H2S was the first airborne, ground scanning radar system. It was developed in Britain during World War II for the Royal Air Force, and was used in various RAF bomber aircraft from 1943 through to the 1990s. It was designed to identify targets on the ground for night and all-weather bombing. The early variants of the transmitter/receiver equipment were officially known as TR3159 (H2S Mk I/ASV VIB) or TR3191 (H2S Mk II).
On 30 January 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 ground mapping for 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, first to 3 cm and then 1.5 cm at which wavelength the system was capable of detecting rain clouds.
On a raid to Cologne on 2/3 February 1943, a Stirling Pathfinder was shot down over the Netherlands. The H2S set it was carrying was damaged but not beyond repair (fortunately for the Germans it was only the second operational use of H2S), and, known as the Rotterdam Gerät, Telefunken was able to reassemble it, with the exception of the PPI display that had been destroyed. Eventually this led to the development of the Funkgerät (FuG) 350 Naxos radar detector, which enabled Luftwaffe night fighters to home on the transmissions of H2S.
Second World War 
After the Battle of Britain, RAF Bomber Command began 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 half the bombs fell on open country. Only one bomb in ten hit the target.
Radio electronics promised some improvement. 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 a night targeting system would not be limited in range to a UK-based transmitter. Taffy Bowen had noticed during his early AI (Airborne Interception) experiments before the war that the radar returns from fields, cities and other areas were 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 (34 mi) away.
The commanders were impressed and, on the first day 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". The genesis of this designation remains somewhat mysterious, with different sources claiming it meant "Height to Slope"; or "Home Sweet Home". The "S" might have also had some connection to "S-band" but it is plausible the abbreviation was deliberately obscure as a security measure. There is also a rumour that it was named after hydrogen sulphide (chemical formula H2S, in connection with its rotten smell), because the inventor realised 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 hadn't thought of it sooner! The "rotten" connection, with a twist, is propounded by R V Jones who relates the tale that, due to a misunderstanding between the original developers and Lord Cherwell, development of the technology was delayed, the engineers thinking that Lord Cherwell wasn't keen on 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 therefore christened the restarted project "H2S" and later, when Cherwell inquired as to what H2S stood for, no one dared tell Cherwell 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 (including R V Jones).
H2S performed its first experimental flight on 23 April 1942, with the radar mounted in a Halifax bomber, the scanning unit installed in the aircraft's belly using the position previously occupied by the mid-under turret, which was by that time seldom installed. The scanning aerial was covered by a distinctive streamlined radome that was later to become a characteristic fitting of RAF heavy bombers. One problem was that to display as uniform a "map" of the terrain as possible, the radar effectively had to have low sensitivity or "gain" for targets directly underneath the bomber, with the gain increasing with target range. The scheme adopted was to tailor the power distribution in the aerial beam according to a cosecant-squared rule, so called after the mathematical function that defined the effective change in gain. The change in the beam 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 aerial reflectors were actually shaped with a cosecant-squared curvature.
The H2S scanner system would later incorporate a gyro-stabilised mounting, reducing the effect of aircraft attitude in pitch and roll upon the reflected signal.
H2S was the TRE's 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 TRE, (just as the British had raided the French coast to seize a German Würzburg radar in Operation Biting). On 25 May 1942, the organization moved from Swanage to Malvern College, about 160 kilometers (99 mi) to the north.
Then 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 huge blow to the program.
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 anode block, containing resonant cavities from which the operation of the device could be deduced, could survive large demolition charges.
The H2S design team did not believe the klystron could do the job, and 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 be proven wrong, though given the widespread parallel development of the cavity magnetron, in hindsight it was not 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 great 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.
It is perhaps interesting to note that the development of the in-service radar sets was so rapid that the developers were forced to use existing plug-and-socket designs to connect the various units of the complete set together. There were no bulkhead mounting male connectors available at this time, and consequently many of the male free connectors at the ends of cable runs carried exposed lethal voltages.
Despite extraordinary efforts, TRE failed to meet the 15 October deadline. By 1 January 1943, 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. Seven of the Pathfinders had to turn back but six marked the target, which was hit by a hundred Lancasters.
Bomber Command didn't use H2S generally until that summer. On the night of 24 July 1943, the RAF began Operation Gomorrah, a large attack on Hamburg. At that time, H2S was also fitted 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 cyclone of fire. About 45,000 people, mostly civilians, were killed.
H2S was vital 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.
After the war, the RAF's Avro Lincoln bombers were also equipped with H2S. In the 1950s, a later variant, H2S Mk.9, formed part of the Navigation and Bombing System (NBS) installed in the RAF's V bomber force,  comprising the Vickers Valiant, Avro Vulcan and Handley Page Victor aircraft. Both the Vulcan and Victor participated in the 1982 Falklands War, using their H2S radars to great effect, and the last of these aircraft were not withdrawn until 1993.
See also 
- Naxos radar detector, created by Germany to spot H2S transmissions
- List of World War II electronic warfare equipment
- RAF History - Bomber Command 60th Anniversary, Campaign Diary: February 1943. Accessed 18 July 2008
- R V Jones, 1978, Most Secret War: British Scientific Intelligence 1939-1945, London: Hamish Hamilton. ISBN 0-241-89746-7 (Published in the USA as The Wizard War: British Scientific Intelligence 1939-1945.)
- "Avro Lincoln". The Spyflight Website. Retrieved 2009-01-14.
- Dillon, John. "NBS". www.john-dillon.co.uk. Retrieved 2009-01-14.
- Gustin, Emmanuel. "Acronyms and Codenames FAQ, L-N". Haze Gray & Underway. Retrieved 2009-01-14.
- "Handley Page Victor - History". Thunder and Lightnings. 2008-10-10. Retrieved 2009-01-14.
- 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 - Cambridge University 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 0-09-151580-7
- E. G. Bowen Radar Days ISBN 0-7503-0586-X
|Wikimedia Commons has media related to: H2S radar|
- Bournemouth University Radar Recollections site
- Picture of an installed H2S unit
- A working H2S Mk 9 and NBS as used in the Vulcan, Victor, and Valiant
- Bomber's Radar - General Survey of the Three Primary Systems Used by Bomber Command - Flight article of September 1945