H2S radar

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An early H2S picture of the Pembroke and Milford Haven area

The 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 was used by RAF bombers for navigation for the first time and so became the first ground mapping radar to be used in combat. Initially it was fitted to Stirling and Halifax bombers and provided a ground mapping capability for both navigation and night bombing.

This development, 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 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.

1 History
2 See also
3 References
4 Notes
5 External links

[edit] History

After the Battle of Britain, RAF Bomber Command began to ramp up night attacks against German cities. Although Bomber Command had reported good results from the raids, an independent analysis based on daylight air reconnaissance performed in the summer of 1940 showed that in reality half the bombs fell on open country. Only one bomb in ten actually hit the intended target.

Radio electronics promised some improvements. The British developed a radio navigation system called "Gee", and then a second medium-range navigation scheme known as "Oboe". 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 had been forgotten.

The idea resurfaced in 1941. Philip Dee's group had 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 H2S radome (top) and its enclosed scanning aerial (bottom) on a Halifax

The commanders were impressed, and on the first day of 1942, the Telecommunications Research Establishment (TRE) set up a team under Bernard Lovell (who later went on to become a leading figure in radio astronomy) to develop an S-band airborne targeting radar, based on AIS. The new targeting radar was designed to fit in an aerodynamic blister on the belly of a bomber, where the antenna would rotate to scan the terrain and feed the reflections to a Plan Position Indicator 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 poor-smelling compound hydrogen sulphide, with the possible interpretation of "it stinks" (reputedly once used by Lord Cherwell to describe the device); 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. There is also a rumour that it was named after hydrogen sulphide, because the inventor realised that had he simply pointed the radar downward instead of towards the sky, he would have an entirely new application for radar, ground tracking instead of simply for identifying air targets, and that it was simply 'rotten' that he hadn't thought of it sooner!^{[citation needed]}

H2S performed its first experimental flight on 23 April, with the radar mounted in a Halifax bomber. Problems remained: in order to display as much 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 English Channel, suggesting the enemy might be planning to raid the TRE, (just as the British had raided the French coast to seize a German Würzburg radar in Operation Biting). On 25 May, the entire organization moved from Swanage to Malvern College, about 160 kilometers (100 miles) to the north.

The Halifax V9977, which crashed in June 1942, killing several key radar technicians

Then an outright disaster occurred. On 7 June 1942, the Halifax performing H2S tests (right) 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 wanted the design team to build H2S around the klystron rather than the magnetron, not wanting to risk the secret of the magnetron falling into German hands. Once the Germans understood it they 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. 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 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. 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 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 for the Allies, on 2 February 1943, a Pathfinder Stirling was shot down near Rotterdam, and the Germans noticed the unusual equipment in its wreckage. 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 Gerät" (Rotterdam Device), as they called it, however the engineers 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 was 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 Göring was shown this, he is said to have exclaimed "My God! the British really can see in the dark"^{[citation needed]}.

Bomber Command didn't use H2S in a major way until that summer. On the night of 24 July 1943, the RAF began Operation Gomorrah, a large-scale systematic attack on Hamburg. At that time, H2S was fitted also to Lancasters, which became a backbone of RAF Bomber Command. 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 a key component in the air battle for Berlin, a series of large raids on the German capital and other cities from November 1943 until March 1944. Berlin was out of range of radio navigation aids such as Gee and Oboe, and often obscured by cloud in the winter, so at the start of the battle it was hoped that H2S would by identifying the many lakes and rivers in the city be a crucial aid to navigation. The H2S sets available at the start of the battle were not able to do so. It was not until after the night of 2 December when the H2S Mark III, which operated on a 3 cm wavelength and could identify open and built up spaces, was successfully used for the first time on operations, that it became possible to bomb Berlin accurately.^[1]

[edit] See also

List of World War II electronic warfare equipment

[edit] References

Sir Bernard Lovell ECHOES OF WAR : The Story of H2S Radar ISBN 0-85274-317-3
Adapted from Microwave Radar At War (1). There is an open source verification for this text on the home page Greg Goebel / In The Public Domain.
A. P. Rowe: One Story of Radar - Camb Univ Press - 1948
Dudley Saward, Bernard Lovell: A Biography - Robert Hale - 1984
Norman Longmate The bombers : the RAF offensive against Germany, 1939-1945, Hutchins & Co, (1983), ISBN 0091515807
E. G. Bowen Radar Days ISBN 0-7503-0586-X