ASV Mark II radar

II equipped Fairey Swordfish that located the German battleship Bismarck in heavy overcast skies, torpedoing her and leading to her destruction the next day.

Tizard was aware that a fighter pilot at night might be able to see a bomber up to 1,000 feet (300 m) at most,[1] however the accuracy of the CH system was such that it could locate an invading plane to no better than about 1 mile (1.6 km) in range, couldn't determine its bearing accurately and was even worse at estimating its elevation.

Watson-Watt met with his researchers in Orford at the local Crown and Castle Hotel, and agreed that the best solution was to introduce a small radar that could be mounted in 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.

[11] 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).

[12] 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 2 to 3 miles (3.2–4.8 km).

Plans were underway to run military exercises in the Channel, including a combined fleet of Royal Navy ships and RAF Coastal Command aircraft, and Watson-Watt wanted to crash the party.

[14] 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.

[16] 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.

[20] 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.

When they investigated, Mullard told the Air Ministry that the tubes were actually built at Philips' factory in Eindhoven, and that attempts to start production in the UK had failed due to problems manufacturing the bases.

[23] In early August, the team was informed that the Air Ministry had ordered 30 AI units and expected Bowen to have them installed in Bristol Blenheim aircraft within 30 days.

[24] To further confuse matters, when the war began on 1 September, the majority of the AMES team was hurriedly sent to a prearranged location at the University of Dundee in Scotland, only to find that nothing had been prepared.

[22] Another chance encounter after the meeting led Bowen to try a new material, polythene, from Imperial Chemical Industries (ICI) which produced excellent coaxial cable and neatly solved the electrical problems they had been having.

[29] By the early part of 1940, Hudsons were arriving at the rate of two or three a week, and the crews were able to quickly fit the sets due to the easy working environment in the large fuselage.

[29] But the device became extremely useful after Squadron Leader Sidney Lugg installed an IFF Mark II transponder at the base, tuned to operate on the ASV frequencies.

For reasons unknown, the contract negotiations required considerable time to finalize, and throughout the production run it battled for precedence with the AI units and Chain Home Low which also made use of the Pye strip.

[39] Shortly after moving to St. Athan in 1939, Hanbury Brown received a request to fit ASV to the Armstrong Whitworth Whitley bomber, which was no longer competitive and was being passed off to other uses.

[40] In early 1940 there was a lengthy debate within the Air Ministry, and the government in general, about whether or not the United States should be told of the many technological developments taking place in the UK.

[44] Here they were surprised to learn that in September 1939 the NRC had started working on an ASV radar using an adapted radio altimeter built by Westinghouse Electric in the US.

This led to a debate on whether to continue development of their own system, whose shorter wavelength would make it more suitable for aircraft use, or to simply build the British unit using Canadian and US tubes.

[48] But the real problem was that the minimum range of the radar was about 1000 yards at best; at shorter distances the returns from the target merged with the leftover signal from the transmitter and became invisible in the electronic noise and scattering off the water.

The same problem affected the AI radars as well, but in that case was far more serious due to the small size of the aircraft targets compared to a U-boat or ship, and the team had invested considerable effort in trying to solve this "minimum range controversy", so far unsuccessfully.

Humphrey de Verd Leigh, an RAF personnel officer, came up with the idea after talking to returning aircrew and learning of the short-range cutoff problem.

Although the Air Ministry was convinced the idea was workable, they decided to re-use an older searchlight design known as the Turbinlite, which was originally intended to be used in a similar role for night fighters.

II was in the midst of achieving some of its greatest successes, in the late summer of 1942 crews returned to base claiming that good detections on German U-boats were followed by the ships disappearing as they moved in for the approach.

[55] A brief reprieve in the effects of Metox was at hand in December 1942, when British codebreakers once again were able to break into the Naval Enigma and U-boat losses began to climb again due to intercepts revealing their positions and orders.

This was combined with a key piece of false information planted by a captured British officer, who claimed their aircraft were equipped with a device to listen for the very weak signals given off by the Metox's intermediate frequency stage.

Early units worked on such long wavelengths that the only available aircraft that was large enough to carry the antennas was this Heyford .
Avro Anson K8758 , as seen from K6260 . The experimental radar on K6260 made the fateful detection of Courageous and Southampton that led to the ASV efforts.
The EF50 made airborne radars practical due to their relatively small size, good frequency response, and good power handling.
The Mk. I units used an antenna arrangement similar to the Mk. II unit seen on this RCAF Douglas Digby at CFB Rockcliffe . This particular aircraft also carried an experimental high-gain antenna under the wings, which cannot be seen here.
Duxford's Short Sunderland mounts the original short-range antennas, now painted bright yellow. By the time this aircraft left service they were used only as receivers for Lucero and BABS .
Compact Yagis were mounted to the wing struts on the Fairey Swordfish . A system like this one was responsible for detecting, and ultimately sinking, the Bismarck .
This Coastal Command Liberator mounts both sets of LRASV antennas. On the nose and under the wings are the Yagi arrays for forward-search, and the port broadside array can be seen just about the roundel on the side of the fuselage. The aircraft two away is equipped with ASV Mk. III.
On the Wellington, the broadside array shared a common transmit array spread along the top of the fuselage.
A crewman cleans the Leigh Light mounted under the starboard wing of an RAF Coastal Command Liberator GR Mk V . The light could be aimed to point at the target without having to point the aircraft directly at it.
The Metox detector was a simple affair, consisting of a cross-shaped antenna that was swung by hand, and a radio receiver inside the submarine. Coastal Command pilots who saw the new antenna nicknamed it the "Biscay Cross".
One of the first fits of ASV Mk. III was on this Vickers Wellington XII MP512 in January 1943.
This Hudson is equipped forward-firing LRASV antennas, with the transmitter on the nose and the receivers under each wing, angled outboard.
This display simulates a typical scene on the ASV Mk. II. At the bottom is a large triangular blip caused by the transmitter signal and the local ground return. Above that is a smaller blip indicating a target at about nine miles range and to the right of the aircraft.