H2S (radar)
H2S was the first airborne, ground scanning radar system. 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. This allowed attacks outside the range of the various radio navigation aids like Gee or Oboe, which were limited to about of range from various base stations. It was also widely used as a general navigation system, allowing landmarks to be identified at long range.
In March 1941, experiments with an early aircraft interception radar based on the 9.1 cm wavelength, cavity magnetron revealed that different objects have very different radar signatures; water, open land and built-up areas of cities and towns all produced distinct returns. In January 1942, a new team was set up to combine the magnetron with a new scanning antenna and plan position indicator display. The prototype's first use in April confirmed that a map of the area below the aircraft could be produced using radar. The first systems went into service in early 1943 as the H2S Mark I and H2S Mark II, as well as ASV Mark III.
On its second operational mission on 2/3 February 1943, an H2S was captured almost intact by German forces, and a second unit a week later. Combined with intelligence gathered from the surviving crew, they learned it was a mapping system and were able to determine its method of operation. When they pieced one together from parts and saw the display of Berlin, near panic broke out in the Luftwaffe. This led to the introduction of the FuG 350 Naxos radar detector in late 1943, which enabled Luftwaffe night fighters to home on the transmissions of H2S. The British learned of Naxos and a great debate ensued over the use of H2S. Later calculations showed that losses after the introduction of Naxos were actually less than before it, and use continued.
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, the H2S Mark III. Almost simultaneously, its American equivalent was introduced as the H2X in October of that year. A wide variety of slightly different Mark III's were produced before the Mark IIIG was selected as the late-war standard. Development continued through the late-war Mark IV to the 1950s era Mark IX that equipped the V bomber fleet and the English Electric Canberra. In the V-force, Mark IXA was tied into both the bombsight and navigation system to provide a complete long-range Navigation and Bombing System. In this form, H2S was last used operationally during the Falklands War in 1982 on the Avro Vulcan. Some H2S Mark IX units remained in service on the Handley Page Victor aircraft until 1993, providing fifty years of service.
Etymology of "H2S"
The radar was originally called "BN", but it quickly became "H2S". The genesis of this remains somewhat contentious, with different sources claiming it meant "Height to Slope"; or "Home Sweet Home". The "S" was already being used by the aircraft interception radar team as a deliberately confusing abbreviation for its operating wavelength in the " " range, which ultimately gave name to the S band. It is widely reported that it was named after hydrogen sulphide, because the inventor realized that had he simply pointed the radar downward instead of towards the sky, he would have a new use for radar, ground tracking instead of for identifying air targets and that it was simply "rotten" that he had not thought of it sooner.The "rotten" connection, with a twist, is propounded by R. V. Jones, director of the Air Ministry's scientific intelligence unit. He relates the tale that, owing to a misunderstanding between the original developers and Frederick Lindemann, science advisor to Winston Churchill, development of the technology was delayed as the engineers thought that Lord Cherwell did not like the idea. Later, when Cherwell asked how the project was progressing, he was most upset to hear that it had been put on hold and repeatedly declared about the delay that "it stinks". The engineers called the resumed project "H2S" and later, when Cherwell inquired what H2S stood for, no one dared tell him that it was named after his phrase. Instead, they pretended, on the spot, that it meant "Home Sweet Home", which was the meaning that Cherwell related to others.
Development
Genesis
After the Battle of Britain, RAF Bomber Command began night attacks against German cities. Although Bomber Command had reported good results from the raids, the Butt Report showed only one bomb in twenty landed within of the target, half the bombs fell on open country, and in some cases, the bombing was seen to fall as far as from the target.Radio electronics promised some improvement and the Telecommunications Research Establishment developed a radio navigation system called "Gee" and then a second known as "Oboe". Both were based on transmitter stations in the UK which sent out synchronized signals. In the case of Gee, an oscilloscope in the aircraft measured the time difference between two signals to determine location. 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. In both cases, the ground-based portion of the system limited range to a line-of-sight, about for aircraft flying at typical mission altitudes. This was useful against targets in the Ruhr, but not the heart of Germany.
Taffy Bowen had noticed during his early 1.5 m wavelength AI radar experiments before the war that the radar returns from fields, cities and other areas were different. This was due to geometry; objects with vertical sides, like buildings or ships, produced much stronger returns than flat objects like the ground or sea. During early tests of the AI system, the operator would often see coastlines at very long distances, and the development team used this as an ad hoc navigation system on several occasions. Bowen had suggested developing a targeting radar based on this principle, but the matter had been forgotten.
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. At this wavelength, the antennas were only a few centimeters long, making radar much easier to fit into an aircraft. The mapping idea resurfaced in March 1941 when Philip Dee's group was developing a new AI radar, christened "AIS" in reference to its "sentimetric" wavelength. During tests in a Blenheim, the team noticed the same sort of effects Bowen had earlier. The set's wavelength, over ten times shorter than the original 1.5 m AI sets, provided much greater resolution and allowed them to pick out individual objects on the ground.
Work begins
In October 1941, Dee attended a meeting of the RAF Bomber Command where the night targeting issue was discussed. Dee mentioned the recent discoveries using AIS. On 1 November, Dee performed an experiment in which he used an AIS radar mounted on a Blenheim to scan the ground. Using this display he was able to pick up the outline of a town away while flying at altitude.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. An initial order for 1,500 sets was placed. It was clear even at this point that a plan position indicator display would be desirable, but this would require a complex scanning parabolic antenna, compared to the very simple set of fixed antennas used in the A-scope system. It was decided to test both systems. In March, it was decided that both H2S and a new centimetric air-to-surface-vessel radar, ASV Mark III, would be built using the same components, simplifying production.
In early tests in April, the superiority of the scanning PPI system was evident, and all work on the older A-scope version ended. H2S performed its first experimental flight on 23 April 1942, with the radar mounted in a Handley Page Halifax bomber, V9977. The scanning unit was installed in the aircraft's belly using the position previously occupied by the mid-under turret, which was by that time seldom installed. The rotating scanner mounting was designed and manufactured by Nash & Thompson. The scanning aerial was covered by a distinctive streamlined radome.
One problem was that the returns from closer objects were much stronger than more distant objects, due to the radar equation. This made the area directly under the bomber much brighter than the surroundings if the signal was not adjusted to account for this. The solution was to adjust the broadcast power according to the cosecant-squared rule, so-called after the mathematical function that defined the effective change in gain. The change was originally produced by fixing an angled metal plate on part of the parabolic reflector of the aerial, as may be seen in the picture of the aerial on a Halifax bomber. Later reflectors were actually shaped with a cosecant-squared curvature, no longer a perfect parabolic section.
File:Halifax V9977.jpg|thumb|right|Halifax V9977 pictured at RAF Hurn while testing the prototype H2S. Its crash in June 1942 destroyed the prototype and killed chief designer Alan Blumlein.
On 7 June 1942, the Halifax performing H2S tests crashed, killing everyone on board and destroying the prototype H2S. One of the dead was Alan Blumlein, the chief designer. Lovell recalled that after inspecting the crash site "it is, perhaps, hardly surprising that I believed this to be the end of the H2S project". 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.