Historical
The world's first IFF, FuG 25a "Erstling" (English: "Debut"), was developed in Germany in 1940. It received the radar frequencies on 125 MHz (Freya radar) and 550-580 MHz (Würzburg radar). To start the identification procedure, the ground operator switched the pulse frequency of his radar from 3,750 Hz to 5,000 Hz.
The airborne receiver decoded that and started to transmit its code. Before departure, two mechanical keys of ten bits each were inserted into the shown reader. The IFF transmitter worked on 168 MHz with a power of 400 Watt (PEP). Unfortunately for the Germans, British military scientists designed and built their own IFF transmitter called "Perfectos", which was designed to trigger a response from any FuG 25a systems in the vicinity. When mounted in an RAF Mosquito, the "Perfectos" device revealed the position of any German nightfighters fitted with an FuG 25a. As a result, the British "Perfectos" device severely compromised German use of the FuG 25a.
Read more about this topic: Identification Friend Or Foe
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