H2S was the first airborne, ground scanning radar system. It was developed for the Royal Air Force's Bomber Command during World War II to identify targets on the ground for night and all-weather bombing, allowing attack outside the range of the various radio navigation aids like Gee or Oboe, which were limited to about 500 kilometres (310 mi). It was also widely used as a general navigation system, allowing landmarks to be identified at long range.
In March 1941, experiments with an early radar based on the cavity magnetron revealed that different objects have very different radar signatures; water, open land and built up areas of cities and towns all produced distinct returns. In January 1942, a new team was set up to combine the current S band magnetron with a new scanning antenna and plan-position indicator display, and the prototype's first use in April confirmed that a map of the area below the aircraft could be produced using radar. The first systems worked at 9.1 cm (3 GHz) like the AI Mk. VIII radar they were developed from, and went into service in 1942 as the TR3159 (H2S Mk. I/ASV VIB) and TR3191 (H2S Mk. II). After it was found the resolution of these sets was too low to be useful over large cities like Berlin, in 1943 work started on a version operating in the X band at 3 cm (10 GHz). A wide variety of these H2S Mk. III versions were produced before the Mk. IIIG was selected as the late-war standard. The US Radiation Laboratory also produced an X band system, the H2X.
On its second operational mission on 2/3 February 1943, an H2S was captured almost intact by German forces. Combined with intelligence gathered from the surviving crew, they learned it was a mapping system and were able to determine its method of operation. When they managed to piece one together from parts and saw the display of Berlin, near panic broke out in the Luftwaffe. This led to the introduction of the FuG 350 Naxos radar detector, which enabled Luftwaffe night fighters to home on the transmissions of H2S. The British learned of Naxos and a great debate ensued over the use of H2S. However, calculations showed that losses during this period were actually less than before; the use of H2S as a night fighter detector saved more bombers than Naxos claimed.
Development continued through the late-war Mk. IV to the 1950s era Mk. IX that equipped the V bomber fleet. The Mk. IX was tied into both the bombsight and navigation system to provide a complete long-range Navigation and Bombing System (NBS). In this form, H2S was last used in anger during the Falklands War in 1982 on the Avro Vulcan. Some H2S Mk. IX units remained in service on the Handley Page Victor aircraft until 1993, providing fifty years of service.
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