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Aviation transponder interrogation modes: Wikis


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An aviation transponder interrogation mode is the format of a sequence of pulses from an interrogating SSR radar, or similar system, and reply format usually referred to as a "Code" from a transponder, 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

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

  • Mode 1 – provides 2-digit 5-bit mission code. (military only – cockpit selectable)[1]
  • Mode 2 – provides 4-digit octal unit code. (military only – set on ground for fighters, can be changed in flight by transport aircraft)[1]
  • Mode 3/A – provides a 4-digit octal identification code for the aircraft, assigned by the air traffic controller. (military and civilian)[2]
  • Mode 4 – provides a 3-pulse reply to crypto coded challenge. (military only)[1]
  • Mode 5 – provides a cryptographically secured version of Mode S and ADS-B GPS position. (military only)[1]
  • Mode C – provides 4-digit octal code for aircraft's pressure altitude. (military and civilian)[2]
  • Mode S – provides multiple information formats to a selective interrogation. Each aircraft is assigned a fixed 24-bit address. (military and civilian)[2]

Mode A and Mode C

When the transponder receives a radar signal 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]

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

Mode S

Another mode called Mode S (Selective) 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.[2] This is the type of transponder that makes the ACAS II (Airborne Collision Avoidance System) and the ADS-B (Automatic dependent surveillance-broadcast) systems function.

Mode S is mandatory in controlled airspace in many countries. Some countries require that all aircraft be equipped with Mode S, even in uncontrolled airspace. However in the field of general aviation, there have been objections to these moves, because of the cost, size, limited benefit to the users in uncontrolled airspace, and, in the case of balloons and gliders, the power requirements for these aircraft that have limited electrical power.

Mode S features

Mode S transponders broadcast information about the aircraft to the Secondary Surveillance Radar (SSR) system, 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

All modern 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 FMS system.

There are 16,777,214 unique ICAO 24-bit addresses (hex codes) available.[3][4] The ICAO 24-bit address can be represented in three digital formats: hexadecimal, octal, and binary. These addresses can be decoded and converted amongst each other online using tools such as those at and to obtain the aircraft's tail number. Whichever format is used, the same information is carried through the signal.

Example of an ICAO 24-bit address:

  • Hexadecimal: AC82EC
  • Decimal: 11305708 (Note: rarely used format)
  • Octal: 53101354
  • Binary: 101011001000001011101100 (Note: occasionally, spaces are added for visual clarity, thus 10101100 10000010 11101100)

(These all correlate to the same aircraft registration, N905NA.)[5][6]

Other features

Mode S TIS Secondary surveillance radar (SSR), or Traffic Information Service, allows a radar installation to send information about nearby traffic back to aircraft, which then displays it on the moving map. Mode S TIS is only available when the aircraft is within radar range of a radar installation that supports it. A Mode S TIS installation combines a Mode S transponder that sends TIS data to a display device – usually a GPS device or Multi Function Display (MFD). Examples of such pairings are a Garmin GTX330D transponder and a GNS530 GPS, and the Garmin G1000 avionics suite.

Mode S capable transponders are also a building block for next generation air traffic control systems, as they can be used to transmit location information for ADS-B and potentially other air traffic control communications. Currently the FAA is deactivating several Mode S TIS equipped stations.

Issues with Mode S transponders

One major issue with Mode S transponders is that pilots have frequently been entering the wrong flight identity into their Mode S transponders.[7] In this case, the capabilities of ACAS II and Mode S SSR can be degraded.[8]


  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 09690054-9-0
  3. ^ UK Civil Aviation Authority – "ICAO 24 bit Aircraft Addresses"
  4. ^ Eurocontrol – "Mode S Technical Overview"
  5. ^
  6. ^
  7. ^ 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.  
  8. ^ 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))


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