H2S (navigation)

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Image of an H2S radar screen taken during the attack on Cologne on October 31, 1944. The course of the Rhine can be clearly seen.

H2S was the code name of a radar device that was used in bombers of the British Royal Air Force (RAF). It was used to find the target under poor visibility conditions, such as bad weather and during night operations.

On January 30, 1943, the H2S was first used in RAF bombers. This was also the first combat mission with a ground detection radar. It was initially used in Stirling and Halifax bombers for navigation and target acquisition at night.

The development of the magnetron made it possible to use ten-centimeter radar (precisely 9.1 centimeter wavelength with which this system initially worked). The wavelength was later reduced to 3.0, then to 1.5 centimeters, which enabled the radar to also detect rain clouds.

In the later course of the Second World War , the German Air Force used the radar detector FuG 350 “Naxos” , with which the night fighters could locate the H2S signals and thus track down the British bombers.

The Americans took over the 1,945 tested from the RAF version Mk VI of H2S device in the microwave range from 8.0 to 12.0 Giga hertz , so-called "X-band" worked. This version, called H2X , delivered significantly sharper images.

Development history

After the Battle of Britain , the RAF began to fly night attacks against German cities with their bombers. Although Bomber Command reported great success with the attacks, independent daylight investigations conducted in 1940 showed that half of the bombs were dropped in the open field and only one in ten bombs actually hit the intended target.

A higher success rate was found with the use of radio navigation . The British initially developed the GEE system , which was operational in March 1942, and a little later (December 1942) the long-range navigation system Oboe . Both GEE and Oboe were passive systems and limited in their range as they required direct radio contact with the stationary transmitters in Great Britain.

A bomber that carried a complete, active radar system would, however, be independent of the range of the stationary transmitters. The British physicist Edward George Bowen had already found in experiments for an air interception radar (AI) before the war that the radar waves that reflected from fields, cities and other areas differ characteristically. At that time, he proposed the development of a radar for target finding, but the idea was forgotten until 1941. A group around Philip Dee was working with a 10 cm / 3 GHz AI radar in a Bristol Blenheim in March of this year . This test setup was called AIS because of its operation in the S-band range. This team also discovered that the radar reflection allowed conclusions to be drawn about the nature of the terrain overflown.

In October 1941, Dee attended a meeting of the Bomber Command about the problem of targeting at night. Then, on November 1, 1941, he carried out an experiment in which the AIS in Blenheim was used to scan the area overflown. He could make out the outlines of a town 55 km away.

The guide was impressed. On New Year's Day 1942, a team was put together at the Telecommunications Research Establishment (TRE) in Swanage under the direction of Bernard Lovell to develop a radar system for target recognition based on the AIS. The new radar was to be housed in a dome ( radome ) on the underside of the bomber. A rotating antenna scanned the surroundings and transmitted the reflection to a picture tube ( PPI display ) on which a black and white map of the surrounding area appeared.

The system was initially called “BN” for “Blind Navigation”, but was soon renamed H2S. There are various interpretations of the exact origin and meaning of this term. Some sources say this means "height to slope", others point to the foul smelling hydrogen sulfide , which had no connection with the system. A development engineer is said to have provided the template for the name H2S with the statement: "They'll stink when we see their cities despite the blackout." It is possible that an obscure name was deliberately chosen for camouflage reasons. According to some sources, H2S stands for "Home, sweet home". Opposing intelligence services should believe on the basis of this code name that it is a navigation device that will bring the bomber back to its starting point.

Halifax antenna system
H2S radar unit on display at the Science Museum in London
Unit 182 (Fishpond) .jpg

On April 23, 1942, the H2S was first tested on a Handley Page Halifax . But there was still a lot to do. The sensitivity or signal amplification had to be adjusted depending on the angle and distance in order to be able to depict the surroundings evenly like a map.

The H2S was a top priority at TRE and Lovell was able to use some of the best engineers, including Alan Blumlein , on it. But there were also obstacles. The secret service reported that a company of German parachutists had been stationed near Cherbourg , who were believed to be planning an attack on the TRE, as the British did in Operation Biting against the German Würzburg radar system . On May 25, the entire organization moved in a hurry from Swanage 160 kilometers north to Malvern College .

Another major setback came when the Halifax on which the tests were conducted crashed on June 7, 1942. The prototype of the H2S was destroyed and the entire crew, including Alan Blumlein, killed.

In addition, Churchill's scientific advisor Frederick Lindemann, 1st Viscount Cherwell , wanted the H2S to be built with a klystron instead of the magnetron. Lord Cherwell was an opinionated, stubborn guy, like Churchill, but also with significantly less self-insight. Most of the people who have had to work with him saw in him a blocker who tries to create problems rather than overcome them.

Lindemann wanted to prevent the secret of the magnetron from falling into German hands, because as soon as they recognized the principle, they would not only try to copy it, but could also quickly develop countermeasures. The klystron was not as powerful as the magnetron, but it could be destroyed very easily in an emergency, while the copper core of a magnetron could withstand even larger charges.

The H2S team doubted the klystron was suitable for the job. When testing an H2S with klystron, the output signal was 20 to 30 times lower. The H2S team protested, arguing that it would take the Germans two years to develop a radar with a wavelength in the centimeter range from a captured magnetron and that there was no reason to believe that they were not already working on such a system work. The first objection turned out to be correct, the second not. The magnetron was already known in Germany, where a patent was applied for as early as 1935. Because of the frequency drift in the magnetron, the German developers used high- key triodes in the transmission stages of their radar systems.

Despite all the problems, Churchill surprisingly demanded the construction of 200 H2S units by October 15, 1942 at a meeting with senior military officials on July 3, 1942. The H2S team was under high pressure, but also had priority in accessing resources. So they could now assert themselves against Lord Cherwell and give up the klystron-based H2S. Despite all efforts, however, it was not possible to achieve the goal by October 15th. By January 1, 1943, however, twelve Stirling and as many Halifax bombers could be equipped with the H2S. On the night of January 30, 1943, thirteen bombers of the Pathfinder Force , which were supposed to drop incendiary devices or flares to mark the target for the stream of bombers, started their first combat mission with H2S, targeting Hamburg . Seven of the scouts had to turn back prematurely, six bombers managed to mark the target, which was then bombed by one hundred Lancaster bombers.

The H2S system was still unknown in Germany at that time. Shot down aircraft were routinely examined carefully. On February 2, 1943, a Boy Scout Stirling was shot down near Rotterdam, and the German troops noticed an unusual device in the wreckage. Apart from the destroyed viewing device with the cathode ray tube , an almost complete H2S device could be recovered. As a result, the investigation of this " Rotterdam device " began at Telefunken in Berlin , but its function could not yet be determined. It wasn't until about a year later that an intact display was recovered from another aircraft and a complete system was installed on one of the Berlin flak towers . When it was switched on, the horrified technicians saw the contours of Berlin on the screen with its numerous bodies of water clearly marked.

Until the summer of 1943 the H2S was only used sporadically. On the night of July 24, 1943, the RAF and the USAAF launched several systematic large-scale operations against Hamburg - Operation Gomorrah . At that time, the Lancaster bombers , which now formed the backbone of the British Bomber Command, were also equipped with the H2S. After the Pathfinder Force aircraft had marked the targets with the help of this radar system, countless high explosive and incendiary bombs followed. This procedure was repeated on July 25th and 27th with the assistance of daytime attacks by the US 8th Air Force . In the firestorm many houses burned to the ground, thereby, arrived about 34,000 people, mainly civilians killed.

The H2S also played an important role in the attacks on Berlin between November 1943 and March 1944. Berlin was beyond the range of British radio navigation with GEE and oboe and also often covered by clouds in winter. It was hoped that the H2S would be able to use the numerous bodies of water in the city as a navigation aid. However, this did not work with the original settings of the H2S. It was only with the H2S Mark III, which worked with a wavelength of 3 cm and was able to distinguish open from built-up areas, that targeted bombs could be dropped there.

For the first time, the H2S used the two-dimensional display of distance and direction on the round radar screen - the “Plan Position Indicator” (PPI), which is still used today.

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