Transponder for aircraft identification

In aeronautics, planes have transponders to help with their identification by radars and also as a system of collision avoidance.

The first transponder systems were developed in aeronautics by the British army and by the American army during the Second World War under the name of Identification friend or foe (IFF) which made it possible to recognize the radar echo of a friend’s device.

transponder is short for transmitter-responder it is a Radio transmitter-receiver automatically responding to an external signal from a radar or a localization system. The transmission-reception is done using a trailing antenna, away from the nacelle, so that the occupants are less exposed to radioelectric radiation.


The system was adapted for civil aviation air traffic control in 1950 using secondary radar (radar beacon) for use in general aviation and commercial aviation.

Secondary radar is called “secondary” to distinguish it from “primary radar” which works by reflecting its radio waves off the surface of the aircraft. Passive (primary) radar works very well when the aircraft is large and all-metal, but with more difficulty, if the aircraft is small and made of composite material. Its range can be reduced by the nature of the terrain, rain, and snow. It can detect unwanted objects like automobiles, hills or trees, antennas, and metal buildings.
Moreover, all passive radars do not give an estimate of the altitude of the aircraft.

The secondary radar makes it possible to overcome these limits but it is dependent on the on-board transponder to answer its interrogations from the ground and thus make the aircraft visible and allow the calculation of its altitude.

primary radar
Primary radar
secondry radar
secondry radar


  • The transponder communicates with the radars only at a minimum altitude and above the reliefs.
  • Its use does not correspond to a short flight and/or at low altitude.


In mode A (Alpha), the coding includes the identification of the aircraft.

In mode C (Charly), the coding also includes the pressure altitude.

In mode S (Sierra), the coding also includes a more precise altitude and other data (eg the flight number, the altitude chosen by the pilot).


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