page_title: ASV Mark II radar
section_title: Leigh light
hierarchical_section_title: ASV Mark II radar / Development / Leigh light
caption_attribution_description: English: Anti-Submarine Weapons: Leigh Light used for spotting U-boats on the surface at night fitted to a Liberator aircraft of Royal Air Force Coastal Command.
context_page_description: 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. 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.
context_section_description: In spite of the system's ability to detect submarines at night, attacking them was not a simple matter. After finding the rough location on the broadside array, the target was plotted on a map and the aircraft maneuvered so it could begin to approach it using the forward-facing antennas. These had less gain and picked up the submarine at shorter ranges, so there was a possibility the submarine could escape as they switched from side to forward approach.
  But the real problem was that the minimum range of the radar was about 1000 yards at best; at shorter distances the returns from the target merged with the leftover signal from the transmitter and became invisible in the electronic noise and scattering off the water. Unfortunately, 1,000 yards was too long a range for the submarine to be spotted visually at night except under perfect conditions, like a full moon. The same problem affected the AI radars as well, but in that case was far more serious due to the small size of the aircraft targets compared to a U-boat or ship, and the team had invested considerable effort in trying to solve this "minimum range controversy", so far unsuccessfully.
  While this work continued, a new solution was introduced. Humphrey de Verd Leigh, an RAF personnel officer, came up with the idea after talking to returning aircrew and learning of the short-range cutoff problem. He built a searchlight into a streamlined container with a lens that spread the beam so it covered an area several degrees wide at a range of 1,000 yards (910 m), about the same angle as the beam of the radar. It would be turned on just as the signal disappeared on the radar screen, lighting the target and allowing the last seconds of the approach to be carried out visually.
  In March 1941 they began trying to fit it to a Vickers Wellington, and after some effort it was flown successfully. Although the Air Ministry was convinced the idea was workable, they decided to re-use an older searchlight design known as the Turbinlite, which was originally intended to be used in a similar role for night fighters. This was not nearly as powerful as Leigh's version, but was smaller and already available in some numbers. In spite of great effort, the Turbinlite never worked satisfactorily. It was not until late in 1941 before the Ministry admitted this and returned to de Leigh's original design. He had continued to develop it in secret during this time.
  The first examples of the Leigh light started appearing in the early summer of 1942. The first success was on 5 July 1942 when a Wellington of No. 172 Squadron RAF sank U-502. From that point on, the combination of ASV Mk. II and Leigh light proved extremely effective. So many submarines were being attacked by the end of the summer that leaving base at night, formerly entirely safe, was now considered suicidal. The Germans were forced to leave their bases during the day so they could at least see the attacking aircraft and put up a fight, but this proved little safer.
caption: A crewman cleans the Leigh Light mounted under the starboard wing of an RAF Coastal Command Liberator GR Mk V. The light could be aimed to point at the target without having to point the aircraft directly at it.
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