IFF
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Meaning: Identification Friend or Foe
A transponder-based system of aircraft identification, used to distinguish friendly craft from enemy craft.
The system essentially uses a "Question and Answer" routine between one aircraft and another. A correct response leads to the questioned aircraft being identified as friendly. In this respect, IFF can be considered a Challenge-response Authentication system.
The interrogation process is undertaken automatically, with no direct pilot / operator intervention necessary.
IFF systems can also be incorporated into missile and or defence systems, thus preventing them from firing on / acquiring "friendly" targets. IFF transponders may also be fitted to civilian aircraft for the same reason.