Identification Friend Or Foe
In telecommunications, identification, friend or foe (IFF) is an identification system designed for command and control. It enables military and national (civilian-located ATC) interrogation systems to identify aircraft, vehicles or forces as friendly and to determine their bearing and range from the interrogator.
IFF was first developed during World War II. The term is something of a misnomer, as IFF can positively identify friendly targets but not hostile ones. If an IFF interrogation receives no reply or an invalid reply, the object cannot be identified as friendly but is not positively identified as a foe.
There are many reasons for friendly aircraft not to reply to IFF, such as battle damage or equipment failure, loss of encryption keys, and wrong encryption keys. Terrain-hugging aircraft are very often poor candidates for microwave line-of-sight systems such as the IFF system. Microwaves cannot penetrate terrain, and very often atmospheric effects (referred to as anomalous propagation) cause timing, range, and azimuth issues.
Read more about Identification Friend Or Foe: Historical, Method of Operation, Modes
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