 ASV Mk. II antennas on a RCAF Consolidated Canso (PBY Catalina). The transmitter is on top of the cockpit, the darker-colored port-side receiver is mounted below and to the right, angled outward. | |
| Country of origin | UK |
|---|---|
| Introduced | 1940 |
| No. built | ~24,600 |
| Type | Sea-surface search |
| Frequency | 176 MHz (VHF) |
| PRF | 400 pps |
| Beamwidth | ~150 degrees |
| Pulsewidth | 2.6 μs |
| Range | 1 to 40 miles (1.6–64.4 km) |
| Precision | ~5 degrees |
| Power | 7 kW |
| Other Names | Type 286, ASE (US Navy), SCR-521 (US Army) |
Radar, Air to Surface Vessel, Mark II, or ASV Mk. II for short, was an airborne sea-surface search radar developed by the UK's Air Ministry immediately prior to the start of World War II. It was the first aircraft-mounted radar of any sort to be used operationally. It was widely used by aircraft of the RAF Coastal Command, Fleet Air Arm and similar groups in the United States and Canada. A version was also developed for small ships, the Royal Navy's Type 286.
The system was developed between late 1937 and early 1939, following the accidental detection of ships in the English Channel by an experimental air-to-air radar. The original ASV Mk. I entered service in early 1940 and was quickly replaced by the greatly improved Mk. II. A single Mk. II was shipped to the US during the Tizard Mission in December 1940, where it demonstrated its ability to detect large ships at a range of 60 miles (97 km). Production was immediately taken up by Philco in the US and Research Enterprises Limited in Canada, with over 17,000 produced for use in the US alone.
It was Mk. II equipped Fairey Swordfish that located the German battleship Bismarck in heavy overcast skies, torpedoing her and leading to her destruction the next day. Mk. II was only partially effective against the much smaller U-boats, especially as the signal faded as the aircraft approached the target and they would lose contact at night. To close the gap, the Leigh light was introduced, allowing the U-boat to be picked up visually after it passed off the radar display. With the introduction of the Leigh light, nighttime U-boat interceptions became common, and turned the German ports in the Bay of Biscay into deathtraps.
A microwave-frequency ASV radar, ASVS, was under development since 1941, but the required cavity magnetrons were in limited supply and priority was given to H2S. The capture of a Mk. II-equipped Vickers Wellington by the Germans led to the introduction of the Metox radar detector tuned to its frequencies. This was soon followed by British pilots reporting submarines diving as the aircraft began to approach. A new design based on H2S, ASV Mk. III, was rushed to service, replacing the Mk. II beginning in 1943. Mk. II remained in use throughout the war in other theatres.