Identification friend or foe

Identification, friend or foe (IFF) is an identification system designed for command and control. It enables military and national (civilian air traffic control) interrogation systems to identify aircraft, vehicles or forces as friendly and to determine their bearing and range from the interrogator. IFF may be used by both military and civilian aircraft.

IFF was first developed during the Second World War, with the arrival of radar, and early friendly fire incidents.

Despite the name, IFF can only positively identify friendly targets, 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 foe. There are in addition many reasons that friendly aircraft may not properly reply to IFF.

IFF is a tool within the broader military action of Combat Identification (CID), "the process of attaining an accurate characterization of detected objects in the operational environment sufficient to support an engagement decision." The broadest characterization is that of friend, enemy, neutral, or unknown. CID not only can reduce friendly fire incidents, but also contributes to overall tactical decision-making.

With the successful deployment of radar systems for air defence during World War II, combatants were immediately confronted with the difficulty of distinguishing friendly aircraft from hostile ones; by that time, aircraft were flown at high speed and altitude, making visual identification impossible, and the targets showed up as featureless blips on the radar screen. This led to incidents such as the "battle of Barking Creek", over Britain, and the "air attack on the fortress of Koepenick", over Germany.

Already before the deployment of their Chain Home radar system (CH), the RAF had considered the problem of IFF. Robert Watson-Watt had filed patents on such systems in 1935 and 1936 respectively. By 1938, researchers at Bawdsey Manor began experiments with "reflectors" consisting of a dipole antennas tuned to resonate at the primary frequency of the CH radars. When a pulse from the CH transmitter hit the aircraft, the antennas would resonate for a short time, increasing the amount of energy returned to the CH receiver. The antenna was connected to a motorized switch that periodically shorted it out, preventing it from producing a signal. This caused the return on the CH set to periodically lengthen and shorten as the antenna was turned on and off. In practice, the system was found to be too unreliable to use; the return was highly dependent on the direction the aircraft was moving relative to the CH station, and often returned little or no additional signal.

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