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 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.

[3] 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.

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.

[6] Success led to contracts with Ferranti for production electronics and Metropolitan Vickers (Metrovick) for the scanning antenna system, which would be known as ASV Mark III.

[9] 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.

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.

[13] 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.

[11] The use of the Wellington with ASV Mark III coincided with the move of the Leigh Light from the wing of the aircraft to a retractable "dustbin" arrangement that extended down through the former belly gun turret ring.

It became clear that there were not enough fitters to keep the units working and in addition to local recruits, a class from the recently formed RAF Station Clinton in Ontario, Canada sent another 110 technicians.

Similar patrols also returned empty-handed until the night of 17 March, when H538 spotted a submarine at 9 miles (14 km) but their Leigh Light failed and they could not press the attack.

[17] Around the time of the introduction of Mark III, the first similar US radar units were arriving, built using magnetron technology introduced to them during the Tizard Mission in late 1940.

This was combined with the arrival of new frigates mounting microwave radars and huff-duff receivers, further hindering U-boat operations; attacking convoys proved almost impossible.

In a mid-May 1943 report to Hitler, he stated: We are at present facing the greatest crisis in submarine warfare, since the enemy, by means of location devices makes fighting impossible and is causing us heavy losses.

[19] In June, U-boats were seen leaving port in flotillas of five or more, providing a higher density of anti-aircraft fire to the point where it was dangerous to approach them, while also reducing the chance of detection per boat.

A Coastal Command captain who had been captured after crashing told a plausible story, apparently entirely of his own creation, that threw the Germans off the scent for months.

An intact example had fallen into German hands during its second operational use when a Short Stirling carrying H2S was shot down over Rotterdam on the night of 2/3 February 1943.

The result was two "blips" on the height scope; by comparing their amplitude, the radar operator could determine the direction of the beacon relative to the nose of the aircraft.

All of these were of little interest in the ASV role, especially the ground-range modifications which were not necessary - due to the low altitudes being flown by these aircraft, the slant range was not too different than the ground distance.

Mark III presented a problem as the nose and belly locations that gave the required all-round view could not be used due to the boat hull of the aircraft.

The radar operator would use Vixen to progressively mute the transmission signals as they approached the submarine, making it appear the aircraft was simply flying by at some distance.

[10] When Mark III was being introduced, its developers at the TRE felt the Germans would quickly extend the frequency response of Metox to see the new signals and the cycle would repeat.

Even then, Naxos was never as effective as Metox and in spite of a few instances of Naxos-aided U-boat escapes, these were the exception and Mark III remained the most widely used system until the end of the war.

At long range, these could be miles on either side and in medium to high sea states, large waves near the submarine would obscure its return.

H2S used a 36 inches (910 mm) reflector designed to spread the signal out in a wide vertical angle to illuminate the area below the bomber as well as in front of it.

The system for ASV modified the design, reducing its width to 28 inches to fit under the nose of the Wellington and reshaping it to send less energy downward.

This was, in theory, over 10° wide as the return might be seen when the antenna was on either side of it, but in practice, the arc tended to be perhaps half that as the signal strength on the edges of the beam was lower.

This did not effect the accuracy of the system during the initial approach as the U-boat was somewhere near the middle of the arc, and when it was near the outside of the display this might be an inch wide and the operator could easily pinpoint the approximate centre.

In later versions this could be addressed by adjusting the unit to push nearby returns out to the edges of the display, using a control originally intended to do the reverse in H2S settings.

Large convoys could be detected at ranges of up to 40 miles (64 km) while flying at an altitude of 500 feet, which meant the ships were well below the radar horizon and the aircraft was invisible to them.