How an airplane is identified when it enters any country airspace?

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.

Transponder codes

A transponder code consists of four digits. It is emitted by an aircraft in response to an interrogation signal from a surveillance radar to assist air traffic controllers in traffic separation.
The four transponder dials range from zero to seven inclusive. The smallest possible code is 0000 and the higher is 7777.

The transponder must not be put on standby without instructions from the controller as this would cause its identification to disappear on the radar.

Distress Codes

Specific codes indicate an anomaly:

–7500: in case of hijacking, (should not be used in the “balloon” activity).
-7600: in case of radio failure.
-7700: in case of distress, general alert.



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

Transponder limits

  • 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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