The introduction of OBOE and H2S into RAF Bomber Command
by Henry R. Black

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.

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.

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 over simplified 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 Prof Lindemann, n 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.  .The OBOE system required two transmitters initially based at Dover and Cromer.

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 at Cromer 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 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, Prof 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 he 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 26TH 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 20th 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 23/24th December1943, 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 24/25th 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 Path Finder operation was on 31st December using two OBOE equipped Mosquitoes for target indicating and followed up by a small force of eight Lancaster's. 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 7th 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 tand warn the workers of an attack and they re 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 7th July 1944 one of 105 Squadron’s Mosquitos fitted with OBOE crashed near Caen and Wg Co 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.

Like the GEE navigating system, OBOE was not strictly a member of the radar family    The position of the aircraft was known from signals transmitted from it. OBOE was a device that under certain conditions enabled a bomb to be aimed with a high degree of accuracy under all conditions of visibility.  The system required the setting up of two interrogating stations known at the time as ‘Cat’ at Dover and ‘Mouse’ at Cromer.   The ‘Cat’ would hold the Mosquito over the Krupp factory 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.

The two men 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.

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 a 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. 

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 who were 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 A.S.V. 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 Years 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.

By March 1941a prototype of an air-air interception was available for testing.  The suggestion was made that by tilting the AI set so that the a 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 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 work shops of Birmingham University, partly to minimise delay and partly for reasons of security.

Cherwell was greatly concerned that he 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 klystrom ceased on July 15th.   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 3rd 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.  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.

At this meeting he insisted to an astonished audience that he must have 200 H2S sets by October 15th 1942.   He made it clear he would brook no reason why it 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 1st January 1943.   The first two squadrons to be fitted with H2S were No 35 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 31st 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 2nd 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.

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 P.O.Ws.  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 Path Finder 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 German defence of the Reich.

The author would welcome comments upon this account through his e-mail address

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