Air-to-surface-vessel radar
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.
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.
Development of the original ASV systems started in 1937 after the team testing an experimental air-to-air radar noticed odd returns while flying near the shore of the North Sea.
[1] To further test the concept, Robert Watson-Watt provided the team with two Avro Ansons that were able to fly out over the North Sea from nearby RAF Martlesham Heath.
[4] These designs had a relatively long minimum range, meaning the submarine targets disappeared from the display just as the aircraft was readying for the attack.
Their effect was dramatic; German U-boats had previously been almost completely safe at night, and could operate out of the Bay of Biscay in spite of it being close to British shores.
By the spring of 1942, Biscay was increasingly dangerous, with aircraft appearing out of nowhere in the middle of the night, dropping bombs and depth charges, and then disappearing again in moments.
Robert Hanbury Brown suggested a new ASV could be quickly introduced by making minor changes to the new H2S radar, mostly to the antenna.
[9] The Germans spent much of the rest of the year using radar detectors at longer wavelengths in a fruitless attempt to find the new ASV.
Further confusion was added by a captured Coastal Command pilot, who related that ASV was no longer used for search, but only in the last minutes of the approach.
[10] This incredible deception not only further delayed the German discovery of the true nature of the problem but also allowed Mark II to once again become effective.
[11] The reason for the long delay in discovering Mark III is somewhat surprising given that a magnetron from H2S fell into German hands almost immediately after it was first used in February 1943.
[12] The magnetron was revealed to the United States in 1940 during the Tizard Mission, and local development began at the MIT Radiation Laboratory in a matter of weeks.
The early DMS-1000 proved to be an excellent unit, but for reasons unknown, the US War Department decided to put the inferior Western Electric SCR-517 into production instead.
Initially, they planned on using the Consolidated B-24 Liberator, which had the range to operate over the Mid-Atlantic Gap, and an example of this aircraft with the DMS-1000 was sent to the UK for testing in early 1942.
One was the introduction of new classes of U-boats with much higher performance, and the other was the use of the schnorkel, allowing even older types to spend most of their time submerged.
[16] The Beaufighter, which became one of Coastal Command's primary strike fighters, had the problem that the fitting of ASV required the removal of some other devices to make room.
This was a very simple system originally intended to indicate a selected range to the pilot, which proved very useful for timing bomb drops.
[21] The opening of the Cold War led to a rapid re-evaluation of this stance, especially as the Soviets were known to be introducing new submarines surpassing even the late-war German designs.
These had been used as very long-range patrol aircraft during the war, and their return to the US left Coastal Command with no suitable airframes to cover the GIUK gap.
[25] The main improvements were the addition of stabilization so the image did not change when the aircraft manoeuvred, and the use of a pressurised radome that kept out humidity and made it suitable for use in tropical areas.
[27][26][28] ASV21 could detect a submarine schnorkel at 15 nautical miles (28 km) "in very favourable conditions but at much shorter range in the sea states normally experienced in the North Atlantic.
A new version of Searchwater, the LAST, was created to provide this coverage when mounted under a Westland Sea King helicopter that was able to operate from a variety of ships.
[28] "Searchwater was a completely new concept, having a high power wideband TWT transmitter and being the first generation of ASV radars to include modern signal and data processing (digital as well as analogue)".
