IFF Mark II was the first operational identification friend or foe system. It was developed by the Royal Air Force just before the start of World War II. After a short run of prototype Mark Is, used experimentally in 1939, the Mark II began widespread deployment at the end of the Battle of Britain in late 1940. It remained in use until 1943, when it began to be replaced by the standardised IFF Mark III, which was used by all Allied aircraft until long after the war ended. The Mark I was a simple system that amplified the signals of the British Chain Home radar systems, causing the aircraft's "blip" to extend on the radar display, identifying the aircraft as friendly. Mark I had the problem that the gain had to be adjusted in flight to keep it working; in the field, it was correct only half the time. Another problem was that it was sensitive to only one frequency and had to be manually tuned to different radar stations. In 1939, Chain Home was the only radar of interest and operated on a limited set of frequencies but new radars were already entering service and the number of frequencies was beginning to multiply. Mark II addressed both these problems. An automatic gain control eliminated the need to adjust the gain, making the device much more likely to be working properly when interrogated. To work with many types of radar, a complex system of motorised gears and cams constantly shifted the frequency through three wide bands, scanning each every few seconds. These changes automated the operation of the device and made it truly useful for the first time; previously, operators could not be sure if a blip was an enemy aircraft or a friendly one with a maladjusted IFF. Originally ordered in 1939, installation was delayed during the Battle of Britain and the system became widely used from the end of 1940. Although the Mark II's selection of frequencies covered the early war period, by 1942 so many radars were in use that a series of sub-versions had been introduced to cover particular combinations of radars. The introduction of new radars based on the cavity magnetron required different frequencies to which the system was not easily adapted. This led to the introduction of the Mark III, which operated on a single frequency that could be used with any radar; it also eliminated the need for the complex gear and cam system. Mark III began entering service in 1943 and quickly replaced the Mark II.
History
Early efforts Before Chain Home (CH) systems began deployment, Robert Watt had considered the problem of identifying friendly aircraft on a radar display. He filed initial patents on such systems in 1935 and 1936. In 1938, researchers at the Bawdsey Manor radar research establishment began working with the first of Watt's concepts. This was a simple "reflector" system consisting of a set of dipole antennas that were tuned to resonate at the frequency of the CH radars. When a pulse from the radar hit them, they would resonate for a short period and cause an additional signal to be received by the station. The antennas were connected to a motorised switch that periodically shorted the antenna out and cancelled the broadcast, causing the signal to turn on and off. On the CH display, this caused the "blip" to periodically lengthen and contract. The system proved highly unreliable; it worked only when the aircraft was at certain locations and flying in certain directions. It was always suspected that this system would be of little use in practice. When that turned out to be the case, the Royal Air Force (RAF) introduced a different system that consisted of a set of tracking stations using HF/DF radio direction finders. The standard aircraft radios were modified to send out a 1 kHz tone for 14 seconds every minute, allowing the tracking stations ample time to measure the aircraft's bearing. Several such stations were assigned to each sector of the air defence system and sent their measurements to a plotting station at sector headquarters. There they used triangulation to determine the aircraft's location. Known as "pip-squeak", the system worked but was very labour-intensive, requiring operators at several stations and at plotting boards in sector HQs. More operators were needed to merge the information from the pip-squeak system with that from the radar systems to provide one view of the airspace. It also meant the pilots were constantly interrupted when talking to their ground controllers. A system that worked directly with the radar was desired.
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