Identification friend or foe
IFF was first developed during World War II, with the arrival of radar, and several friendly fire incidents.
[6] 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,[7][8][9] and the air attack on the fortress of Koepenick over Germany.
[10][11] Already before the deployment of their Chain Home radar system (CH), the RAF had considered the problem of IFF.
By 1938, researchers at Bawdsey Manor began experiments with "reflectors" consisting of dipole antennas tuned to resonate to the primary frequency of the CH radars.
They were 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.
Mark II had a series of separate tuners inside tuned to different radar bands that it stepped through using a motorized switch, while an automatic gain control solved the problem of it sending out too much signal.
Mark II was technically complete as the war began, but a lack of sets meant it was not available in quantity and only a small number of RAF aircraft carried it by the time of the Battle of Britain.
By 1941, a number of sub-models were introduced that covered different combinations of radars, common naval ones for instance, or those used by the RAF.
But the introduction of radars based on the microwave-frequency cavity magnetron rendered this obsolete; there was simply no way to make a responder operating in this band using contemporary electronics.
In 1940, English engineer Freddie Williams had suggested using a single separate frequency for all IFF signals, but at the time there seemed no pressing need to change the existing system.
With the introduction of the magnetron, work on this concept began at the Telecommunications Research Establishment as the IFF Mark III.
Mark III transponders were designed to respond to specific 'interrogators', rather than replying directly to received radar signals.
The system also allowed limited communication to be made, including the ability to transmit a coded 'Mayday' response.
Before a flight, the transceiver was set up with a selected day code of ten bits which was dialed into the unit.
British military scientists found a way of exploiting this by building their own IFF transmitter called Perfectos, which were designed to trigger a response from any FuG 25a system in the vicinity.
But it requires a complete transmitter for the responder side of the circuitry, in contrast to the greatly simplified regenerative system used in the British designs.
When the Mark II was revealed in 1941 during the Tizard Mission, it was decided to use it and take the time to further improve their experimental system.
The main difference between this and earlier models is that it worked on higher frequencies, around 600 MHz, which allowed much smaller antennas.
This led to a US–British effort to make a further improved model, the Mark V, also known as the United Nations Beacon or UNB.
As development continued it was decided to introduce an encoding system known as the "Selective Identification Feature", or SIF.
This was during a period of great expansion of the civilian air transport system, and it was decided to use slightly modified Mark X sets for these aircraft as well.
By placing this function on the IFF, the same information could be returned for little additional cost, essentially that of adding a digitizer to the aircraft's altimeter.
[citation needed] It had long been considered a problem that the IFF responses could be triggered by any properly formed interrogation, and those signals were simply two short pulses of a single frequency.
If the result of that operation matches the value dialled into the IFF in the aircraft, the transponder replies with a Mode 3 response as before.
[16] By 1943, Donald Barchok filed a patent for a radar system using the abbreviation IFF in his text with only parenthetic explanation, indicating that this acronym had become an accepted term.
[17] In 1945, Emile Labin and Edwin Turner filed patents for radar IFF systems where the outgoing radar signal and the transponder's reply signal could each be independently programmed with a binary codes by setting arrays of toggle switches; this allowed the IFF code to be varied from day to day or even hour to hour.
