ASV (Air-to-Surface-Vessel radar) Mk XI, being radar equipment for the detection of vessels at sea. A rotating aerial in the form of a parabolic reflector permitted continuous all-round scanning on the sea in the vicinity and enabled Plan Position Indication (PPI) to be employed on the Cathode Ray Tube (CRT). Thus, all targets within range could be kept under continuous and simultaneous observation and the bearing of each could be accurately determined.

Facilities were provided for the recognition of long-range radar beacons and also beam-approach and Identification Friend or Foe (IFF) apparatus.

This equipment was developed in 1942-1943 and used in Royal Air Force (RAF) and Fleet Air Arm (FAA) aircraft from early 1944 onwards for the protection of the Atlantic and North Russian convoys, in mine-laying operations off the Norwegian coast and in patrol duties in British home waters.

Its primary function, like that of the ASV Mk. II, was to detect objects on the surface of the sea such as ships, submarines and coastlines but, my virtue of the employment of 3cm radiation, it possessed many advantages over the earlier equipment.