Radar, Air to Surface Vessel, or ASV radar for short, is a classification used by the Royal Air Force (RAF) to refer to a series of aircraft-mounted radar systems used to scan the surface of the ocean to locate ships and surfaced submarines. The first examples were developed just before the opening of World War II and they have remained a major instrument on patrol aircraft since that time. It is part of the wider surface-search radar classification, which includes similar radars in ground and ship mountings.
The first ASV was developed after the accidental detection of wharves and cranes while testing an air-to-air radar in 1937. For a variety of reasons, ASV was easier to develop than the air-to-air variety of the same systems, and the first operational use of the Mark I followed in early 1940. A cleaned-up and repackaged version, ASV Mark II, replaced it at the end of the year, but the system was not widespread until late in 1941.
ASV was useful for detecting U-boats at night, but the target had to be seen to be attacked, a problem that was addressed with the Leigh light with rapidly increasing success. As German U-boat losses shot up in 1942, they concluded the RAF was using radar to detect them and responded with the Metox radar detector. The RAF responded by deploying the microwave-frequency ASV Mark III, which the Germans were unable to detect until the U-boat fleet had already been decimated. A series of other ASVs were developed for different aircraft as the war progressed.
In the post-war era, several new ASV radars were developed, notably ASV Mark 7A, ASV Mark 13 and ASV Mark 21. By the late 1960s the original terminology was no longer being used, and the last major entries in the series were known simply as Searchwater and Seaspray.