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An aviation transponder interrogation mode is the format of a sequence of pulses from an interrogating Secondary Surveillance Radar (SSR) or similar Automatic dependent surveillance-broadcast (ADS-B) system. The reply format is usually referred to as a "code" from a transponder, which is used to determine detailed information from a suitably equipped aircraft.

In its simplest form, a "Mode" or interrogation type, is generally determined by pulse spacing between two or more interrogation pulses. Various modes exist from Mode 1 to 5 for military use, to Mode A, B, C and D and Mode S for civilian use.

Interrogation modes[edit]

Several different RF communication protocols have been standardized for aviation transponders:

1 provides 2-digit 5-bit mission code (cockpit selectable).[1]
2 provides 4-digit octal unit code (set on ground for fighters, can be changed in flight by transport aircraft).[1]
3 A provides a 4-digit octal identification code for the aircraft, set in the cockpit but assigned by the air traffic controller. Mode 3/A is often combined with Mode C to provide altitude information as well.[2]
C provides the aircraft's pressure altitude and is usually combined with Mode 3/A to provide a combination of a 4-digit octal code and altitude as Mode 3 A/C, often referred to as Mode A and C.[2]
4 provides a 3-pulse reply, delay is based on the encrypted challenge.[1]
5 provides a cryptographically secured version of Mode S and ADS-B GPS position.[1]
S provides multiple information formats to a selective interrogation. Each aircraft is assigned a fixed 24-bit address.[2]

Mode A and Mode C[edit]

When the transponder receives an interrogation request it sends back a transponder's squawk code. This is referred to as Mode 3A or more commonly Mode A. A transponder code can be paired with pressure altitude information, which is called Mode C.[2] Pressure altitude is obtained from an altitude encoder, a separate self-contained unit mounted in the aircraft. The altitude information is passed to the transponder using a modified form of the Gray code called a Gillham Code.

Mode 3A and C are used to help air traffic controllers to identify the aircraft on a radar screen and to maintain separation.[2]

Mode S[edit]

Another mode called Mode S (Select) is designed to help avoiding overinterrogation of the transponder (having many radars in busy areas) and to allow automatic collision avoidance. Mode S transponders are compatible with Modes A & C SSR system.[2] This is the type of transponder that makes the ACAS II (Airborne Collision Avoidance System) and the ADS-B systems function. The format of the messages is documented in ICAO Doc 9688, Manual on Mode S Specific Services.[3]

Mode S features[edit]

Upon interrogation, Mode S transponders transmit information about the aircraft to the SSR system, to TCAS receivers on board aircraft and to the ADS-B SSR system. This information includes the call sign of the aircraft and/or the transponder's permanent ICAO 24-bit address in the form of a hex code.

ICAO 24-bit address[edit]

Mode S equipment on aircraft are assigned a unique ICAO 24-bit address or (informally) Mode-S "hex code" upon national registration and this address becomes a part of the aircraft's Certificate of Registration. Normally, the address is never changed, however, the transponders are reprogrammable and, occasionally, are moved from one aircraft to another (presumably for operational or cost purposes), either by maintenance or by changing the appropriate entry in the aircraft's Flight management system.

There are 16,777,214 (224-2) unique ICAO 24-bit addresses (hex codes) available.[4][5] The ICAO 24-bit address can be represented in three digital formats: hexadecimal, octal, and binary. These addresses are used to provide a unique identity normally allocated to an individual aircraft or registration.

As an example, following is the ICAO 24-bit address assigned to the Shuttle Carrier Aircraft with the registration N905NA:[6][7]

  • Hexadecimal: AC82EC
  • Octal: 53101354
  • Binary: 101011001000001011101100 (Note: occasionally, spaces are added for visual clarity, thus 1010 1100 1000 0010 1110 1100)
  • Decimal: 11305708

These are all the same 24-bit address of the Shuttle Carrier Aircraft, represented in different digital formatting (see above).

Issues with Mode S transponders[edit]

One major issue with Mode S transponders is that pilots have frequently been entering the wrong flight identity into their Mode S transponders.[8] In this case, the capabilities of ACAS II and Mode S SSR can be degraded.[9]

Extended Squitter[edit]

In 2009 the ICAO published an "extended" form of Mode S[10] with more messages formats to use with ADS-B; it was further refined in 2012.[11]


  1. ^ a b c d NATO STANAG 4193
  2. ^ a b c d e f Peppler, I.L.: From The Ground Up, pages 238–239. Aviation Publishers Co. Limited, Ottawa Ontario, Twenty Seventh Revised Edition, 1996. ISBN 0-9690054-9-0
  3. ^ Manual on Mode S Specific Services (2 ed.). International Civil Aviation Organization. 2004. ISBN 92-9194-407-6. 
  4. ^ "UK Civil Aviation Authority – "ICAO 24 bit Aircraft Addresses"". Caa.co.uk. 2007-10-02. Retrieved 2013-02-07. 
  5. ^ "Eurocontrol – "Mode S Technical Overview"". Eurocontrol.int. Retrieved 2013-02-07. 
  6. ^ Ralf D. Kloth, DL4TA, Ludwigsburg, DE. "Aircraft on HFDL - ICAO 24 bit ID". Kloth.Net. Retrieved 2013-02-07. 
  7. ^ airframes.org (2005-08-11). "Aircraft Database". Airframes.Org. Retrieved 2013-02-07. 
  8. ^ International Civil Aviation Organization (March 2005). "The Third Meeting of Automatic Dependent Surveillance – Broadcast (ADS-B) Study and Implementation Task Force (ADS-B TF/3)" (PDF). Retrieved 2008-03-28. 
  9. ^ Eurocontrol – Aeronautical Information Circular (AIC) – ICAO 24-Bit Aircraft Addresses and Aircraft Identification Reporting (in Minutes from The Third Meeting of the Automatic Dependent Surveillance – Broadcast (ADS-B) Study and Implementation Task Force (ADS-B TF/3))
  10. ^ ICAO (2008). ICAO Doc 9871, Technical Provisions for Mode S and Extended Squitter (1 ed.). International Civil Aviation Organization. ISBN 978-92-9231-117-9. 
  11. ^ ICAO (2012). ICAO Doc 9871, Technical Provisions for Mode S and Extended Squitter (2 ed.). International Civil Aviation Organization. ISBN 978-92-9249-042-3. 

Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Aviation_transponder_interrogation_modes — Please support Wikipedia.
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