ASV Mark III radar
Radar, Air-to-Surface Vessel, Mark III, or ASV Mk. III for short, was a surface search radar system used by RAF Coastal Command during World War II. It was a slightly modified version of the H2S radar used by RAF Bomber Command, with minor changes to the antenna to make it more useful for the anti-submarine role. It was Coastal Command's primary radar from the spring of 1943 until the end of the war. Several improved versions were introduced, notably the ASV Mark VI, which replaced most Mk. IIIs from 1944 and ASV Mark VII radar, which saw only limited use until the post-war era.
Coastal Command's first radar was ASV Mark I, which began experimental use in 1939. Minor improvements were made for the Mark II in 1940 but it was not widely available until late 1941. Having realized that the RAF was using radar to detect their U-boats, in the summer of 1942 the Germans introduced the Metox radar detector to listen for their signals. This gave the submarine a warning of the aircraft approach long before the submarine became visible on the aircraft's radar display. The RAF noticed this in early autumn when crews reported with increasing frequency that they would detect submarines that would disappear as they approached.
An ASV working in microwave frequencies using the new cavity magnetron had been under development for some time at this point, known as ASVS, but had not matured for various reasons. Robert Hanbury Brown suggested using H2S for ASV but this had been rejected by Bomber Command, who wanted all the sets for themselves. Brown continued development with EMI and presented it again in late 1942 when Metox negated the earlier marks of ASV. Obstruction by Bomber Command led to more delays and it was not until March 1943 that the first dozen aircraft were operational. Deliveries were rapid after this point and Mk. II had been largely replaced by the end of the summer.
The Germans had no way to detect the signals from Mark III, which operated in the 10 cm band compared to the 1.5 m wavelength of the Mk. II. Further confusion was caused by a captured RAF officer who stated they carried a device that could detect the Metox radar detector. Combined with other anti-submarine technologies introduced around the same time, submarine losses shot up in the late spring of 1943. By the time that the Germans realised what the British had done, the German U-boat force was almost destroyed and the Battle of the Atlantic was entering its final phase. Naxos, a microwave detector, was introduced in October 1943 but it was nowhere near as sensitive as Metox and had little effect on events; Mark III continued to guide the majority of Coastal Command's fleet until the end of the war.
Development
Mark II
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 English Channel. 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 address this problem, further testing was carried out on two Avro Anson patrol aircraft. The system was crude, with a simple dipole antenna being held out a window and swung by hand to find returns.For several reasons, the 1.5 m wavelength of the radar system worked better over water than land; notably the large area and flat vertical sides of the ships made excellent radar targets. After some additional development of suitable antennas, the system was largely ready for production by early 1939. Production quality sets were available in late 1939 and entered operational service in January 1940, becoming the first aircraft radar system to be used in combat. A somewhat improved version, Mark II, followed in 1941.
The ASV designs had a relatively long minimum range, meaning the submarine targets disappeared from the display just as the aircraft was closing for the attack. At night this allowed the submarines to escape. This problem was solved by the Leigh Light, a searchlight that lit up the submarines during the last seconds of the approach. By early 1942, ASV Mark II and the Leigh Light had been installed on large numbers of aircraft, just in time for the winter hiatus to end. German U-boats had previously been safe at night and could operate from the Bay of Biscay in spite of it being close to British shores. By the spring of 1942, Biscay was a deathtrap; aircraft would appear out of nowhere in the middle of the night, drop bombs and depth charges, and then disappear again in moments.
The Germans defeated ASV Mark II by the end of 1942 with the introduction of the Metox radar detector. This amplified the radar's pulses and played them into the radio operator's headphones. With experience, the operators could tell whether the aircraft was approaching or just flying by. It provided this warning long before the echoes off the submarine became visible on the aircraft's display, allowing the U-boat to dive and escape detection.
ASVS, original Mark III
After the early 1940 invention of the cavity magnetron, which produced microwaves at around 10 cm, all of the British forces began development of radars using these devices. Among these were the Air Ministry teams who had developed AI and ASV turned their attention to AIS, the S standing for "senitmetric". Tests in April 1941 with early lash-up devices against showed they could detect semi-submerged submarines at several miles range.In June 1941 a formal application was made to Robert Watson-Watt to form a separate group to develop an ASVS. This was initially a version of the Mark II with the minimal conversions needed to use the magnetron as the transmitter. This would otherwise operate like the Mark II, with the relative strength of the returns on two antennas being used to determine the rough bearing of the target; if the return on the left antenna was slightly stronger, the target was somewhere to the left of the nose of the aircraft.
During this same period, the TRE was also developing the new H2S radar for Bomber Command. H2S featured a plan-position indicator, which produced a map-like 360° two-dimensional display of the ground below the aircraft. The PPI also greatly eased the operator workload for most other radar tasks as well, because they could see the area around the aircraft at a glance instead of having to manually scan back and forth across areas of interest. ASVS soon adopted the PPI as well, using a cathode ray tube display and a second, range-only display, on a CRT.
H2S had been developed for the new four-engine bombers being introduced at that time, while Bomber Command's older designs like the Wellington were being cast off to Coastal Command. The new bombers, like the Handley Page Halifax, had a large ring cut out of the belly of the bomber to mount a gun turret, and the H2S antenna was produced to fit into this ring. The Wellington's version of the turret cut-out was much smaller, so the main conversion required was to shrink the antenna from to wide. With that exception, the units were similar to the H2S Mark I.
Philip Dee noted that the first flight on Wellington T2968 did not take place until December 1941 and it was not until 13 January 1942 that he noted "ASV saw Titlark at 12 miles". Success led to contracts with Ferranti for production electronics and Metropolitan Vickers for the scanning antenna system, which would be known as ASV Mark III. Ferranti had a prototype ready by the summer of 1942, although they predicted that the first deliveries would not be ready before the spring of 1943.
Testing ASVS
T2968 continued tests until 24 February and on 7 March 1942 was sent to RAF Ballykelly in Northern Ireland to carry out competitive tests against other ASV developments. One was the Mark IIA which had a new transmitter that increased broadcast power from 7 to 100 kW. This was found to increase detection range against surfaced submarines to about and even when the submarine was semi-submerged, just the conning tower above water. This was about twice the effective range of the original Mark II. However, this also greatly increased the amount of clutter as the returns from waves were similarly magnified. A second unit used a similar high-power transmitter that operated on a 50 cm wavelength rather that 1.5 m but this was shown to have no advantages over the basic Mark II.In contrast, the ASVS set showed dramatic improvements. Performance against convoys was when the aircraft was flying at only 500 feet, in spite of the radar horizon being only at that altitude. Other aircraft were visible at and surfaced submarines at. The ASVS was immediately chosen as the new operational requirement, with the 50 cm set also being ordered as a backup. As it became clear the magnetron was going to work, the 50 cm system was cancelled.
H2S, new Mark III
was convinced the H2S radar being developed for RAF Bomber Command could be adapted for anti-shipping work, simply by changing the antenna to one suited to an aircraft flying at rather than. He continued working on this project with the primary developers of H2S, EMI.By late 1942, Metox had been introduced and Ferranti reported that the Mark III would not be available in numbers for some time. Brown's H2S-based adaptation was largely complete and it would be possible to have a small number of hand-built units installed by the end of 1942. This system, working at 10 cm, would be invisible to Metox. The TRE team in charge of ASVS was not under the control of Dee, so he was happy to point out problems with their design. On 25 September 1942, at a meeting at the DCD, Dee pointed out that the AI and ASV teams were developing separate systems that were, from a signals perspective, almost identical. The only major difference was that ASV had larger displays. He proposed abandoning the Ferranti system and using the H2S-based system.
The meeting took place during a furious debate over the use of the magnetron; if an aircraft carrying H2S was shot down, it would fall into German hands and be quickly reverse engineered. Frederick Lindemann was especially vocal against the use of the magnetron in H2S and demanded they use a klystron instead. The klystron was already known to the Germans and so fragile that it would be unlikely to survive any crash. A similar concern did not exist for ASV, where the magnetron would fall into the water if shot down. This made ASV a much safer choice for deployment of the very few magnetron units available. The commander of Bomber Command, Arthur "Bomber" Harris, objected, claiming his bombers would do much more damage to the German U-boat fleet by bombing their pens in France than Coastal Command would by hunting them down at sea. The meeting ended with Coastal Command being awarded priority for the magnetron-based units. On 30 September, Ferranti was ordered to stop work on their design in favour of the H2S-based system, also to be known as Mark III.
The disputes with Bomber Command were magnified by problems within Coastal Command, due to the upset that the original Mark III project had been cancelled by the Air Ministry without consulting Coastal Command. That the H2S-based system could be available immediately did not seem to impress the higher echelons of the Command. Adding to the confusion, the commander of Coastal Command, Philip Joubert de la Ferté, visited the radar development teams at the TRE and told them he did not believe in ASV, which led to demands to see it in action. More confusion followed when the TRE teams suggested fitting the new radar on four-engine airframes. These would provide ample room for the installations and superb range over the North Atlantic. On 8 December 1942, a meeting was called over the topic but Joubert refused to intercede in favour of the TRE and they were told to continue with the two-engine Wellington.