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In telecommunications, identification, friend or foe (IFF) is an identification system designed for command and control. It enables military and national (civilian-located ATC) 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 World War II. The term is something of a misnomer, as IFF can positively identify friendly targets but 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 a foe. There are 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 fratricide, but also contributes to overall tactical decision-making. 
With the successful deployment of radar systems for the defence of the British Isles, the British were immediately confronted with difficulty in 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.
Already before the deployment of the radar system, the RAF had implemented a tracking system for directing their own forces; aircraft were tracked by triangulating their position from fixed positions, using intermittent "pip-squeak" signals from each aircraft. This system was used during the Battle of Britain to track 'friendly' formations. It used the standard HF radio fit of RAF fighters, plus a control box that essentially contained a timer, so that a pre-arranged signal was 'squawked' at given intervals - to be picked up by the HF/DF tracking stations.
With the introduction of radar, researchers endeavoured to integrate a working IFF system with the radar system. Initial patents for both passive and active radar IFF were filed in the UK by Robert Watson-Watt in 1935 and 1936 respectively. By 1938, researchers at Bawdsey Manor had come up with 'reflectors' mounted on the aircraft designed to reflect the Chain Home wave frequencies - giving friendly forces a distinctive profile on the radar screens. The design, however, proved impractical, as other difficult-to-control factors, including the angle of the incident radar beam with the plane's flight-path, influenced the magnitude of the reflected signal making a consistent friend or foe designation hard to achieve.
The first active IFF transponder was the 'IFF Mark I' and was put into operation in 1939. On receipt of an interrogation from the Chain Home radar system (20-30 MHz), consisting of a radar distinctive 'peaked' pulse tuned to a specific frequency, the unit would respond with a signal of steadily increasing amplitude, allowing the radar operators to identify it as friendly.
Flaws in the Mark I version, including the need to constantly adjust it during flight, were rectified somewhat with the Mark II sets; these could also be interrogated by alternate fequency band transmissions, including those from mobile sets in use with the navy and army.
In 1940, English engineer Frederic Calland Williams began work on the Mark III system at the Telecommunications Research Establishment, which was to become the standard for the Western Allies for most of the war. 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. The IFF sets were designed and built by Ferranti in Manchester to Williams' specifications. Equivalent sets were manufactured in the US, initially as copies of British sets, so that allied aircraft would be identified upon interrogation by each other's radar.
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 watts (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.
The IFF of World War II and Soviet military systems (1946 to 1991) used coded radar signals (called Cross-Band Interrogation, or CBI) to automatically trigger the aircraft's transponder in an aircraft illuminated by the radar. Radar-based IFF is also called secondary radar, with primary radar bouncing an RF pulse off of the aircraft to determine position. George Charrier, working for RCA, filed for a patent for such an IFF device in 1941 that required the operator to perform several adjustments to the radar receiver in order to suppress the image of the natural echo on the radar receiver in order to visually examine the IFF signal.
By 1943, Donald Barchok filed a patent for a radar system using the acronym IFF in his text with only parenthetic explanation, indicating that this acronym had become an accepted term. 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.
An IFF system must be standardised so that all allied users can recognise each other.
The United States and other NATO countries started using a system called Mark XII in the late twentieth century; Britain had not until then implemented an IFF system compatible with that standard, but then developed a program for a compatible system known as successor IFF (SIFF).
Mode 1 – provides 2-digit octal mission code. (military only – can be changed in flight)
Mode 2 – provides 4-digit octal unit code. (military only – can't be changed in flight)
Mode 3/A – provides a 4-digit octal identification code for the aircraft, assigned by the air traffic controller. (military and civilian)
Mode 4 – provides a 3-pulse reply to crypto coded challenge. (military only)
Modes 4 and 5 are designated for use by NATO forces.