page_title: IFF Mark II
section_title: Mark II
hierarchical_section_title: IFF Mark II / History / Mark II
caption_attribution_description: USAAF pilots with an RAF Supermarine Spitfire at Gibraltar in 1942. Original caption: "United States Army Air Force men in a Spitfire observing the performance of another one in flight. Left to right: they are Colonel Harold B. Willis, Boston, Massachusetts, Major Marvin L. McNickle, Doland, South Dakota, and Captain A. E. Vinson of Monticello, Mississippi."
context_page_description: 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.
context_section_description: Beyond the operational problems with the Mark I, a more serious issue was the growing number of new radar systems being deployed. Even as the Mark I was being tested, the RAF, Royal Navy and British Army were introducing new systems, spanning a wide range of frequencies from the RAF's 200 MHz systems used on night fighters and Chain Home Low to the Army's 75 MHz gun-laying radars and on to the CH at 20 to 30 MHz. Attempting to manually tune among these would be impractical and impossible if the aircraft were visible to more than one radar, which was increasingly the case.
  A solution was already under development in early 1939, similar to the Mark I but employing tuned circuits sensitive to many radar sets. It used a "complicated system of cams and cogs and Geneva mechanisms" to switch among the bands by connecting to oscillators covering a band and then used a motorised tuning capacitor to sweep through the frequency range within that band. To ensure the signal was the right strength and did not cause squitter, an automatic gain control was added. These changes eliminated the need for tuning or gain adjustments in flight, greatly improving the chance it would respond correctly to a radar. Only periodic adjustments on the ground were needed to keep it working properly.
  An order for 1,000 sets was sent to Ferranti in October 1939 and they had completed the first 100 sets by November. The rapid expansion of the RAF precluded a significant proportion of its force being equipped by the time of the Battle of Britain in mid-1940. In any case, the action took place mostly over southern England, where IFF would not be very useful as the CH stations were positioned along the coast and could see the fighters only if they were out over the English Channel. There was no pressing need to install the systems and pip-squeak continued in use during the battle.
  The lack of IFF led to problems including friendly fire; the Battle of Barking Creek in September 1939 would not have occurred if IFF had been installed. It also meant that enemy aircraft could not be identified if they were close to known RAF aircraft. In July 1940, the Germans began to take advantage of this by inserting their bombers into formations of RAF bombers returning from night missions over Europe. To the ground operators these appeared to be more RAF aircraft and once they crossed the coast there was no way to track them. Even if one of the rare Mark I sets was available, the unreliability of their signals made it difficult for controllers to trust it.
  As the Battle of Britain ended, Mark II was rapidly installed in RAF aircraft. Its installation on the Supermarine Spitfire required two wire antennas on the tail that slowed the top speed by 2 miles per hour (3.2 km/h) and added 40 pounds (18 kg) of weight. Pip-squeak was still used for areas over land where CH did not cover, as well as an emergency guidance system. Mark II also found a use on Royal Navy ships, where it was produced as the Type 252 so that ships could identify each other by radar.
  A Mark II set was taken to the US as part of the Tizard Mission in November 1940. US researchers were already working on their own IFF system of some complexity. They realised the importance of using a common IFF system and in early 1941 they decided to install Mark II in their own aircraft. Production was taken up by Philco with an order for 18,000 sets as the SCR-535 in July 1942. The system was never entirely reliable.
caption: The IFF antenna can be seen on the left of this photo, meeting the fuselage in the RAF roundel. The lengthy antennas, which had to be placed on both sides of the fuselage, slowed the Spitfire by about 2 miles per hour (3.2 km/h). Rock of Gibraltar in background.
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