Identification friend or foe

Lore & Background

Before the deployment of Chain Home radar, the RAF had considered the problem of IFF. Robert Watson-Watt had filed patents on such systems in 1935 and 1936. By 1938, researchers at Bawdsey Manor began experiments with reflectors consisting of dipole antennas tuned to resonate to the primary frequency of the Chain Home 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, causing the return on the CH set to periodically lengthen and shorten. In practice, the system was found to be too unreliable to use.

IFF Mark I

The first active IFF transponder was the IFF Mark I, used experimentally in 1939. It used a regenerative receiver tuned to the signal from the CH radar (20–30 MHz), amplifying it so strongly that it was broadcast back out the aircraft's antenna, producing a lengthened blip on the CH display. In testing, it was found that the unit would often overpower the radar or produce too little signal.

IFF Mark II

IFF Mark II was introduced in early 1940 with separate tuners stepped through by a motorized switch and automatic gain control. Mark II was technically complete as the war began, but a lack of sets meant it was not available in quantity. IFF Mark III became the standard for the Western Allies for most of the war, designed to respond to specific interrogators rather than replying directly to received radar signals.

Germany

Germany developed the FuG 25a Erstling in 1940, tuned to the low-VHF band at 125 MHz used by the Freya radar. Before a flight, the transceiver was set up with a selected day code of ten bits.

The ground operator switched the pulse frequency of his radar from 3,750 Hz to 5,000 Hz, and the airborne receiver decoded that and started to transmit the day code. The IFF transmitter worked on 168 MHz with a power of 400 watts (PEP). British military scientists exploited this by building their own IFF transmitter called Perfectos, designed to trigger a response from any FuG 25a system.

Reader's Guide

IFF is a tool within the broader military action of combat identification (CID), the characterization of objects detected in the field of combat with sufficient accuracy to support operational decisions. The broadest characterizations are those of friend, enemy, neutral, or unknown. CID can not only reduce friendly fire incidents but also contributes to overall tactical decision-making. IFF can only positively identify friendly aircraft or other forces; if an IFF interrogation receives no reply or an invalid reply, the object is not positively identified as foe.

Friendly forces may not properly reply to IFF for various reasons, such as an equipment malfunction, being near combat but not involved, or being a civilian light general aviation aircraft which may not carry a transponder. After World War II, IFF technology was adopted by civilian aircraft to assist air traffic control by identifying individual aircraft on radar displays and improving the reliability of tracking information. IFF sets were highly classified; many of them were wired with explosives in the event the aircrew bailed out or crash landed.