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The Luftwaffe in 1941, were confident that they were ahead of the British in terms of radar developments. Yet by the end of the war this lead had been lost and the Allies were supreme.

This is an account of the development and entry into service of two of the more important radar devices, OBOE and H2S. OBOE and H2S, together with the navigational aid GEE, radically improved the performance of Bomber Command by increasing the tonnage of bombs that landed in the target area. RAF Bomber Command had begun to realise that its bombers were not able to find and hit specific military targets such as airfields or armament factories. A military investigation of 1940, undertaken by the RAF, revealed that just one in five aircraft was succeeding in dropping its bombs within five miles of its target. Under such circumstances, the bombing offensive could only be effective if it was directed at targets as big as cities.

At the same time, however, the radiation emitted by H2S in particular, provided the means whereby German fighters and ground radar systems could detect the presence of RAF heavy bombers. In securing greater efficiency, risks and casualties were increased. The use of H2S quickly led to the development of many other radar devices, developed by both sides, and based upon the detection of radiation from airborne radar sets.

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The history of the air war against Germany, is very complex, with many major developments influencing its progress. Post-war accounts present a misleading and oversimplified picture of events.

Each new development which assisted Bomber Command in the short term, was quickly nullified as Germany produced a suitable jamming system.

Design teams on both sides were eventually forced to accept that their technology or equipment would, sooner or later, find its way into enemy hands, thus enabling them to produce a counter-measure. In some cases, fear of capture of equipment or knowledge of its development, actually delayed some new devices being brought into service.

The visual bombing of Germany and occupied Europe had one major limitation in both winter and summer; cloud over the target area. Inadequate navigational systems in the early years prevented a large proportion of a bomber force, failing to reach, and/or identify, the target. Thus a high percentage of bombs was wasted by hitting the wrong target or simply falling on open countryside.

By early 1942, the navigational device GEE (a radio navigation system which measured the time delay between two radio signals to produce a fix.) improved the efficiency of Bomber Command by enabling more bombers to reach the target area.

However, this could still present problems if cloud below 20,000 feet obscured the target. If the meteorologists forecast cloud over the target there was no alternative but to stand down practically the entire bomber force. This waste of opportunity was unacceptable, and the finding a solution, was of the utmost urgency.

The early failure by RAF bombers to locate their targets in sufficient numbers, caused Churchill, and his prime scientific adviser, Professor Frederick Lindemann, (later 1st Viscount Cherwell), to place a high priority upon the development of improved methods of navigation to enable the bombing of targets through cloud or haze.

OBOE

OBOE was a British aerial blind bombing targeting system based on radio transponder technology suggested by Alec Harley Reeves with his co-worker Francis Edgar Jones at the Telecommunications Research Establishment. This was part of a planned study on blind bombing.

Using triangulation to determine relative location, the system consisted of a pair of radio transmitters on the ground, which sent signals that were received and re-transmitted by a transponder in a bomber aircraft (typically a Mosquito, Halifax, Stirling or Lancaster). By comparing the time each signal took to reach the aircraft, its location could be calculated to a fair degree. In this way, a bomber could be directed blindly over a predetermined target.

Radar has the potential to measure distances very accurately and this accuracy is not reduced with range but, like the GEE navigating system, OBOE was not strictly a member of the radar family. The system required the setting up of two transmitter stations known at the time as ‘Cat’ at Walmer near Dover and ‘Mouse’ at Trimingham near Cromer. Prior to a mission, a circle was drawn around the primary OBOE transmitter at Walmer ('Cat'), so that it passed over the target, with the primary transmitter in the exact centre, and the radius of the circle noted.

The transponder-equipped bomber(s) would then attempt to fly one at a time, along the circumference of this circle towards the target. By keeping careful track of the range between the airborne transponder and the transmitter, the Oboe operator in England would use the equipment to see if the bomber had strayed from the correct course, and if so, give the pilot instructions on how to regain it.

While the primary transmitter could determine if the aircraft was flying on the circumference of the circle, it was impossible to tell what point it was at. To enable this, the range from the secondary transmitter at Trimingham ("Mouse") was also measured, and a circle drawn around it. Where the two circles intersected determined the position the bomber. By repeatedly doing this, the progress of the bomber could be tracked, and when it reached the point where the primary line passed over the target, a coded signal was sent alerting the crew to drop their bombs. The path was only 35 yards (32M) wide, thus allowing for much greater accuracy than other systems such as GEE. The curved path of the aircraft was quite evident to German radar operators, who came to call the system 'Boomerang' after the arc segment left on their displays as the aircraft appeared and disappeared out of range. OBOE’s limitation of one aircraft at a time led to several new systems. Among these were the GEE-H System, which used the existing GEE Equipment with OBOE-like transmitters. GEE-H could guide about 80 aircraft at on

The OBOE system was capable of achieving extremely high accuracy, much higher than could be reached with conventional bomb sights and Professor Lindeman was fond of reminding Churchill that the more accurate the bombing, the fewer bombers would be required to win the war thus releasing a great number of workers and saving vast quantities of scarce war materials.

But despite its success, Lindemann was unimpressed due to its ‘optical line of sight system’ affected by the curve of the earth. This imposed a maximum range of about 270 miles, restricting the effective range to little further than the German Ruhr at a time when he was strongly advocating more raids upon distant targets such as Berlin.

The use of OBOE proved to be an important development for the later Pathfinder Force. Its use made a major contribution to the devastation of the Ruhr until the end of the war. To achieve the longer range required the bombers necessarily needed to fly at higher altitudes. With the Stirling limited to 14,000 feet and the Halifax to 18,000 feet the Mosquito was chosen to carry the devise in the role as target marker.

The Luftwaffe pre-war had anticipated the Pathfinders of the RAF by using their ‘Y’ beams in an early form of ‘Pathfinding’ attacks. The system was also used by the German Kampf-gruppe 100. So confident were the Germans in this system that they neglected to teach their observers the principles of dead reckoning navigation.

Lord Cherwell became almost obsessed in his desire for a system, effective at greater ranges. Many of the doubts expressed about OBOE were due to the short wavelength that had to be used. 1.5m was first chosen and aircraft on this wavelength required an equivalent of an optical path and therefore had to fly at great heights. The advantage of using shorter wave radar was that the range obtained was always greater and it was more difficult to jam.

The OBOE differed from GEE in one important characteristic in that there was a limitation to the number of aircraft it could control. The aircraft had to be continuously monitored by its ground stations and therefore it could only initially control six aircraft per hour and as there were only three stations, the maximum was only eighteen aircraft per hour.

It had been used first by Stirlings against the Battle cruisers in Brest as a blind bombing device. The small number of aircraft it could control suggested a better use was for path finding Mosquitoes who could mark accurately the targets for the main bomber force to attack.

For Bomber Harris, 1943 would be the beginning of a new phase in Bomber Command’s history. On 26 November 1942 the first of 2 pairs of OBOE ground stations commenced operations that enabled 109 Squadron, the first squadron to be equipped with OBOE Mosquitos, to commence training. This squadron was commanded by Wing Commander H.E. Bufton, brother of Sidney Bufton who was to have many famous administrative battles with Bomber Harris to establish Pathfinder Squadrons to lead Bomber Command's major attacks.

The first OBOE Mosquitoes were ready for operations on 20 December 1942. The first attack was intended to be the power station at Lutterade but it was not a success as only one aircraft was able to locate the target.

The purpose of the raid was to calibrate their OBOE installations. Further calibration raids were carried out in December and January 1943, which resulted in the Krupp’s works in Essen being damaged. The final OBOE calibration raid was successfully carried out on the cadet school and night fighter airfield at St. Trond in Belgium.

On 23/24 December 1943, five OBOE Mosquitoes attacked targets in Essen, Hamborn, Meiderich and Rheinhausen but because of haze, the results were difficult to determine. The aircraft had found their targets using GEE. Those aircraft attacking Essen were able to drop 50% of their bombs on the main Krupps factory. A day later on 24/25 December OBOE Mosquitoes again attacked Essen and their bombs hit the northern part of the Krupps factories. Both these operations were without loss. Thus for the first time the giant Krupps factories were damaged by bombs.

The first trial Pathfinder operation was on 31 December using two OBOE equipped Mosquitoes for target indicating and followed up by a small force of eight Lancasters. This operation was against Düsseldorf. Of nine bombs dropped, six hit industrial premises but without serious damage being caused. On the same night two more OBOE equipped Mosquitoes attacked a night fighter control room at Floriennes Airfield in Belgium, dropping six bombs from 28,000 feet, and hitting the building.

On 7 January 1943, there was a meeting between senior Luftwaffe officers and the Directors of Krupps, who were greatly disturbed that Mosquito bombers were reaching Essen undetected. They were able to fly across the town of Essen and Krupps undetected, regardless of the weather and industrial haze, dropping bombs with great accuracy in hitting the works. The air raid sirens were failing to sound and warn the workers of an attack and they were getting concerned. The question they asked was: 'Is the enemy using some kind of infra-red homing device?' but the German specialists were able to confirm that the Mosquitos were flying at 30,000 feet on a beam originating from England.

March 1943 signalled the opening of the RAF Campaign to wipe out the munitions industries of the Ruhr and by Spring 1943, 50,000 labourers had had to be drafted in from constructing the Atlantic Wall to repair bomb damage in the Ruhr.

The British planners in July 1942 had expected that the system would be free of jamming for little more than a month but to their surprise, eighteen months would elapse before it was seriously impaired. The Germans had falsely identified OBOE used to control the British version of E-boats. By then, the British had developed a Mk2 version of OBOE jointly with American teams and this extended its life beyond ‘D’ Day. However, the OBOE equipped Mosquito loss-rate proved to be very small at less than one-quarter per cent. The Germans remained mystified by the high-level attacks of the OBOE equipped Mosquitos and were unable to shoot one down to examine the radar equipment*. They were able to deduce the wavelength of the OBOE system and were considering methods of jamming it. Their radar experts had, however, come to the conclusion that the RAF had alternative wavelengths available which would have precluded jamming.

*Whenever a new radar device was proving to be promising, consideration had to be given to the effects of it falling into enemy hands through crashed aircraft and the possibility of German jamming it before its full value could be realised. On 7 July 1944 one of 105 Squadron’s Mosquitos fitted with OBOE crashed near Caen and Wing Commander Edward Barton was dispatched in another Mosquito to investigate. The aircraft was landed close to the crashed aircraft and the precious OBOE device was removed before the Germans arrived at the site. Unfortunately, the navigator had bailed out and the pilot of the Mosquito was killed in the crash before the secret equipment could be destroyed.

Later the Luftwaffe were able to plot the Mosquitos using OBOE, with a radar system given the name of ‘Flammen’. It was some months before British intelligence could link the Flammen plots with the use of OBOE. Later the Germans did find a method of jamming which entailed a sweep of the frequencies used for OBOE. This led to a complete failure of a raid on Rheinhausen which in turn lead to the RAF bringing into service OBOE Mk ll and a Mk lll which worked on the centimetre bands. OBOE Mk 1 continued to be used as camouflage for the other marks of OBOE. As the Allied armies advanced across Europe mobile OBOE transmitters followed in their wake.

As the war ended the Germans did have some success in jamming all marks of OBOE but by then its job had been completed.

Intricately interwoven with both OBOE and H2S was the story of WINDOW and its German equivalent DUPPEL. This will be the subject of a later article.

H2S

H2S was introduced at approximately the same time as OBOE. It was originally known as ‘BN’ for blind navigation. Since these latter initials indicated the potential use for this radar system a name change was inevitable. The name was said to have been chosen by Lord Cherwell to suggest ‘Home Sweet Home’ as homing onto a target; not H2S (Hydrogen Sulphide) which was commonly believed to be its origin.

Aircrew Remembered Senior Editor (a former RAF radar specialist) writes: 'Having done all my advanced radar training on H2S I can say with some degree of certainty that this is yet another variation on the theme. Sir Robert Watson-Watt was instrumental in the development of ASV and H2S and the explanation of the origins of the term H2S is recorded in his book 'Three Steps to Victory' which is a personal account.

Its origins are somewhat convoluted but had to do with what is called 'Column 7' and 'Column 9' items for a modification to an aircraft. Column 7 items are aircraft wiring and equipment mounting trays installed during manufacture and Column 9 are the black boxes. Modified aircraft initially flew without the black boxes and to provide for any enemy curiosity about the content of a crashed aircraft, aircrew were told these particular items were for a new homing system to lead them 'Home Sweet Home' i.e. H2S. The book cites that it was one of Watson-Watt's lab technicians and not Lord Cherwell that coined the phrase off the cuff when asked what the equipment was.'

Radar pioneer, Albert Percival Rowe, reported that late in October 1941 at a ‘Sunday Soviet’ the subject chosen for discussion was how Bomber Command could attack cloud-obscured targets. At this meeting, Lord Cherwell was insisting upon Bomber Command having the means operate over a greater range from its British bases. He considered GEE and OBOE to be of limited value because of it being a line of sight device and its requirement for ground transmissions from bases in Britain. Some discussion took place as to whether following German electric power lines was a practical proposition. This particular meeting ended without a satisfactory proposal having been made but the ground had been prepared for the birth of new ideas.

However, later that week of October 1941, there was an inter-team meeting between physicist, Philip Ivor Dee, working on centimetric wavelength and the team headed by fellow physicist, Herbert Wakefield Banks Skinner Skinner, working on the basic problems of the same wavelength. They recalled that while working at Leeson House above Swanage, echoes had been received from the town. It was already known that with a centimetre ASV set, a map of the sea could be displayed in an aircraft, which would also show any vessel sailing across it. It was then thought that radar echoes could be obtained from a town, which would show buildings lakes, and countryside.

There seems to be some confusion as to which scientist first suggested the ideas behind H2S. Watson-Watt, in his book, proposed Welsh physicist, Edward George "Taffy" Bowen, who had written to A.P. Rowe in 1937-1940 indicating that some discrimination of echoes might be possible, although it was thought that Bowen at that time had recommended the use of a longer wavelength

On New Year's Day 1942, A.P. Rowe named Bernard Lovell to head the H2S development team. Lovell was reluctant, since he was fully involved in an Air Interception (A.I) set for fighter aircraft, and for reasons unknown, he even found it objectionable. After the war, he became a leading radio astronomer and his name will forever be associated with Jodrell Bank Radio Astronomy Station.

H2S radar installation

H2S installed in RAF Bomber

By March 1941 a prototype of an air-air interception was available for testing. The suggestion was made that by tilting the AI set so that the rotating centimetric beam could scan the area beneath the bomber and produce a picture of the area. An A.I. set was quickly modified at Christchurch Airfield which would scan the ground below and in front of the aircraft. The following day a flight was made over Southampton and Salisbury, which showed promise. This was the day that H2S was born.

Cherwell at this early stage was insisting on having the early production aircraft fitted with the new device; a mere seven months after its birth. This was an impossible target considering that from this very short period had to be deducted development time and setting up the production of sets and aircraft to carry them. To add to the pressure being built up over the urgent requirement to develop H2S, it became apparent in 1942 that a move away from the vulnerable South Coast of Britain was becoming essential. All this was to provide a further impediment to H2S development.

After considering several sites the decision was made to take over and modify the buildings of Malvern College, the boy’s public school in Worcestershire. The location of the school was perfect for the development of H2S, in that the school was on a hill overlooking the town, while at the same time, the town was of sufficient size to absorb the 1000 staff of the new research station, now to be known as the Telecommunications Research Establishment (TRE) Later they would move again to a dedicated site elsewhere in the town which remains its base to this day.

The version of H2S that entered service depended upon a new device or component called the high-powered cavity magnetron (vacuum tube). This was designed by John Randall and Harry Boot at Birmingham University to become the heart of the device. It started development with a power of 500 watts and was later increased to 10,000 watts.

It was highly secret, and considerable concern was expressed that it should not fall into the hands of the Germans. It was central to the centimetric revolution and was one of those treasured items taken to the USA by the (Henry Thomas) Tizard Delegation, officially the British Technical and Scientific Mission, in 1940. The complex prototype devices were developed in the workshops of Birmingham University, partly to minimise delay and partly for reasons of security.

Cherwell was greatly concerned that the magnetron could fall into the hands of the Germans prematurely. It proved to be almost impossible to destroy by using detonators in aircraft in danger of crashing. He, therefore, advocated using a klystron (a specialized linear-beam vacuum tube) in place of the magnetron. Details of this device had already been published in the scientific press. He considered that the version based upon the magnetron would take longer to develop. To get around the problems of the lack of power of the klystron he suggested that the bombers could use their GEE to approach the target and then use their H2S to bomb the target. He would accept a high system failure rate and depend upon at least some H2S-equipped aircraft blind bombing their target.

The H2S team strongly disagreed with Cherwell as the klystron lacked the power required and considered that such poorly developed equipment would not be acceptable. The controversy with Cherwell at one stage resulted in a decision to develop H2S in two versions using a klystron as well as a cavity magnetron. Eventually, Cherwell accepted the situation and work on the klystron ceased on 15 July. Sets fitted with magnetrons were accepted as standard.

The arrival of the new device was widely welcomed and quickly received the enthusiastic backing of the Prime Minister. He convened a now famous meeting on 3 July 1942 which was attended by Rowe, Lovell, Dee, Watson- Watt as well as Churchill’s military chiefs and American representatives

These Americans were astonished when first meeting Churchill to find he had direct contact with his scientific advisors. At the time, this did not happen with their President, and it certainly did not happen with Hitler and Göering. The German leaders remained suspicious throughout the war of both intellectuals and scientists. This outlook was to have a crippling effect upon the German conduct of the war.

A tragic accident occurred on Sunday 7th June 1942 when the Handley Page Mk ll aircraft used for H2S trials crashed in the Wye valley killing eleven engineers and military personnel. EMI had assigned to the development of H2S , their best engineer, Alan Blumlein, and he was included amongst those killed. Blumlein was undoubtedly an electronics engineer of exceptional brilliance who had before the war invented stereo sound and developed key technologies which made television possible.

Alan Blumlein is remembered and honoured with a Blue Plaque at his former home in Ealing, London.

The loss of Blumlein was considered by Bernard Lovell and others to be a national disaster. Several other members of the H2S development team were lost in the accident. The early Halifax aircraft were inherently unstable under certain conditions and this had already resulted in a number of accidents, often fatal. The investigation which followed revealed that an improperly tightened tappet nut on the starboard outer engine came loose as the result of a special inspection just before the flight, and caused the engine to catch fire, which destroyed the wing and led to the crash.

At a key meeting, an astonished audience was faced with the demand that 200 H2S sets must be made available by 15 October 1942. It was made clear that no reason would be acceptable as to why this should not be achieved. The radar representatives declared it to be impossible, as the production models would never be ready in time. They pointed out the inevitable delays imposed by the removal of the research facilities to Malvern and the loss of the Halifax with important members of their research team. It was all to no avail, and to meet Churchill’s requirements, a special ‘crash’ programme was put in hand. Research teams and manufacturers alike were ordered to equip two squadrons of bombers by October 1942.

Lovell’s team designed a 360° rotating scanner which was positioned on the lower face of the fuselage behind the wing and encased in a Perspex dome 2,500 mm long and 1,200 mm wide. To accommodate it a major modification was required to the Halifax and the Lancaster. This resulted in a temporary delay in the production of the four-engine bombers.

Shortly after, the practicality of H2S was demonstrated, the Americans were informed of the development. At first, they could not reproduce the results obtained by the British, possibly because they were using their ASV-based centimetric equipment at too low an altitude. Eventually, independent development of the H2S by the Americans resulted in an improved version working in the 3 cm band with an improved aerial system called H2X which gave greatly improved resolution. It was hoped that if this system could be combined with the Norden bombsight phenomenal accuracy would be achieved., However, it was soon realised that for a bombardier to hit his proverbial barrel a miracle was required.

Air Marshall Sir Arthur Harris had already been advised that H2S would be available to his bombers after 1 January 1943. The first two squadrons to be fitted with H2S were 35 Squadron equipped with Halifaxes and 7 Squadron with Stirlings.

For Germany, the military situation had deteriorated with the loss of Stalingrad in the East and reverses in the Western Desert. The arrival of OBOE and H2S, coincided with the establishment of new directives on targets from the Combined Chiefs of Staff at Casablanca. He also had to accept that his Stirling and Halifax aircraft had shortcomings in relation to operating height and speed; both being inferior to the Lancaster. His complaints regarding the Stirling eventually lead to their production being curtailed.

In view of its complexity, the Air Ministry initially restricted production of H2S to Pathfinder Squadrons. Harris resisted this policy in favour of all aircraft being fitted with the system. The first raid using the new H2S devices, carried out on 31 January 1943, against Hamburg, was not a complete success with a proportion of the bombs being wasted. The second raid, on 2 February 1943, was on Cologne where Bomber Command was unfortunate in losing an H2S equipped Pathfinder Stirling R9264 from No 7 Squadron, shot down by a night fighter near Rotterdam.

R9264 in Allied Losses Database

Thus the Germans obtained an example of H2S which, although damaged, the firm, Telefunken, was able to repair but before the Germans were able to complete their testing, an air raid on the Telefunken works destroyed it. By chance, the same night, a Halifax from 35 Squadron which crashed near Rotterdam, yielded yet another H2S set. These examples now named ‘Rotterdam’, astonished the German engineers by demonstrating that the British had been working on centimetric devices for many months. German research work in this field had been postponed by orders from higher command. Placed now in a bomb-proof flak-tower for protection; some months passed before German engineers were fully able to establish the function of the H2S radar set, assisted by information provided by captured PoWs. Eventually, radiation emitted by the device led the Germans to develop a series of new devices, which enabled Luftwaffe fighters to home in on the bombers, though it was some months before this was known.

The installation of the new radar devices into Bomber Command aircraft did not immediately guarantee success on all targets. The period from February to July 1943 saw many German towns and cities being attacked, among them Berlin. Over this period the crews including the Pathfinders still required additional training and experience and this resulted in only partial success on a number of occasions. Weather often obscured the target, preventing adequate bombing photographs being taken.

By the end of July 1943, a decision had been made for the RAF to carry out a series of very heavy raids upon Hamburg. At this point, Harris was confident that his aircraft crews and radar devices, had reached the stage of readiness when a major raid would be most effective. For the first time, the bombers of the USAAF were invited to attack the city by day. Marking of the target would be by H2S, as Hamburg was beyond OBOE range. Hamburg was a good target for H2S as it was a coastal city. GEE Mk ll was fitted to many of the bombers giving additional frequencies for the navigator to use. ‘Window’ was introduced for the first time, it had been ready since April 1942; to confuse the otherwise excellent German Würzburg ground-based radar and the lighter air borne Lichtenstein radar. Twenty Pathfinder aircraft using H2S released target indicators and 750 bombers released their bombs over a period of 50 minutes. It was during these raids that the dreaded firestorms took place.

The destruction of Hamburg caused considerable concern in the major German cities and Speer amongst others began to have doubts as to the outcome of the war, a view still not shared by Adolf Hitler.

The output by the Germans of twin and single-engine night fighters had increased significantly. More importantly, the twin-engine fighters were beginning to be fitted with radar air-to-air sets that could home onto the radiation emitted by the H2S radar sets.

It was thought by the Germans that by erecting metal decoys on the ground in open country surrounding major targets the echoes received by the bombers could persuade their crews that they were flying over a city. It was hoped that the bombs thus released, would fall in open countryside. Similarly, these decoys could be designed to float on large lakes. But the echoes received by the H2S sets, were, in practice, inadequate to deceive the bombers and the scheme was a failure.

For a time, the Germans did devise a method of jamming which would affect the H2S, but shortly after it was introduced, a Mk ll version of H2S entered service which worked on a higher frequency than the earlier version, and so the jamming was ineffective.

For the RAF and the Dominion air forces, tragedy and many bloody battles remained ahead. The entry into service of H2S and other systems that resulted in the emission of radiation, gave the Germans the opportunity to develop radar systems that detected the presence of Bomber Command aircraft as soon as they left their bases. This resulted in the loss of many aircraft and their crews before the battle was over.

Eventually, the large numbers of long-range Allied fighters that escorted Allied bombers night and day and the shortage of fuel caused by the bombing of their refineries and fuel dumps finally ended the German defence of the Reich.

ORIGINAL VERSION

The Luftwaffe in 1941, were confident that they were ahead of the British in terms of radar developments. Yet by the end of the war this lead had been lost and the Allies were supreme.

This is an account of the development and entry into service of two of the more important radar devices. The entry into service of OBOE and H₂S, together with the navigational aid GEE, radically improved the performance of Bomber Command by increasing the tonnage of bombs that landed in the target area. Britain’s air force had began to realise that its bombers were not able to find and hit specific military targets such as airfields or armament factories. A 1940 military investigation undertaken by the British revealed that just one in five aircraft was succeeding in dropping its bombs within five miles of its target. Under such circumstances, the bombing offensive could only be effective if it was directed at targets as big as cities.

Most bombs missed by miles

At the same time however, the radiation emitted by H₂S in particular, provided the means whereby German fighters and ground radar systems could detect the presence of RAF heavy bombers. In securing greater efficiency, risks and casualties were increased. The use of H₂S quickly led to the development of many other radar devices that would be developed on both sides based upon the detection of radiation from airborne radar sets.

--xOx--

The history of the Air War against Germany is a very complex one with many strands of major developments that influenced its progress. Post-war accounts present a misleading and oversimplified picture of the events that are present in many accounts published since that time. A new development would assist Bomber Command for a time, then the Germans would design a jamming system that would largely nullify it. The design teams on both sides would have to accept that sooner or later their latest radar would fall into their opponent’s hands, leading to a counter-development. In some cases, fear of capture would delay the entering into service of a new device.

The visual bombing of Germany and occupied Europe had one major limitation winter and summer; clouds would obscure the target. Inadequate navigational systems would prevent a large proportion of bombers failing to reach or identify their targets, leading to a high proportion of their bombs being wasted by falling on open country or the wrong target.

By early 1942, the navigational device GEE, would revolutionise the efficiency of Bomber Command to enable more bombers to reach the target area. This could still present problems if cloud below 20,000 feet obscured the target. There was no alternative, when the meteorologist’s forecast cloud over the target to standing down almost the entire bomber force. This waste of opportunity was intolerable to Bomber Command.

The early failure by Bomber Command to locate their targets in sufficient numbers caused Churchill, supported by Professor Lindemann, to place a high priority upon the development of improved methods of navigation and to enable them to bomb their targets through cloud or haze.

OBOE was a scheme put forward by A.H. Reeves with his co-worker F.E. Jones at Telecommunications Research Establishment. This was part of a planned study on blind bombing. Radar has the potential to measure distances very accurately and this accuracy is not reduced with range. Like the GEE navigating system, OBOE was not strictly a member of the radar family. The system required the setting up of two interrogating stations known at the time as ‘Cat’ at Walmer near Dover and ‘Mouse’ at Trimingham near Cromer. The ‘Cat’ would hold the Mosquito over the Krupp factory - for example - on a circular path of 200 mile radius and the Mouse would signal the point over the factory when the aircrew should release the bomb to hit the target. The ‘Mouse’ signal could also be used to release the bomb.

OBOE was a British Aerial Blind Bombing Targeting System in WW2, based on Radio Transponder Technology. Using triangulation to determine relative location, the system consisted of a pair of radio transmitters on the ground, which sent signals which were received and retransmitted by a transponder in a bomber aircraft (typically a Mosquito, Halifax, Stirling or Lancaster). By comparing the time each signal took to reach the aircraft, its location could be calculated to a fair degree. In this way, a bomber could be directed blindly over a predetermined (numerous targets were pre-calculated and kept on file). Prior to a mission, a circle was drawn around the Primary Oboe Transmitter at Walmer, near Dover in East Kent, so that it passed over the target, with the Primary Transmitter in the exact Centre, and the radius of the circle noted.

The transponder-equipped bomber(s) would then attempt to fly along the circumference of this circle towards the target. By keeping careful track of the range between the airborne transponder and the transmitter, the Oboe Operator in England would use the equipment to see if the bomber strayed from the path of the circle, and give the pilot Instructions on how to regain it. While the Primary Transmitter could tell that the Aircraft was in the Circle, it was impossible to tell at what point it was. For this, the range from the Secondary Transmitter was also measured, and a circle drawn around it; the bomber would be at the point where the 2 circles Intersected. By repeatedly doing this, the progress of the bomber could be tracked, and when it reached the point where the Primary Line passed over the target, a coded signal was sent alerting the crew to drop their bombs. The path was only 35 yards (32M) wide, allowing for much greater accuracy than other systems like GEE. The curved Path of the aircraft was quite evident to German radar operators, who came to call the system 'Boomerang' after the arc segment left on their displays as the aircraft appeared and disappeared out of range. OBOE’s limitation of 1-Aircraft at a time led to several new systems. Among these were the GEE-H System, which used the existing GEE Equipment with OBOE-like transmitters. GEE-H could guide about 80 aircraft at once.

The two men aboard the bomber operating the system could release a bomb with an accuracy of 10 yards which was greater than could ever be achieved by heavy bomber aircrew but the final result would depend upon the characteristics of the individual bomb or target indicator.

At Dover, the OBOE ground-based transmitters sent out a stream of pulses to the aircraft. The aircraft carried an airborne transmitter, which was activated by the ground-based transmitter. The airborne transmitter returned a stream of pulses to the ground-based transmitter which measured the distance between the two sets.

The second station (Mouse) was located on the coast road between Cromer and Mundesley, 1 kilometre east of the village of Trimingham on the Norfolk coast. This radar station was established on the cliff top by the British Army in the latter part of 1941, originally to detect German E-Boats and low-flying aircraft and was then equipped with a CD Mk.4 radar.

'Mouse' station on Norfolk coast

As previously described, OBOE located precisely the aircraft being flown on a circular path based on Dover. When the aircraft reached the computed bomb release point a signal was sent to release the bomb.

OBOE station, painting by William Thomas Rawlinson. Courtesy IWM

OBOE system was capable of achieving extremely high accuracy, much higher than could be reached with conventional bomb sights. Professor Lindeman was fond of reminding Churchill that the more accurate the bombing, the fewer bombers would be required to win the war thus releasing a great number of workers and saving vast quantities of scarce war materials.

It may be surprising therefore, to know that a system which achieved this kind of accuracy engendered controversy almost to the end of the war. The OBOE system was unusual amongst radar devices in that it was initially not welcomed when it was first made, by official sources outside the research station; this criticism persisted even after trials had showed it had promise. A.P. Rowe stated in his book ‘One Story of Radar’ that it continued to be a controversial subject right to the end of the war.

The use of OBOE proved to be an important development for the later Pathfinding Force. Its use made a major contribution to the devastation of the Ruhr. to the end of the war. One objection was that it was thought suicidal to fly an aircraft on the required circular course. In practice this objection did not allow for the fact the Mosquito could fly at heights and speeds that proved almost impossible to intercept. As a result OBOE equipped Mosquitoes losses were very low.

At the time, Professor Lindemann himself was less than encouraging since the OBOE was an ‘optical line of sight system’ affected by the curve of the earth. This meant a maximum range of about 270 miles which meant that it could be extended little further than the German Ruhr. At the time, he was strongly advocating more raids upon distant targets such as Berlin. It was not really suitable for use in heavy bombers therefore owing to the need, to fly at great heights to achieve the range required.. The Stirling was limited to 14,000 feet and the Halifax to 18,000 ft. It was for this reason that the Mosquito was chosen to carry this device in their role as target markers.

Prof Lindemann, later Lord Cherwell, was Churchill’s science adviser, but to many of his colleagues, he was a difficult man to work with. There came a time when Tizard and his entire Air Defence committee resigned rather than work with him. Watson-Watt was an exception in that at a time when cathode ray tubes were first introduced, Lindemann encouraged Watson- Watt to incorporate them in his radar devices. Afterwards, Watson-Watt stated there would have been no effective radar without the use of these components and the support of Lindemann

It was similar in concept to the German Knickerbein system used by the Germans in 1940, which could be rendered useless by British jamming and their developments the ‘X’ and ‘Y’ which suffered a similar fate.

The Luftwaffe pre-war had anticipated the Pathfinders of the RAF by using their ‘Y’ beams in an early form of ‘Pathfinding’ attacks. The system was also used by the German Kampf-gruppe 100. So confident were the Germans in this system that they neglected to teach their observers the principles of dead reckoning navigation.

Lord Cherwell continuing pressing for a system that could become effective at greater ranges; to( including no doubt, Berlin) and it became almost an obsession with him.

Many of the doubts expressed about OBOE were due to the short wavelength that had to be used. 1.5m was first chosen and aircraft on this wavelength required an equivalent of an optical path and therefore had to fly at great heights. The advantage of using shorter wave radar was that the range obtained was always greater and it was more difficult to jam.

The OBOE differed from GEE in one important characteristic in that there was a limitation to the number of aircraft it could control. The aircraft had to be continuously monitored by its ground stations and therefore it could only initially could only control six aircraft per hour as there were only three stations the maximum was only eighteen aircraft per hour.

It had been used first by Stirlings against the Battle cruisers in Brest as a blind bombing device. The small number of aircraft it could control suggested a better use was for path finding Mosquitoes who could mark accurately the targets for the main bomber force to attack.

For Bomber Harris, 1943 would be the beginning of a new phase in Bomber Command’s history. On 26^TH November 1942 the first of 2 pairs of OBOE ground stations commenced operations that enabled 109 Squadron, the first OBOE equipped Mosquitos to commence their training. This squadron was commanded by Wg Com H.E. Bufton brother of Sidney Bufton who was to have many famous administrative battles with Bomber Harris to establish the Pathfinder Squadrons to lead Bomber Command major attacks.

The first OBOE Mosquitoes were ready for missions on 20^th December 1942. The first attack was intended to be Lutterade but it was not a success as only one aircraft was able to locate the power station.

The purpose of this raid was to calibrate their OBOE installations. Further calibration raids were carried out in December and January 1943, which resulted in the Krupp’s works in Essen being damaged. The final OBOE calibration raid was successfully carried out on the cadet school and night fighter airfield at St Trond in Belgium.

On 23/24^th December 1943, five OBOE Mosquitoes attacked targets in Essen, Hamborn, Meiderich and Rheinhausen but because of haze, the results were difficult to determine. The aircraft had found their targets using GEE. Those aircraft attacking Essen were able to drop 50% of their bombs on the main Krupps factory. A day later on 24/25^th December OBOE Mosquitoes again attacked Essen and their bombs hit the northern part of the Krupps factories. Both these operations were without loss of Mosquitoes. Thus for the first time the giant Krupps factories were damaged by bombs.

The first trial Pathfinder operation was on 31^st December using two OBOE equipped Mosquitoes for target indicating and followed up by a small force of eight Lancasters. This operation was against Düsseldorf. Out of nine bombs, dropped six hit industrial premises without serious damage being caused. On the same night two more OBOE-equipped Mosquitoes attacked a night fighter control room at Floriennes Airfield in Belgium, dropping six bombs from 28.000 ft and hitting the building.

On the 7^th January 1943, there was a meeting between senior Luftwaffe officers and the Directors of Krupps, who were greatly disturbed that Mosquito bombers were reaching Essen undetected. They were able to fly across the town of Essen and Krupps undetected, regardless of the weather and industrial haze, dropping bombs with great accuracy in hitting the works. The air raid sirens were failing to sound and warn the workers of an attack and they were getting concerned. The question they had to ask was: 'Is the enemy using some kind of infra-red homing device?' but the German specialists were able to confirm that the Mosquitos were flying at 30,000 ft on a beam originating from England.

Whenever a new radar device was considered to be promising, consideration had to be given to the effects of it falling into German hands through crashed aircraft and the possibility of German jamming it before its full value could be realised.

On 7^th July 1944 one of 105 Squadron’s Mosquitos fitted with OBOE crashed near Caen and Wing Commander Edward Barton was dispatched in another Mosquito to investigate. The aircraft was landed close to the crashed aircraft and the precious OBOE device was removed before the Germans arrived at the site. Unfortunately, the navigator had bailed out and the pilot of the Mosquito was killed in the crash before the secret equipment could be destroyed.

March 1943 signalled the opening of the RAF Campaign to wipe out the munitions industries of the Ruhr.

By Spring 1943, 50,000 labourers would have to be drafted in from constructing the Atlantic Wall to repair bomb damage in the Ruhr.

The British planners in July 1942 expected that the system would be free of jamming for little more than a month but to their surprise, eighteen months would elapse before it was seriously impaired. The Germans had falsely identified OBOE used to control the British version of E-boats. By then, the British had developed a Mk2 version of OBOE jointly with American teams and this extended its life beyond ‘D’ Day. However, the OBOE equipped Mosquito loss-rate proved to be very small at less than one-quarter per cent. The Germans remained mystified by the high-level attacks of the OBOE-equipped Mosquitos and were unable to shoot one down to examine the radar equipment. They were able to deduce the wavelength of the OBOE system and were considering methods of jamming it. Their radar experts had, however, come to the conclusion that the RAF had alternative wavelengths available which would have precluded jamming.

Later the Luftwaffe were able to plot the Mosquitos using OBOE, with a radar system given the name of ‘Flammen’. It was some months before British intelligence could link the Flammen plots with the use of OBOE. Later the Germans did find a method of jamming which entailed a sweep of the frequencies used for OBOE. This led to a complete failure of a raid on Rheinhausen which in turn lead to the RAF bringing into service OBOE Mk ll and a Mk lll which worked on the centimetre bands. OBOE Mk 1 continued to be used as camouflage for the other marks of OBOE. As the Allied armies advanced across Europe mobile OBOE transmitters followed in their wake.

As the war ended the Germans did have some success in jamming all marks of OBOE but by then its job had been completed.

Intricately interwoven with both OBOE and H₂S was the story of WINDOW and its German equivalent DUPPEL. This will be the subject of a later article.

H₂S was introduced at approximately the same time as OBOE. It was originally known as ‘BN’ for blind navigation. Since these latter initials indicated the potential use for this radar system a name change was inevitable. The name was said to have been chosen by Lord Cherwell to suggest ‘Home Sweet Home’ as homing onto a target; not H₂S ( Hydrogen Sulphide) which was commonly believed to be its origin.

AircrewRemembered Senior Editor (a former RAF radar specialist) writes: 'Having done all my advanced radar training on H2S I can say with some degree of certainty that this is yet another variation on the theme. Sir Robert Watson-Watt was instrumental in the development of ASV and H2S and the explanation of the origins of the term H2S is recorded in his book 'Three Steps to Victory' which is a personal account.Its origins are somewhat convoluted but had to do with what is called 'Column 7' and 'Column 9' items for a modification to an aircraft.

Column 7 items are aircraft wiring and equipment mounting trays installed during manufacture and Column 9 are the black boxes. Modified aircraft initially flew without the black boxes and to provide for any enemy curiosity about the content of a crashed aircraft, aircrew were told these particular items were for a new homing system to lead them 'Home Sweet Home' i.e. H2S. The book cites that it was one of Watson-Watt's lab technicians and not Lord Cherwell that coined the phrase off the cuff when asked what the equipment was.'

A.P. Rowe reports that late in October 1941 at a ‘Sunday Soviet’ the subject chosen for discussion was how Bomber Command could attack cloud-obscured targets. At this meeting, Lord Cherwell was insisting upon Bomber Command having the means to have a greater range of operations from its British bases. He considered GEE and Oboe to be of limited value because of it being a line of sight device and its requirement for ground transmissions from bases in Britain. Some discussion took place as to whether following German electric power lines was a practical proposition. This particular meeting ended without a satisfactory proposal having been made but the ground had been prepared for the birth of new ideas.

However later that week of October 1941, there was a inter-team meeting between P.I. Dee working on centimetric wavelength and the team headed by H.R. Skinner working on the basic problems of the same wavelength. They recalled that while working at Leeson House above Swanage, echoes had been received from the town. It was already known that with a centimetre ASV set, a map of the sea could be displayed in an aircraft, which would also show any vessel sailing across it. It was then thought that radar echoes could be obtained from a town, which would show buildings lakes, and countryside.

There seems to be some confusion as to which scientist first suggested the ideas behind H₂S. Watson-Watt in his book suggested Bowen who had written to A.P. Rowe in 1937-1940 suggesting that some discrimination of echoes might be possible, although it was thought that Bowen at that time had suggested the use of a longer wavelength

On New Year's Day 1942 A.P. Rowe named Bernard Lovell to head the H2S development team. Lovell was reluctant since he was fully involved in an Air Interception (A.I) set for fighter aircraft and for reasons unknown he even found it objectionable. After the war, he became a leading radio astronomer and his name will forever be associated with Jodrell Bank Radio Astronomy Station.

H2S installed in RAF Bomber

By March 1941 a prototype of an air-air interception was available for testing. The suggestion was made that by tilting the AI set so that the rotating centimetric beam could scan the area beneath the bomber and produce a picture of the area. An A.I. set was quickly modified at Christchurch Airfield which would scan the ground below and in front of the aircraft. The following day a flight was made over Southampton and Salisbury, which showed promise. This was the day that H₂S was born.

Cherwell at this early stage was insisting on having the early production aircraft fitted with the new device; a mere seven months after its birth. This was an impossible target when it is appreciated that out of this very short period had to be deducted development time and setting up the production of sets and aircraft to carry them. To add to the pressure being built up over the urgent requirement to develop H₂S, it became apparent in 1942 that a move away from the vulnerable South Coast of Britain was becoming essential. All this was to provide a further impediment to H₂S development.

After considering several sites the decision was made to take over and modify the buildings of Malvern College, the boy’s public school. The location of the school was perfect for the development of H₂S in that the school was on a hill overlooking the town while at the same time, the town was of sufficient size to absorb the 1000 staff of the new research station, now to be known as The Telecommunications Research Establishment (TRE).. Later they would move again to a dedicated site elsewhere in the town which remains its base to this day.

The version of H₂S that entered service depended upon a new device or component called the high-powered cavity magnetron. This was designed by Randall and Boot at Birmingham University to become the heart of the device. It started development with a power of 500 watts and was later increased to 10,000 watts.

It was highly secret and considerable concern was expressed that it should not fall into the hands of the Germans. It was central to the centimetric revolution and was one of those treasured items taken to the USA by the Tizard Delegation in 1940. The complex prototype devices were developed in the workshops of Birmingham University, partly to minimise delay and partly for reasons of security.

Cherwell was greatly concerned that the magnetron could fall into the hands of the Germans prematurely. It proved to be almost impossible to destroy by using detonators in aircraft in danger of crashing. was. He, therefore, advocated using a klystron in place of the magnetron. Details of this device had already been published in the scientific press. He considered that the version based upon the magnetron would take longer to develop. To get around the problems of the lack of power of the klystron he suggested that the bombers could use their GEE to approach the target and then use their H₂S to bomb the target. He would accept a high system failure rate and depend upon at least some H2S-equipped aircraft blind bombing their target.

The H₂S team strongly disagreed with Cherwell as the klystron lacked the power required and considered that such poorly developed equipment would not be acceptable. The controversy with Cherwell at one stage resulted in a decision to develop H₂S in two versions using a klystron as well as a cavity magnetron. Eventually, Cherwell accepted the situation and work on the klystron ceased on July 15^th. Sets fitted with magnetrons were accepted as standard.

The arrival of the new device was widely welcomed and quickly received the enthusiastic backing of the Prime Minister. He convened a now famous meeting on July 3^rd 1942 which was attended by Rowe, Lovell, Dee, Watson- Watt as well as Churchill’s military chiefs and American representatives

These Americans were astonished when first meeting Churchill to find he had direct contact with his scientific advisors. At the time, this did not happen with their President and it certainly did not happen with Hitler and Göering. The German leaders remained suspicious throughout the war of both intellectuals and scientists. This outlook was to have a crippling effect upon the German conduct of the war.

A tragic accident occurred on Sunday 7^th June 1942 when the Handley Page Mk ll aircraft used for H₂S trials crashed in the Wye valley killing eleven engineers and military personnel. EMI had assigned to the development of H₂S , their best engineer Alan Blumlein and he was included amongst those killed. Blumlein was undoubtedly an electronics engineer of exceptional brilliance who had before the war invented stereo sound and developed key technologies which made television possible.

Alan Blumlein is remembered and honoured with a Blue Plaque at his former home in Ealing, London.

The loss of Blumlein was considered by Bernard Lovell and others to be a national disaster. Several other members of the H₂S development team were lost in the accident. The early Halifax aircraft were inherently unstable under certain conditions and this had already resulted in a number of accidents, often fatal. The investigation which followed revealed that an improperly tightened tappet nut on the starboard outer engine came loose as the result of a special inspection just before the flight and caused the engine to catch fire, which destroyed the wing and led to the crash.

At a key meeting, an astonished audience was faced with the demand that 200 H₂S sets must be made available by October 15^th 1942. It was made clear that no reason would be acceptable as to why this should not be achieved. The radar representatives declared it to be impossible as the production models would never be ready in time. They pointed out the inevitable delays imposed by the removal of the research facilities to Malvern and the loss of the Halifax with important members of their research team. It was all to no avail, and to meet Churchill’s requirements, a special ‘crash’ programme was put in hand. Research teams and manufacturers alike were ordered to equip two squadrons of bombers by October 1942.

Lovell’s team designed a 360° rotating scanner which was positioned on the lower face of the fuselage behind the wing and encased in a perspex dome 2,500 mm long and 1,200 mm wide. To accommodate it a major modification was required to the Halifax and the Lancaster. This resulted in a temporary delay in the production of the four-engine bombers.

Shortly after the practicality of H₂S was demonstrated, the Americans were informed of the development. At first, they could not reproduce the results obtained by the British, possibly because they were using their ASV-based centimetric equipment at too low an altitude. Eventually, independent development of the H₂S by the Americans resulted in an improved version working in the 3 cm band with an improved aerial system called H₂X which gave greatly improved resolution. It was hoped that if this system could be combined with the Norden bombsight phenomenal accuracy would be achieved., However, it was soon realised that for a bombardier to hit his proverbial barrel a miracle was required.

Air Marshall Sir Arthur Harris had already been advised that H₂S would be available to his bombers after 1^st January 1943. The first two squadrons to be fitted with H₂S were No 35 Squadron equipped with Halifaxes and No 7 Squadron with Stirlings. For Germany, the military situation had deteriorated with the loss of Stalingrad in the East and reverses in the Western Desert. The arrival of OBOE and H₂S, coincided with the establishment of new directives on targets from the Combined Chiefs of Staff at Casablanca. He also had to accept that his Stirling and Halifax aircraft had shortcomings in relation to operating height and speed; both being inferior to the Lancaster. His complaints regarding the Stirling eventually lead to their production being curtailed.

The Air Ministry initially restricted production of H₂S in view of its complexity, to Pathfinder Squadrons. Harris resisted this policy in favour of all aircraft being fitted with the system. The first raid was carried out on the 31^st January 1943 with the new H₂S devices against Hamburg which was not a complete success with a proportion of the bombs being wasted. The second raid on 2^nd February 1943 was on Cologne where Bomber Command was unfortunate in losing an H₂S equipped Pathfinder Stirling R9264 from No 7 Squadron. A night fighter near Rotterdam shot this aircraft down.

R9264 in Allied Losses Database

Thus the Germans obtained an example of a H₂S which although damaged, the firm Telefunken was able to repair. Before the Germans were able to complete their testing an air raid on the Telefunken works destroyed it. By chance, the same night a crashed Halifax from 35 Squadron yielded yet another H2S set. These examples now named ‘Rotterdam’ astonished the German engineers by demonstrating that the British had been working on centimetric devices for many months. German research work in this field had been postponed by orders from higher command. Placed now in a bomb-proof flak-tower for protection; some months passed before German engineers were fully able to establish the function of the H₂S radar set, assisted by information provided by captured PoWs. Eventually, radiation emitted by the device led the Germans to develop a series of new devices, which enabled Luftwaffe fighters to home in on the bombers, though it was some months before this was known.

The installation of the new radar devices into Bomber Command aircraft did not immediately guarantee success on all targets. The period February to July 1943 saw many German towns and cities being attacked, among them Berlin. Over this period the crews including the Pathfinders still required additional training and experience and this resulted in only partial success on a number of occasions. Weather often obscured the target, preventing adequate bombing photographs being taken.

By the end of July, a decision had been made for the RAF to carry out a series of very heavy raids upon Hamburg. At this point, Harris was confident that his aircraft crews and radar devices had reached the stage of readiness when a major raid would be most effective. For the first time, the day bombers of the USAAF would be invited to join the attack by day. Marking the target would be by H₂S, as Hamburg was beyond OBOE range. Hamburg was a good target for H₂S as it was a coastal city. GEE Mk ll was fitted to many of the bombers giving additional frequencies for the navigator to use. ‘Window’ was introduced for the first time, it had been ready since April 1942; to confuse the otherwise excellent German Würzburg ground-based radar and the lighter air borne Lichtenstein radar. Twenty Pathfinder aircraft using H2S released target indicators and 750 bombers released their bombs over a period of 50 minutes. It was during these raids that the dreaded firestorms took place.

The destruction of Hamburg caused considerable concern in the major German cities and Speer amongst others began to have doubts as to the outcome of the war, a view still not shared by Adolf Hitler.

The output by the Germans of twin and single-engine night fighters was increased significantly. More importantly, the twin-engine fighters were beginning to be fitted with radar air-to-air sets that could home onto the radiation emitted by the H₂S radar sets.

It was thought by the Germans that by erecting metal decoys on the ground in open country surrounding major targets the echoes received by the bombers could persuade their crews that they were flying over a city. It was hoped that the bombs thus released, would fall in open countryside. Similarly, these decoys could be designed to float on large lakes. But the echoes received by the H₂S sets were in practice inadequate to deceive the bombers and the scheme was a failure.

For a time the Germans did devise a method of jamming which would affect the H₂S but shortly after it was introduced, a Mk ll version of H₂S entered service which worked on a higher frequency than the earlier version and so the jamming was ineffective.

For the RAF and the Dominion air forces, tragedy and many bloody battles remained ahead. The entry into service of H₂S and other systems that resulted in the emission of radiation gave the Germans the opportunity to develop radar systems that detected the presence of Bomber Command aircraft as soon as they left their bases. This resulted in the loss of many aircraft and their crews before the battle was over.

Eventually, the large numbers of long-range Allied fighters that escorted Allied bombers night and day and the shortage of fuel caused by the bombing of their refineries and fuel dumps finally ended the German defence of the Reich.

This article was stimulated by an article written by Henry Black around 2008 and published at qsl.net for which permission has been sought. Reference was also made to the Pathfinder Craig blog and to a blog with information on Alan Blumlein. Additions and enhancements have been made by AircrewRemembered Editors

• Memories from RAF Bomber Command including OBOE, H2S
• The Defection of Herbert Schmid: capture of German airborne radar
• Key capture of German airborne radar equipped Ju-88
• Animation showing every Allied bomb dropped in WW2