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H2s RadarThe H2S radar was used in bombers of RAF Bomber Command, it was designed to identify targets on the ground for night and all-weather bombing. On January 30 1943 H2S radar is used by RAF bombers for navigation for the first time and so became the first ground mapping radar to be used in combat. Initally it was fitted to Stirling and Halifax bombers and provided a ground mapping capability for both navigation and night bombing. This develoment, using ten-centimeter radar, (actually 9.1 cm) was possible thanks to the development of the cavity magnetron. later versions of H2S reduced the wavelength used, first to 3 cm and then 1.5 cm, at which wavelength the system was capable of detecting rain clouds. Later in the World War II the Luftwaffe night fighters used Naxos radar detectors to home in on the transmissions of H2S. The Americans adapted the X-Band version of H2S (H2S Mk VI) as H2X radar which they regarded as a significant improvement and which was tested by the RAF Bomber Command in 1945. Text from an open souce Taken from Microwave Radar At War (1). There is a open source verification for this text on the home page Greg Goebel / In The Public Domain. After the Battle of Britain, RAF Bomber Command began to ramp up night attacks against German cities. Unfortunately, although Bomber Command reported grand results from the raids, an independent analysis based on daylight air reconnaissance performed in the summer of 1940 showed that half the bombs fell on open country. Only one bomb in ten actually hit the intended target. Radio electronics promised some relief. The British developed a radio navigation system called "Gee", mentioned in an earlier chapter, and then a second long range navigation scheme known as "Oboe", both discussed in detail in a later chapter. Gee and Oboe were limited in range to a line of sight to the transmitters. A bomber carrying its own, self-contained night targeting system would not be limited in range to a UK-based transmitter. Taffy Bowen had noticed during his early AI experiments before the war that the radar returns from fields, cities, and other areas were distinctively different. He had suggested development of targeting radar, but the matter was forgotten in the chaos. The idea resurfaced in 1941. Philip Dee's group had got a 10 cm / 3 GHz AI flying in a Blenheim in March of that year. The experimental set was known as "AIS" in reference to its S-band operation. During tests of the AIS, Dee's team rediscovered that radar reflections could reveal different types of terrain. In October 1941, Dee attended a meeting of the RAF Bomber Command where the night targeting issue was discussed. After the meeting, on 1 November 1941 Dee performed an experiment in which he used an AIS radar mounted on a Blenheim to scan the ground. He was able to pick up the outline of a town 55 kilometers (35 miles) away. The brass were impressed, and on the first day of 1942, the TRE set up a team under Bernard Lovell to develop an S-band airborne targeting radar, based on AIS. The new targeting radar was designed to fit in a blister on the belly of a bomber, where the antenna would rotate to scan the terrain and feed the reflections to a PPI display, producing a map of sorts of the land below the bomber. The targeting radar was originally designated "BN (Blind Navigation)", but quickly became "H2S". This acronym remains somewhat mysterious, with different sources claiming it meant "Height to Slope"; the smelly compound hydrogen sulfide, with the possible interpretation of "it stinks"; or, with a little rearrangement, "Home Sweet Home". The "S" might have also had some connection to "S-band", but it is plausible the acronym was deliberately obscure and misleading as a security measure. H2S performed its first experimental flight on 23 April, with the radar mounted in a Handley-Page Halifax bomber. There was much still to be done. For example, in order to display as a uniform a "map" of the terrain as possible, the radar had to have low sensitivity or "gain" for targets directly underneath the bomber, with the gain increasing with the angle of the radar away from vertical. This scheme would become known as "cosecant-squared" scanning, after the mathematical function that defined the change in gain. H2S was the TRE's top priority, and Lovell's team had use of the brilliant Alan Blumlein and other top EMI engineers, but there were snags. Intelligence reports had revealed the Germans had stationed a company of paratroopers near Cherbourg, across the channel, suggesting the enemy might be planning to raid the TRE. On 25 May, the entire organization moved out in another mad, infuriating fire drill from Swanage to Malvern College, about 160 kilometers (100 miles) to the north. Fortunately, this would prove to be the last move. As if this weren't bad enough, then an outright 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 major blow to the program. Furthermore, Churchill's science advisor Lord Cherwell, known earlier as Professor Frederick Lindemann, wanted the design team to build H2S around the klystron rather than the magnetron. Lord Cherwell was opinionated, obstinate, contrary, something like Churchill himself but without quite as many redeeming features. Most people who had to deal with Lord Cherwell regarded him, with some justification, as an obstructionist who tried to create problems instead of figuring out how to overcome them. He was not always wrong by any means, but he was usually annoying. Lord Cherwell did not want the secret of the magnetron to fall into German hands. Once the Germans understood it, they would not only try to duplicate it, but could quickly develop countermeasures against it. The klystron wasn't as powerful as the magnetron, but it could be much more easily destroyed in an emergency than a magnetron. A magnetron's copper core could survive even large self-destruct charges. The H2S design team did not believe the klystron could do the job, and in fact tests of an H2S built with klystrons instead of the cavity magnetron showed a drop in output power by a factor of 20 to 30. 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 weren't working on the technology already. The first concern would prove correct; the second would fortunately be proven wrong, though given the widespread parallel development of the cavity magnetron, in hindsight it wasn't an unreasonable assumption. Despite all the problems, on 3 July 1942 Churchill held a meeting with brass and the H2S group, where he shocked the radar designers by demanding the delivery of 200 H2S sets by 15 October 1942. Bomber Command had to have H2S. The H2S design team was under extreme pressure, but they were given priority on resources. The pressure also gave them an excellent argument to convince Lord Cherwell that the klystron-based H2S program be finally dropped. Despite the extraordinary efforts of the TRE, there was no way to meet the 15 October deadline. By 1 January 1943, however, twelve Short Stirling and twelve Halifax bombers had been fitted with H2S. On the night of 30 January 1943, thirteen "Pathfinder" bombers, which dropped incendiaries or flares on a target to "mark" it for other bombers following in the bomber "stream", took off to give H2S its introduction to combat by marking the German city of Hamburg for a strike. Seven of the Pathfinders had to turn back, but six marked the target successfully, which was hit by a hundred Lancasters. The Germans did not know about H2S at the time. Unfortunately, on 2 February 1943, a Pathfinder Stirling was shot down near Rotterdam, and the Germans noticed the unusual gear in its wreckage. The British had been clever with electronics, and the Germans were careful to look for anything out of the ordinary in RAF aircraft forced down in the Reich. Most of the H2S set was recovered except for the display, and German engineers began to work on the "Rotterdam Gert" (Rotterdam Device), as they called it, however the engineeers were puzzled as to what the equipment actually did. The equipment remained a puzzle, until about a year later a working display was recovered from another aircraft and the complete equipment set-up on one of Berlin's immense concrete flak-towers. When the equipment was switched on and the onlookers saw the display they were horrified, the display recognizably showing Berlin's other flak towers and surrounding area. When Hermann Gring was shown this, he is said to have exclaimed "My God! the British really can see in the dark" Bomber Command didn't use H2S in a big way until that summer. On the night of 24 July 1943, the RAF began OPERATION GOMORRAH, a large-scale systematic attack on Hamburg. With the target marked by Pathfinders using H2S, RAF bombers hit the city with high explosive and incendiary bombs. They returned on the 25th and the 27th, with the USAAF performing two daylight attacks in between the three RAF raids. Large parts of the city were burned to the ground by a terrifying cyclone of fire. About 45,000 people, mostly civilians, were killed. H2S was noteworthy for introducing the Plan Position Indicator, or PPI the rotating-map display that is now familiar to radar operators the world over. One of the scientists working on H2S was Bernard Lovell, who later went on to become a leading figure in radio astronomy and who was instrumental in the founding of the Jodrell Bank radio telescope. See also References - A. P. Rowe: One Story of Radar - Camb Univ Press - 1948
- Dudley Saward, Bernard Lovell: A Biography - Robert Hale - 1984
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