA web site at https://www.easa.europa.eu/. JAA documents are also available on the EUROCONTROL site at http://www.eurocontrol.int/ .
JAA Temporary Guidance Leaflet (TGL) 13 Revision 1.
D.10 EASA.
Publications are available on the EASA web site at https://www.easa.europa.eu/.
AMC 20-13 Certification of Mode S Transponder Systems for Enhanced Surveillance.
EASA Certification Specification and Acceptable Means of Compliance – Airborne Communication Navigation and Surveillance, Book 1 and Book 2.
Appendix E E-1 APPENDIX E. DEFINITIONS AND ACRONYMS.
E.1 Definitions. - The following definitions are specific to this AC and may differ from definitions contained in other references.
Active surveillance. - Using TCAS interrogations and subsequent replies to update or acquire a TCAS track.
Advisory. - Message given to alert the flight crew of converging aircraft and/or a potential collision.
Air traffic control radar beacon system (ATCRBS). - Secondary surveillance radar system with ground based interrogators and airborne transponders capable of operation on Modes A and C.
Alert. - Indication (aural or visual) that informs the flight crew in a timely manner about converging aircraft or potential collision.
Automatic dependent surveillance broadcast (ADS-B). - A function on an aircraft or vehicle that automatically broadcasts its own aircraft's identity, state vector (horizontal and vertical position and velocity), associated quality and performance parameters (accuracy and integrity) and other information. Broadcast links include the 1090 MHz extended squitter (ES) and the universal access transceiver (UAT) broadcast on 978MHz.
Coast. - Condition which occurs when TCAS II does not receive a reply to an interrogation from an intruder for which it has established a track, resulting in the logic continuing the track based on previous track characteristics.
Comm-B. - A 112-bit Mode S reply containing a 56-bit MB message field containing the extracted transponder register.
Corrective resolution advisory. - Advises the pilot to either deviate from current vertical speed, such as CLIMB when the aircraft is level, or to maintain an existing climb or descent rate.
In TCAS II V7.0 and V7.1, a “Maintain Rate RA” is classified as a corrective RA solely to provide a green fly-to arc or zone on a vertical speed indicator.
Discrete. - Separate, complete and distinct signal.
Failure. - Inability of a system, subsystem, unit, or part to perform within previously specified limits.
False advisory (TA or RA). - Advisory caused by a false track or TCAS II malfunction.
Hybrid Surveillance. - Combined use of active and passive surveillance to update a TCAS track.
Incorrect resolution advisory. - RA occurring when a threat is present, but, because of a failure of the installed TCAS II, Mode S transponder, or associated sensors, commands a maneuver that reduces separation to the threat.
Appendix E E-2 Intruder. - Aircraft satisfying the TCAS II traffic advisory detection criteria.
Missing resolution advisory. - An RA that does not occur, or that occurs later than the CAS logic indicates is necessary, when a threat is present.
Mode A. - Type of secondary surveillance radar (SSR) equipment or mode of operation that replies by selected 4096 code (nonaltitude) when interrogated.
Mode C. - Type of secondary surveillance radar (SSR) equipment or mode of operation that replies with aircraft altitude information when interrogated.
Mode S. - Type of secondary surveillance radar (SSR) equipment that replies to Mode A and Mode C ground interrogations, a discrete address, and other aircraft information in response to interrogations from the ground or air.
Other traffic. - Aircraft more than ±1200 feet vertical or 6 nautical miles (NM) from own aircraft that are neither RA nor TA.
Passive surveillance. - The use of airborne position messages to update a TCAS track.
Preventive resolution advisory. - Requires a pilot to avoid certain deviations from current vertical rate (for example, a Do Not Climb RA when the aircraft is level).
Proximate traffic. - Aircraft within 6 NM in range and within ±1,200 feet vertically from own aircraft, but does not meet the TCAS II thresholds of a TA or RA.
Resolution advisory. - Aural voice and display information provided by TCAS II to a flight crew, advising that a particular maneuver should, or should not, be performed to attain or maintain minimum safe vertical separation from an intruder.
Resolution display. - Shows vertical guidance depicting areas to “fly to” and/or avoid above or below the TCAS II equipped aircraft.
Sense. - A direction that an RA may take (either Climb or Descend) relative to the existing flight path of own aircraft.
Threat. - An intruder that satisfies the threat detection logic and thus requires an RA.
Track. - Estimated position and velocity of a single aircraft based on correlated surveillance data reports.
Traffic. - Aircraft with an operating transponder capable of being tracked and displayed by a TCAS-equipped aircraft.
Traffic advisory (TA). - Aural voice and display information from TCAS II to a flight crew, identifying the location of nearby traffic meeting certain minimum separation criteria.
Traffic display. - Horizontal position of transponder-equipped aircraft relative to the TCAS II equipped aircraft.
Transponder register. - Transponder register means a transponder data buffer containing different pieces of information. It has 56 bits which are divided into different fields. The definition of the transponder registers can be found in ICAO Doc 9871 edition 2 and in RTCA/DO-181E, Minimum Operational Performance Standards for Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/Mode S) Airborne