In telecommunication, identification, friend or foe (IFF) is a system using electromagnetic transmissions to which equipment carried by friendly forces automatically responds, for example, by emitting pulses, thereby distinguishing themselves from enemy forces.

The primary use of the term is to describe an aviation subsystem that permits military aircraft, and both civilian and military air-traffic controllers, to identify aircraft as enemy and friendly, and track them.

The fundamental mechanism is the use of special radar signals to automatically trigger an IFF transponder in a friendly aircraft "painted" by the radar. A transponder responds,

  • in a civilian aircraft, by sending a code identifying it;
  • in a military aircraft, by sending, only when detecting a code indicating the radar is part of the friendly-forces' network, a similarly coded signal.

Major benefits of IFF include:
  • Avoiding "friendly-fire" or "fratricidal" attacks
  • Reducing attacks on civilian aircraft
  • Improving, by coordination among friendly forces, decisions about tactics.

The secondary surveillance radar (SSR) system used in modern air traffic control systems is an outgrowth of the military IFF system used during World War II. The IFF equipment carried by modern military aircraft is compatible with the transponder system used for civilian air traffic control.


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