page_title: H2S (radar)
section_title: Work begins
hierarchical_section_title: H2S (radar) / Development / Work begins
caption_attribution_description: English: The H2S radome (top) and the enclosed, rotating, scanning aerial (bottom) on a Handley Page Halifax. Imperial War Museum? - picture scanned by me Ian Dunster 08:36, 22 September 2005 (UTC) from the; The Radar War article in the November 1978 issue of Aeroplane Monthly and uncredited.
context_page_description: 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 350 kilometres. 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 Airborne Interception radar based on the 9.1 cm S band 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 Mk. I and H2S Mk. II, as well as ASV Mark III.
context_section_description: 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 35 miles (56 km) away while flying at 8,000 feet (2,400 m) altitude.
  The commanders were impressed and, on 1 January 1942, the Telecommunications Research Establishment (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 (PPI) 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-Surface-Vessel radar (ASV) radar, ASV Mk. 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 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.
  Then disaster occurred; 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, and his loss was a huge blow to the programme. 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.
caption: The H2S radome (top) and its enclosed scanning aerial (bottom) on a Halifax. The angled plate fixed to the top of the reflector modified the broadcast pattern to make nearby objects less bright on the display.
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