IFF Mark III, also known as ARI.5025 in the UK or SCR.595 in the US, was the Allied Forces standard identification friend or foe (IFF) system from 1943 until well after the end of World War II. It was widely used by aircraft, ships, and submarines, as well as in various adaptations for secondary purposes like search and rescue. 500 units were also supplied to the Soviet Union during the war. Mark III replaced the earlier Mark II which had been in service since 1940. Mark II had an antenna that received signals from radar systems, amplified them, and returned them. This caused the blip on the radar display to become larger, indicating a friendly aircraft. As the number of radar systems on different frequencies proliferated through the mid-war period, the number of models of Mark II had to do the same. Aircraft could never be sure their IFF would respond to the radars they flew over. Freddie Williams had suggested using a single separate frequency for IFF as early as 1940, but at that time the problem had not become acute. The introduction of microwave radars based on the cavity magnetron was the main reason for adopting this solution, as the Mark II could not easily be adapted to respond on these frequencies. In 1942, a new frequency band, between 157 and 187 MHz, just below most VHF radars, was selected for this role. The only downside to this design is that the radar itself no longer provided the trigger signal for the transponder, so a separate transmitter and receiver was needed at the radar stations. The Mark III began to replace the Mark II in 1942 and 1943, in a somewhat lengthy switchover period. It was also used as the basis for several other transponder systems such as Walter and Rebecca/Eureka, which allowed suitably equipped aircraft to home in on locations on the ground. These found use for dropping paratroopers and supplies in Europe, locating downed aircraft, and other roles. Several newer IFF designs were trialled, but none of them offered enough of an advantage to warrant a switchover. Mark III was replaced by IFF Mark X over an extended time starting in 1952.
History
IFF Mark I and II
IFF Mark I was the first IFF system to see experimental use, with a small number of units installed in 1939. Mark I was a simple system that listened for signals on the 5 meter band used by Chain Home radars and responded by sending out a short pulse on the same frequency. At the Chain Home station, this signal would be received slightly after the reflection of the station's own broadcast signal, and was more powerful. The result was that the aircraft's blip on the radar display grew larger and stretched out. The same blip would be produced if the radar was tracking a group of targets in formation, so the transponder also had a motorized switch that turned the signal on and off, causing the blip to oscillate on the display. Mark I was used only experimentally, with about 50 sets completed in total. The problem with Mark I was that it operated only on the 23 MHz Chain Home frequency. By 1939 there were already several other radars being introduced that operated on different frequencies, notably the 75 MHz used by the GL Mk. I radar and the 43 MHz used by the Royal Navy's Type 79 radar. To address this, development of the IFF Mark II began in October 1939 and the first units were available in early 1940. This used a complex mechanical system to select among several separate radio tuners and sweep through each one's band of frequencies, ensuring it would hear the radar signal from any of the systems in service at some point in the 10 second cycle. Mark II was the first system to be operationally deployed, and was widespread by late 1940. Even as Mark II was being deployed, it was clear that the number of radars being introduced would shortly present a problem even for that system. In 1940 Freddie Williams had suggested that the IFF systems should work on their own frequency band instead of trying to listen for every possible radar that might come along. This would also have the advantage that the radio electronics would be much simpler, eliminating the complex mechanical switch and multiple tuners. At the time it was not considered a serious enough problem to warrant a change, instead the Mark II's would be given different tuners depending on which radars they were expected to encounter. It was not long before there was a profusion of different versions of the Mark II covering different combinations of radars.
IFF Mark III
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