ASV Mark II radar

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 . 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. II equipped Fairey Swordfish that located the German battleship in heavy overcast skies, torpedoing her and leading to her destruction the next day. Mk. II was only partially effective against the much smaller U-boats, especially as the signal faded as the aircraft approached the target and they would lose contact at night. To close the gap, the Leigh light was introduced, allowing the U-boat to be picked up visually after it passed off the radar display. With the introduction of the Leigh light, nighttime U-boat interceptions became common, and turned the German ports in the Bay of Biscay into deathtraps.

A microwave-frequency ASV radar, ASVS, was under development since 1941, but the required cavity magnetrons were in limited supply and priority was given to H2S. The capture of a Mk. II-equipped Vickers Wellington by the Germans led to the introduction of the Metox radar detector tuned to its frequencies. This was soon followed by British pilots reporting submarines diving as the aircraft began to approach. A new design based on H2S, ASV Mk. III, was rushed to service, replacing the Mk. II beginning in 1943. Mk. II remained in use throughout the war in other theatres.

Development

Background

Early during the development of the first British radar system, Chain Home (CH), Henry Tizard became concerned that the CH system would be so effective that the German air force (''Luftwaffe'') would be forced to turn to night bombing. Tizard was aware that a fighter pilot might be expected to see a bomber at about at most, whereas the accuracy of the CH system was perhaps . He wrote a memo on the topic on 27 April 1936 and sent it to Hugh Dowding, who was at that time the Air Member for Research and Development, and copied Robert Watt at the CH research center at Bawdsey Manor in Suffolk.

Watt met with his researchers at the local Crown and Castle pub, and agreed that the best solution was to introduce a small radar that could be mounted in a night fighter. If the airborne radar had a range of about 5 miles, CH could be tasked with getting the fighter into the general area, and then the fighter's own radar could take over and guide them until the enemy could be seen visually. "Taffy" Bowen asked to take on the project, and formed a small team to consider the problem in August 1936. They gave the concept the name RDF2, as Chain Home was at that time known as RDF1. This would later be known as " Airborne Interception radar", or AI for short.

The major problem faced by the Airborne Group was the problem of wavelength. For a variety of reasons, an antenna with reasonable gain has to be on the same order of length as the wavelength of the signal, with the half-wave dipole being a common solution. CH worked at wavelengths on the order of 10 metres, which called for antennas about long, far too large to be practically carried on an aircraft. Through 1936 the team's primary concern was the development of radio systems operating at much shorter wavelengths, eventually settling on a set working at 6.7 m, based on an experimental television receiver built at EMI.

Discovery

In early 1937 the Airborne Group received a number of Western Electric Type 316A doorknob vacuum tubes. These were suitable for building transmitter units of about 20 W continual power for wavelengths of 1 to 10 m. Percy Hibberd built a new push–pull amplifier using two of these tubes working at 1.25 m wavelength; below 1.25 m the sensitivity dropped off sharply. Gerald Touch converted the EMI receiver to the same frequency by using it as the intermediate frequency portion of a superheterodyne circuit. The new sets were fitted to a Handley Page Heyford in March 1937.

On its first flight the set demonstrated very limited range against aircraft. However, while flying the aircraft about, the operators saw odd returns appearing on the display. They finally realized these were from the wharves and cranes at the Harwich docks miles south of Bawdsey. Shipping also appeared, but the team was unable to test this very well as the Heyford was forbidden to fly over water.

With this accidental discovery of ship detection, the team was given two Avro Anson maritime patrol aircraft, ''K6260'' and ''K8758'', along with five pilots stationed at nearby RAF Martlesham Heath to test this role. Early tests demonstrated a problem with noise from the ignition system interfering with the receiver, but this was soon resolved by fitters at the Royal Aircraft Establishment (RAE).

On its first real test on 17 August, Anson ''K6260'' with Touch and Keith Wood aboard immediately detected shipping in the English Channel at a range of . This was particularly impressive given the very low power of the transmitter, about 100 W per pulse.

Demonstration

By this time, Watt had moved to Air Ministry headquarters in London. He heard of the successful test and called the team to ask if they would be available for a demonstration in early September. Plans were underway to run military exercises in the Channel, including a combined fleet of Royal Navy ships and RAF Coastal Command aircraft, and Watt wanted to crash the party. On the afternoon of 3 September 1937 ''K6260'' successfully detected the battleship , the aircraft carrier and the light cruiser , receiving very strong returns.

The next day they took off at dawn and, in almost complete overcast, detected ''Courageous'' and ''Southampton'' at a distance of . As they approached the ships the Anson eventually became visible through the clouds, and the team could see the ''Courageous'' launching aircraft in a futile effort to intercept them. The weather was so bad that the operators had to use the radar as a navigation system to find their way home, using the reflection off seaside cliffs.

The promise of the system was not lost on observers; Albert Percival Rowe of the Tizard Committee commented that "This, had they known, was the writing on the wall for the German Submarine Service."

Continued development

For the next year, Bowen's team found themselves working much more on the ASV than AI. Much of this involved the development of new antenna systems, more advanced than the system on the Anson where a dipole was held outside the escape hatch and rotated by hand to hunt for signals. Among the experiments was a motorized rotating dipole that scanned the entire area around the aircraft and displayed angles as the X-axis and range on the Y-axis. This appears to be the first example of what is today known as a B-scope.

ASV proved easy to develop for a variety of reasons. One was that the host aircraft tended to be very large, so equipment size and weight were not as critical as it was in the much smaller night fighters. It was also easier to move around in these aircraft while fitting the equipment. Another reason was that these aircraft tended to fly at slower speeds, which meant that larger antennas could be used for better reception without seriously affecting aircraft performance. The early units used standard quarter-wave dipoles mounted on the nose area, but these were later extended to three-quarter wave in production units.

But the major reason that ASV was easier to develop than AI was the behaviour of the very high frequency (VHF) radio waves when interacting with water. In the case of AI, when the radar's signal hit the ground it tended to scatter in all directions, sending some part of the signal back towards the aircraft. Although only a small portion of the original signal was returned, the ground was essentially infinite in size so this ''ground return'' was still much more powerful than the reflection from a target. An aircraft flying at the typical German bomber altitude of could only see aircraft within 15,000 feet, anything beyond that was hidden in the ground return. This was a much shorter range than the 5 miles needed to close the gap with Chain Home.

In comparison, when the same signal hit the water it tended to reflect rather than scatter, sending the majority of the signal forward and away from the aircraft. The only time the signal could be seen is when the aircraft approached the water very closely when some of it would strike the water just in front of the aircraft and scattering off waves would cause a ground return. Even then the signal was relatively small compared to the huge ground return seen in the AI case, and only caused problems within about of the aircraft, although this could grow to as much as in high sea states. This would turn out to be an important limitation in practice, but one that was ultimately solved in a roundabout fashion.

Finally, the shape of the targets as seen from the radar were ideal for detection. The side of the ship, rising vertically from the surface of the water, created a partial corner reflector. Radio signals hitting the target directly were returned to the receiver, but so was any signal reflecting forward off the water close to the ship, as this signal would also strike the ship and reflect back to the receiver. Whereas aircraft were difficult to detect beyond about , ships could be easily detected at distances on the order of . Any vertical surface worked in this way, including seaside cliffs, which could be picked up at very long range and proved to be extremely useful for navigation.

New tubes

AI and ASV developed in parallel for some time. In May 1938 the team received the Western Electric 4304 tubes which replaced the 316As doorknobs in the transmitter and improved transmit power to 2,000 W. In testing this proved to increase detection range on ships to , although in the AI role the range was little improved.

While the transmitter problem was considered solved with the new tubes, the team had significant problems with the receivers. A Metrovick employee had been told to begin building the receivers and asked for an example, but the team had only one airworthy receiver and had to give them an old hand-assembled bench model with the instructions that it wasn't to be used for a production design. Sure enough, Metrovick returned a design based on this model, which proved useless. The team also contacted Cossor and provided complete details of the required design, but when they returned their first attempt six months later it was completely unusable. When they asked for improvements, Cossor never responded, too busy with other work.

While waiting for the Metrovick and Cossor receivers to arrive, there was a chance encounter between Bowen and his former professor at King's College, Nobel prize winner Edward Appleton. In early 1939, Appleton mentioned to Bowen that Pye Electronics had also been interested in the BBC's experimental 45 MHz television service and had built receivers that they still might have on-hand. Bowen visited the company in April or May, and found that they had "scores and scores" of the receivers in a production-ready form. When they tested them, they were found to be far superior to the EMI models.

Much of the improvement in the Pye receiver was due to the use of a new type of tube developed by Philips, the EF50 "Miniwatt", which had been designed specifically for efficient VHF use. The tubes were labeled Mullard, Philip's UK subsidiary. When they investigated, Mullard told the Air Ministry that the tubes were actually built at Philips' factory in Eindhoven