introduction of OBOE and H2S into RAF Bomber Command
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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
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.
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.
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
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.
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.
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.
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.
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.
purpose of this raid was to calibrate their OBOE
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.
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.
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.
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.
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.
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.
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
1943 signalled the opening of the RAF Campaign to wipe out the munitions
industries of the Ruhr.
Spring 1943, 50,000 labourers would have to be drafted in from constructing the
Atlantic Wall to repair bomb damage in the Ruhr.
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.
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.
war ended the Germans did have some success in jamming all marks of OBOE
but by then its job had been completed.
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.
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.
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..
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.
seems to be some confusion as to which scientist first suggested the ideas
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
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.
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
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
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
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.
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.
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.
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.
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..
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.
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.
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
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
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
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
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
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
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
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
For a time the Germans did devise a method of jamming which would affect
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