ASV Mark III radar

Radar, Air-to-Surface Vessel, Mark III, or ASV Mk. III for short, was a surface search radar system used by RAF Coastal Command during World War II. It was a slightly modified version of the H2S radar used by RAF Bomber Command, with minor changes to the antenna to make it more useful for the anti-submarine role. It was Coastal Command's primary radar from the spring of 1943 until the end of the war. Several improved versions were introduced, notably the ASV Mark VI, which replaced most Mk. IIIs from 1944 and ASV Mark VII radar, which saw only limited use until the post-war era. Coastal Command's first radar was ASV Mark I, which began experimental use in 1939. Minor improvements were made for the Mark II in 1940 but it was not widely available until late 1941. Having realized that the RAF was using radar to detect their U-boats, in the summer of 1942 the Germans introduced the Metox radar detector to listen for their signals. This gave the submarine a warning of the aircraft approach long before the submarine became visible on the aircraft's radar display. The RAF noticed this in early autumn when crews reported with increasing frequency that they would detect submarines that would disappear as they approached. An ASV working in microwave frequencies using the new cavity magnetron had been under development for some time at this point, known as ASVS, but had not matured for various reasons. Robert Hanbury Brown suggested using H2S for ASV but this had been rejected by Bomber Command, who wanted all the sets for themselves. Brown continued development with EMI and presented it again in late 1942 when Metox negated the earlier marks of ASV. Obstruction by Bomber Command led to more delays and it was not until March 1943 that the first dozen aircraft were operational. Deliveries were rapid after this point and Mk. II had been largely replaced by the end of the summer. The Germans had no way to detect the signals from Mark III, which operated in the 10 cm band compared to the 1.5 m wavelength of the Mk. II.