Air-to-Surface Vessel radar


Radar, Air-to-Surface Vessel, or ASV radar for short, is a classification used by the Royal Air Force to refer to a series of aircraft-mounted radar systems used to scan the surface of the ocean to locate ships and surfaced submarines. The first examples were developed just before the opening of World War II and they have remained a major instrument on patrol aircraft since that time. It is part of the wider surface search radar classification, which includes similar radars in ground and ship mountings.
The first ASV was developed after the accidental detection of wharves and cranes while testing an air-to-air radar in 1937. For a variety of reasons, ASV was easier to develop than the air-to-air variety of the same systems, and the first operational use of the Mark I followed in early 1940. 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 not useful for detecting U-boats at night, a problem that was addressed with the Leigh light. As German U-boat losses shot up in 1942, they concluded the RAF was using radar to detect them and responded with the Metox radar detector. 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. A series of other ASVs were developed for different aircraft as the war progressed.
In the post-war era, several new ASV radars were developed, notably ASV Mark 7A, ASV Mark 13 and ASV Mark 21. By the late 1960s the original terminology was no longer being used, and the last major entries in the series were known simply as Searchwater and Seaspray.

WWII developments

Initial concept

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 further test the concept, Robert Watson-Watt provided the team with two Avro Ansons that were able to fly out over the English Channel from nearby RAF Martlesham Heath. After several successful flights over the summer of 1937, 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

For a variety of reasons, the 1.5 m wavelength of the radar system worked better over water than land, and the large size and flat vertical sides of the ships made excellent radar targets. 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 detection. 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. With experience, the operators could tell whether the aircraft was approaching or just flying by. It provided this warning long before the echos from the submarine became visible on the aircraft's display, 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 all of the small numbers of magnetrons for their H2S, 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 very high-frequency 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 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 next 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 allowed Mark II to once again become effective but further delayed the German discovery of the true nature of the problem.
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. For reasons that are not recorded, the magnetron was either unknown to the 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 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.
The TRE was sure the Germans would soon detect Mark III and render it ineffective as well, so they responded with a new ASV Mark VI that was essentially a more-powerful Mark III. The key trick to Mark VI was the "Vixen" device that allowed the operator to progressively mute the output as they approached the U-boat, hopefully fooling the radio operator into believing they were flying away. Mark VI never fully replaced Mark III in service, as truly effective detectors did not become available until the U-boat fleet had largely been destroyed. The failure of Naxos and later devices led to morale problems in the U-boat force.
Another solution to the problem of being detected was to change frequencies. From 1943, both the UK and US began developing magnetrons that worked on even shorter wavelengths, first in the X-band at 3 cm wavelength, and later in the K-band at 1.25 cm. The UK-developed 3 cm version for the Liberator became ASV Mark VII, while the US version based on ASG was known as AN/APS-15 and given the UK designation ASV Mark X. It was expected the latter would be available in December 1943. The similar AN/APS-3 was mounted to Catalinas and named ASV Mark VIII.

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.

Post-war developments

With the ending of World War II in 1945, the British believed another war was at least a decade off, and put little effort into new radar systems. However, 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.
Adding to the problem was the loss of the large numbers of Liberator aircraft with the ending of lend-lease. 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. A solution was found by adapting surplus Bomber Command Avro Lancaster bombers with Mark VII to become the Lancaster GR.3. The use of Roman numerals had become passé by this point, and these units were referred to as ASV Mark 7A, remaining in service until 1954.
A more suitable custom-built patrol aircraft was a priority and led to an adapted version of the Lancaster known as the Avro Shackleton. This mounted the ASV Mark 13, a further development of a war-era Mark XIII. The main improvements were the addition of stabilization so the image did not change when the aircraft manoeuvred, and the use of a pressurized radome that kept out humidity and made it suitable for use in tropical areas. In service, the Mark 13 proved unreliable.
EMI developed a replacement, ASV Mark 21, which began to replace Mk. 13 on the Shackleton beginning in 1951, fully replacing it by 1954. Mark 21 was generally similar to the earlier designs. Mk. 21 was also selected for the Mark II models of the Canadair CP-107 Argus, replacing the American AN/APS-20 of the Argus Mark I's. The Argus was widely described as the best anti-submarine aircraft of its era. Lacking a new design, the Mk. 21 also equipped the early models of the Hawker Siddeley Nimrod, although some of these were replaced starting in 1979. The Mk.21 remained in service on the Argus until the last example retired in 1981.

Later developments

The war-era radar classifications became less relevant in the 1970s as radar units increasingly became multi-purpose as opposed to being dedicated to a single role. Newer designs, even dedicated naval surveillance designs, were not assigned numbers in the ASV lineage.
The first such example is the Seaspray, a small unit designed to be mounted on the Westland Lynx. This was originally developed in concert with the Sea Skua missile to allow the Lynx to attack fast attack craft at long range from their carrier ships. It has since been sold around the world and used in a variety of roles. The latest versions, Seaspray 7000, are completely rebuilt and share only the name with the original models.
Another example is the Searchwater, which was designed to replace the Mk. 21 in a new version of the Nimrod, the MR2. These began arriving in 1979. In 1978, the Royal Navy retired its fleet carriers, losing the Fairey Gannet AEW.3 airborne early warning aircraft. 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. Several greatly improved versions followed, part of the Searchwater 2000 series.

System list

From Watts and Smith: