Initial concept K8758 demonstrated ASV in commanding fashion in September 1937. 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. They eventually realized these were the docks and cranes at the
Harwich docks miles south of them. Shipping also appeared, but the team was unable to test this very well as their
Handley Page Heyford was forbidden to fly over water. 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. The testing was crude; a small
dipole antenna was hand held outside one of the escape hatches and rotated looking for when the signal disappeared, indicating the antenna was aligned with the target ship. This was not easy as the signal naturally fluctuated. The first successes were in August 1937. After several successful flights over the summer, Watt asked the team if they could be ready for a demonstration in September. On 4 September, the system was used to detect
Royal Navy ships on manoeuvres in almost complete overcast. The weather was so bad they had to use the radar pattern from local sea-side cliffs to navigate home.
Albert Percival Rowe of the
Tizard Committee later commented that "This, had they known, was the writing on the wall for the German Submarine Service."
Mark I and II Production quality sets were available in 1939 and entered operational service in early 1940, becoming the first radar system to be mounted on an aircraft in a combat setting. A somewhat improved version, Mark II, followed in 1941, which saw tens of thousands of units produced in the UK, Canada, US and Australia. 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. At night, this allowed the submarines to escape attack by maneuvering when an aircraft could be heard. This was solved by the
Leigh Light, a
searchlight that lit up the submarines during the last seconds of the approach. By early 1942, Mark II and the Leigh Light were finally available on large numbers of aircraft. 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. The Germans ultimately solved the problem of Mark II with the introduction of the
Metox radar detector. This amplified the radar's pulses and played them into the radio operator's headphones. It provided this warning long before the echos from the submarine became visible on the aircraft's display. With experience, the operators could tell whether the aircraft was approaching or just flying by, allowing the U-boat to dive and escape detection. By the end of 1942, Mark II had been rendered ineffective.
Mark III The introduction of the
cavity magnetron in early 1940 led to efforts to develop microwave-frequency versions of the various radars then in use, including a new ASV under the name ASVS for "Sentimetric". A prototype was available from
Metrovick in the summer of 1942, but they predicted it would not be widely available until summer 1943. It was at this point that the Metox started to become effective.
Robert Hanbury Brown suggested a new ASV could be quickly introduced by making minor changes to the new
H2S radar, mostly to the antenna. This started a furious debate between
RAF Bomber Command, who wanted every H2S for their bombers, and
RAF Coastal Command, who wanted them for submarine hunting. After several changes in policy, the first
ASV Mark III's began arriving in March 1943, and had largely replaced the Mark II in front-line units by the end of the summer. The Germans had no way to detect these signals, and their submarines were repeatedly attacked with no warning. The losses were so great they took to leaving port in the day, but the RAF responded with
strike aircraft patrols and losses shot up once again. In August, shipping losses to submarines was the lowest since November 1941, and more U-boats were sunk than cargo ships. 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. Instead, their aircraft were using a receiver tuned to the Metox
intermediate frequency that allowed them to detect the submarines at as much as . This led to an urgent 13 August 1943 message from German Naval High Command ordering that submarines turn off their Metox. 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. Accepting this as an accurate report, it must rank among the most successful deceptions in military history. The amount of Intermediate Frequency signal just before the detector (where it is at its maximum) would be under a watt in power. Any such signal, from within an equipment cabinet inside a submarine, could only reach the outer air by conduction back past the converter stage, up the antenna feedline and finally by radiation from the antenna. Readers with common levels of theoretical electronics understanding may consider this scenario unlikely. The successful deception must be considered a masterstroke of strategic importance. 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. Sources disagree on the reason; the magnetron was either unknown to the German Navy, or they did not believe it could be used against U-boats. It was not until late 1943 that a naval version of the
Naxos radar detector arrived, having originally been developed to allow German
night fighters to track the RAF's H2S radars. Naxos provided very short detection range, about , too short to be really useful. Better detectors arrived very late in the war, but by that time the U-boat force had largely been destroyed.
Other WWII developments 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. US development was not subject to the infighting in the RAF, but suffered its own series of setbacks and confusion. 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. Meanwhile,
Philco had been developing a system for
blimps, the ASG, which was much better than the SCR-517. The RAF decided that UK-built aircraft would be fitted with their Mk. III, while any US aircraft in British service would use US sets. 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. Another 30 arrived with a mix of DMS-1000, SCR-517 and ASG. However, when Bomber Command decided the
Boeing B-17 Flying Fortress was unsuitable for bombing operations, the Air Ministry ordered Coastal Command to take over their existing orders in spite of them having a shorter range that was unsuitable for closing the Gap. Coastal Command was able to have the radar switched to the ASG, which they operated under the name ASV Mark V.
Late-war developments In October 1944, the Germans introduced two innovations that were extremely worrying. 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. This made the X-band versions of ASV a requirement, as they had the resolution needed to detect the
schnorkel. On 22 November 1944, it was decided to deploy new 3 cm-band ASV's, with both the UK and US developing versions. However, these demonstrated poor performance against the
schnorkel, and experiments with these new systems were still underway when the war ended. In the immediate post-war era, development of the system continued as an
air-sea rescue system, as it could detect
life rafts even if they did not carry a
transponder. In order to upgrade the
Fairey Swordfish, which had previously used the early Mark II radars, the Mark X was further adapted as the Mark XI. This used a new narrow
radome that fit between the Swordfish's
landing gear. The radome's location made the carriage of a
torpedo impossible, so these aircraft were fit with eight
RP-3 rockets with armor-piercing warheads to damage or puncture the U-boat making it impossible to dive, and
flares to mark the location for follow-up attacks by other aircraft carrying
depth charges. Further developments of this system led to the Mark XIII, used on
de Havilland Mosquitos,
Bristol Beaufighters and
Bristol Brigands. 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. Previously they had carried a long-distance radio for remaining in contact with their base, as well as a
Gee system for navigation. Neither could be safely removed, and the desire for a much smaller ASV for this role developed. This was fulfilled with the Mark XVI, built in the US as LHTR and supplied under
lend-lease. This was a very simple system originally intended to indicate a selected range to the pilot, which proved very useful for timing bomb drops. Trials were carried out in August 1944 and experimental fits were made to the Beaufighter, Mosquito and
Fleet Air Arm Avengers. ==Post-war developments==