H2S (radar)
It was developed for the Royal Air Force's Bomber Command during World War II to identify targets on the ground for night and all-weather bombing.
After it was found the resolution of the early sets was too low to be useful over large cities like Berlin, in 1943 work started on a version operating in the X band at 3 cm (10 GHz), the H2S Mk.
The "S" was already being used by the aircraft interception radar team as a deliberately confusing abbreviation for its operating wavelength in the "sentimetric [sic]" range, which ultimately gave name to the S band.
Oboe used a transponder in the aircraft to reflect the signals back to the UK where operators carried out the same measurements on much larger displays to produce more accurate values.
[8] In 1940, John Randall and Harry Boot, PhD students at the University of Birmingham, devised a new microwave-frequency vacuum tube known as the cavity magnetron that output thousands of watts of radio signal at 9 cm wavelength.
[2] The commanders were impressed and, on 1 January 1942, the TRE set up a team under Bernard Lovell to develop an S-band airborne targeting radar based on AIS.
[14] Also killed in the crash were Blumlein's colleagues Cecil Oswald Browne and Frank Blythen; a TRE scientist Geoffrey S. Hensby, and seven RAF personnel.
While the ability to bomb in all weather at great distances was obviously useful to Bomber Command, the loss of an H2S aircraft would potentially reveal the secret of the magnetron to the Germans.
[16] Unlike a klystron, which is made mostly of glass and fragile metal parts, the magnetron was built out of a single block of copper that would be extremely difficult to destroy with any reasonable demolition charge.
[2] The H2S team also protested that it would take the Germans two years to develop a centimetric radar once the cavity magnetron fell into their hands and that there was no reason to believe they were not working on the technology already.
During this same period, it had been noticed that German submarines had been fitted with a radar detector, later known to be the FuMB 1 Metox 600A, which allowed them to detect Coastal Command's ASV sets operating on the older 1.5 m band.
[22] Worse, the team working on airborne radars ended up at a tiny private airstrip in Perth, Scotland that was entirely unsuitable for development.
When reports were received that "seventeen train loads" of paratroopers had been stationed near Cherbourg, directly across the English Channel from Christchurch, near panic broke out in the Air Ministry, and yet another emergency move was made.
[25] Despite all the problems, on 3 July 1942 Churchill held a meeting with his military commanders and the H2S group, where he surprised the radar designers by demanding the delivery of 200 H2S sets by 15 October 1942.
To further improve operations, on 12 March it was decided that Bomber Command would receive more of the available spares, as it was believed that they would need to make up for higher casualty rates.
[42] When the photos reached the desk of Bomber Command's Deputy Commander-in-Chief Robert Saundby, he immediately sent a message to the Air Ministry demanding that they be installed with all possible speed.
[47] George Beeching had been assigned the task of fitting H2S to the Stirling, and in early 1943 he managed to obtain a single 3 cm magnetron from Herbert Skinner's AI group working on the Boeing.
On the 10 miles (16 km) range setting, used during the bomb run, returns covered the entire display and there were no clear outlines of large objects on which to navigate.
[29] By the middle of 1944, the war in Europe was clearly entering its final stages, and the RAF began making plans to begin attacks on Japan with the Tiger Force group.
On its second combat mission, during the raid on Cologne on the night of 2/3 February 1943, one of the Stirlings carrying H2S was shot down near Rotterdam by the crew of Oblt Frank & Fw Gotter.
A quickly adopted countermeasure was put in place by installing small corner reflectors around the city, producing bright spots on the display in areas that would otherwise be empty, like lakes and rivers.
[58] This development was slowed by the German electronic industry's decision to stop researching microwaves shortly before Rotterdam Gerät literally fell from the sky.
Roderich transmissions were timed roughly with the scanning speed of the H2S antenna, causing a pattern to appear similar to a pinwheel that made it difficult to see the ground between its pulses.
[58] This limited the availability of the Funkgerät (FuG) 350 Naxos radar detector to a handful of operational examples, which enabled Luftwaffe night fighters to home on the transmissions of H2S.
[68] Further improvements in magnetron and receiver design during the war led to the ability to use even shorter wavelengths, and in the summer of 1943 the decision was made to begin development of versions operating in the K band at 1.25 cm.
This would improve the resolution by more than a factor of two over the X band versions, and was especially interesting as a system for low-level bombing where the short local horizon limited the amount of territory visible on the display and would require guidance on smaller objects like particular buildings.
[78] More rigorous testing demonstrated that the experimental set was only really useful when the aircraft was flying under 3,000 feet (910 m) and had a maximum effective detection range on the order of 3 to 4 miles (4.8–6.4 km).
[80] Using the larger whirligig reflector and a slotted waveguide allowed the angular beamwidth to be reduced to 1.5 degrees, a great improvement over the World War II models.
In this case, the idea was not to improve resolution but to provide much more rapid updates of the selected area, which was needed in order to account for the much higher speed of the aircraft.
The last use in combat was made by the Vulcans of the Operation Black Buck flights in 1982 during the Falklands War, which used the system as the primary navigation and bombing aid throughout the 7,000-mile (11,000 km) round trips to and from Ascension Island.
