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IFF is an abbreviation of the phrase "identification of friend or foe." Since radar is used to direct gun fire or bombing at night, or in daylight when the target is beyond the range of visibility, it is obviously important that there be radar equipment that will serve to identify instantly any aircraft or ship as friend or foe before attack.

Two basic components in any identification system are: (1) the transpondor, carried by the aircraft or ship to be identified; (2) the interrogator-responsor, or questioning device, located aboard a ship, at a ground station, or in another aircraft.

The transpondor on a plane (or ship) is a combination radio transmitter and receiver, packaged in one small box. Normally, the transpondor is in a receiving condition, and will continue to function as a receiver until it receives a radio signal. Then, the transpondor will automatically become a transmitter and, as such, will transmit a signal in reply to that received. (The transpondor is said to have been triggered.)

In the identification systems the transpondor when functioning as a receiver mechanically sweeps a band (or bands) of frequencies within a few seconds. If during a sweep through the frequency band a radio signal within that band is received, the transpondor will send back the received signal within a fraction of a second, and will transmit the signal on the same frequency on which the signal was received. In effect, then, the transpondor acts as a selective mirror to radio signals.

The interrogator-responsor is also a transmitter and receiver. The transmitter (interrogator) is designed to permit triggering a transpondor. The receiver (responsor) gets the automatic transmitted reply from the transpondor, and by means of additional equipment displays this transmitted reply on a radar scope.