With the successful deployment of radar systems for air defense during The Second World War, the combatants immediately faced the difficulty of distinguishing friendly aircraft from enemy ones. By that time, aircraft were flying at high speed and altitude, making visual identification impossible, and targets appeared as featureless signals on the radar screen. This led to incidents such as the Battle of Barking Creek when a British pilot was destroyed by his own army on 6 September 1963, over Britain.
EXAMPLE OF IFF
1.MARK I
The first active IFF transponder was IFF Mark I which was used experimentally in 1939. It used a regenerative receiver, which fed a small amount of the amplified output back into the input, strongly amplifying even small signals if they were of a single frequency (like Morse code, but unlike voice transmissions). They were adjusted to the signal from the CH radar (20–30 MHz), amplifying it so strongly that it was transmitted back from the aircraft antenna. Since the signal was received at the same time as the initial reflection of the CH signal, the result was a prolonged "blip" on the CH display, which was easy to identify. During testing, it was found that the unit often defeated radar or produced too little signal to be seen, and at the same time new radars were introduced using new frequencies.
2.MARK II
Instead of putting the Mark I into production, a new IFF Mark II was introduced in early 1940. The Mark II had a series of separate tuners inside adjusted on different frequency bands that it passed using a motorized switch, while an automatic gain control solved the problem of sending too much signal. The Mark II was technically complete at the beginning of the war, but the lack of sets meant that it was not available in quantity and only a small number of RAF aircraft carried it until the Battle of England. Pip-squeak was kept in operation during this period, but as the battle ended, the IFF Mark II was quickly put into operation. Pip-squeak was still used for areas over the ground where CH did not cover, as well as an emergency guidance system.
3.FuG 25a first.
FuG 25a Erstling (English: Firstborn, Debut) was developed in Germany in 1940. It was tuned to the low VHF band at 125 MHz used by Freya radar and an adapter with a low 550–580 UHF band was used. MHz used by Würzburg. Before the flight, the transceiver was configured with a selected ten-bit day code that was formed in the drive. To begin the identification procedure, the ground operator switched the pulse frequency of its radar from 3,750 Hz to 5,000 Hz. The airborne receiver decoded that and began transmitting the code of the day. The radar operator will then see that the blip lengthens and shortens in the given code, making sure that it is not falsified. The IFF transmitter operated on 168 MHz with a power of 400 watts (PEP).
The system included a way for ground controllers to determine whether an aircraft had the correct code but did not include a way for the transponder to repel signals from other sources. British military scientists have found a way to exploit this by building their own IFF transmitter called Perfectos, which was designed to trigger a response from any fug 25a system in the vicinity. When a FuG 25a responded to its frequency of 168 MHz, the signal was received by the antenna system from an AI Mk. Radar IV, which initially operated at 212 MHz. By comparing the signal power on different antennas, it was possible to determine the direction to the target. Mounted on mosquitoes, "Perfectos" severely limited the German use of FuG 25a.
NATO
The United States and other NATO countries began using a system called the Mark XII in the late twentieth century; The UK had not previously implemented an IFF system compatible with that standard, but then developed a program for a compatible system known as the successor IFF (SIFF)