By early 1941, the ASV Mk 2 systems were in production. In fact there were about 100 ASV-fitted aircraft in service with Coastal Command by the end of 1941 and the first successful sinking of a U-Boat (U 206) (using ASV) took place on November 30th 1941. The number of successful 'kills' began to rise but the U-Boats continued to hold sway throughout 1942.

In June 1942, the introduction of the 'Leigh Light'; a very powerful search beam attached to the aircraft aided the bomb aimer on the final bomb run onto a surfaced U-Boat. U-502 was sunk with the aid of the Leigh light. As a counter measure against ASV, the Germans quickly developed a 'jamming' unit known as METOX.

Fortunately, by March 1943 the 'all new' ASV Mk3 units, based upon 10 cm wavelengths were available and it is this equipment more than anything else that turned the balance of superiority away from the U-Boat and towards the searching aircraft and ships. However, their allocation to Coastal command was not achieved without a political battle being fought within the Government. Sir Henry Tizard and his colleague P.M.Blackett wanted ASV Mk3 to go to Coastal Command first but Churchill, Watson Watt and Professor Lindemann advocated that Bomber Command should be equipped with this new system first and that Coastal Command would have to wait its turn.

Eventually, by August 1943, enough sets were installed into long-range patrol aircraft to turn the tide against the U-Boat. These planes were now equipped with every device at the allies disposal and their U-Boat patrols were
becoming very successful.