Aviation transponder interrogation modes

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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.

[edit] Interrogation modes

  • Mode 1 — provides 2-digit 5-bit mission code (military only — cockpit selectable).
  • Mode 2 — provides 4-digit octal unit code (military only — either set on the ground or changed in flight depending on the particular aircraft type).
  • Mode 3/A — provides a 4-digit octal identification code for the aircraft, known as a squawk code, assigned by the air traffic controller (military and civilian).
  • Mode 4 — provides a 3-pulse reply (dependent upon a valid 32-bit cryptographic coded challenge), military only.
  • Mode 5 — provides cryptographic secure capabilty similar to Mode S including transmission of ADS-B and GPS position (military only).
  • Mode C — provides a 10-bit binary Gray code for the aircraft's pressure altitude (military and civilian).
  • Mode S — originally envisioned as a data packet standard in both uplink data (1030 MHz) and downlink data (1090 MHz) formats, also used to provide a radar design wherein the transponder responds to selective interrogations (each aircraft can be assigned a fixed 24-bit address for selective interrogation purposes). The downlink data format can also be utilized independently to squitter information such as position and velocity (AIS-P/TailLight). (military and civilian).

For civilian flights according to ICAO the modes of operations are A, C and S. The A mode is based on a 4-digit code using numbers between 0 and 7 assigned by the ATC and set by the pilot enabling identification and monitoring. Mode C transmits pressure altitude, read automatically from the aircraft altimeter. The mode S is triggered by a mode S interrogation and can provide the particular information that is requested by the interrogation signal. For modes A and C, all aircraft receiving the interrogation signal will reply, whereas mode S allows aircraft to be addressed individually. In modern ATC systems the data appear with alphanumeric characters in a tag or label linked to the flight position symbol on the radar screen.