Radar and the Air War – Part 8 – by John Heys G3BDQ

A Personal View, by John Heys G3BDQ.

Part 8 – The Oboe System (das Bumerangverfahren – Boomerang System)

Earlier in this series I mentioned the unhappy fact that when the War started in 1939 Britain was unable to accurately find bomber targets in enemy territory, with bomb-loads being dropped some miles away from the intended target. The Germans however had a clever system that was based upon the well known Lorenz ‘Blind Landing’ equipment. They used receivers in their bombers that looked like the Lorenz equipment and worked on a frequency close to 30MHz. It was this equipment code-named Knickebein (literally ‘advocat’ or ‘crooked leg’) which let their bomber stream destroy the heart of Coventry on the night of 14/15th November 1940. Eventually subtle British jamming techniques were developed which upset the German system and led to major navigational errors
that simulated ‘beam bending.’ The RAF greatly improved their air navigational skills by the use of Gee and H2S radars but a really accurate ‘fix’ on a particular target was still rather elusive. Soon however this was all to change when Oboe was developed.

Oboe used the pulses from two ground transmitting stations to determine the distances an aircraft was from those stations and to signal the exact time when a bomb-load or batch of flares was to be released and land on the ground with astounding accuracy. The system could only work with an individual aircraft at any one time so Oboe was used for special target bombing or more usually for dropping ground or air-suspended marker flares.

Oboe was developed by A H (Alec) Reeves and F E (Frank) Jones together with a team of radio engineers that was set up at TRE in May 1941. Reeves was the pulse expert of Standard Telephones and Cables Ltd, and Frank Jones was a permanent employee researcher at TRE.

How Oboe Worked

The basic idea was to fly an aircraft at a constant range from one ground based transmitter by sending pulses to the aircraft which picked them up and sent back a return pulse. This enabled the ground station to discover the range of the aircraft by a time measurement of the return pulses. When flying at a constant range from the ground station the aircraft would fly along a circular path that was centred upon the ground station. This ‘circle’ was arranged to be near the target but a little short of it, for a released bomb-load lands ahead of the point from which it was released. The second transmitter station was used to determine when the aircraft should actually release its bomb-load. There was one obvious weakness to this system; the aircraft’s signal could be homed in on by an enemy night fighter. This problem was largely solved by the use of fast high-flying (30,000 ft) Mosquito aircraft equipped with the Oboe.

Oboe was the most precise bombing system used during WW2 and it was so accurate that the geodetic alignment of our Ordnance Survey with European maps had to be determined before a decision on the timing measurements could be made.

CAT and MOUSE

One of the ground stations was code-named CAT and it controlled the circular track of the aircraft by interrogating its on board transponder (a repeater.) The return pulse was displayed on a CRT which had a delayed magnified timebase (strobing) on which one mile filled the whole 12 inch screen. The precise range then could be set up by an adjustable marker pulse. The CAT station kept the return pulses exactly in line with the marker strobe by automatically transmitting dots or dashes; dots if the aircraft range was less than the target range and dashes if it was greater. The dots and dashes merged into a continuous signal when the aircraft’s range and position along the circular track coincided.

The dot – dash signal was transmitted by pulse width modulation of the primary CAT pulses. This system could show a deviation from the required range from the circular track to an accuracy of plus or minus 17 yards and the aircraft could fly along the circular track that was just 35 yards wide when above the target.

The MOUSE ground station signalled the aircraft at precise intervals until the load release point was reached, when a release signal was sent. This was five dots followed by a dash.  The MOUSE station could follow the aircraft’s progress and allow for airspeed, wind-speed and the bomb ballistics. The MOUSE computer took all these variables into account to ensure the correct release point.

More Oboe Details

The first successful raid using Oboe was on the 20th December 1942 when Sqdn. Leader H E Bufton in a Mosquito of 109 Squadron dropped bombs on top of the power station at Lutterade in Holland. This was followed by a raid on Florennes in Belgium and on March 5th 1943 Essen was successfully bombed.

There were several Oboe stations around the south and east coasts between Clenadon in Durham and Sennen in Cornwall, any of which could perform as either CAT or MOUSE. The first operational stations were at Trimingham near Cromer and Hawkshill near Walmer, Kent. The early Oboe stations operated on a frequency of 200 MHz with pulse space modulation, but later they worked on the S-Band (10cm wavelength) and used pulse width modulation. Two typical ground stations were Winterton in Norfolk and Kingsdown near Dover.

Some Facts & Figures

The exactness of timing was remarkable a MOUSE station could monitor and measure the aircraft’s velocity within half a mile per hour. The very accurate determination of the speed of the radio waves meant that a mean figure of 186,234 miles per second was used in calculations instead of the normal adoptive speed of 186,240 miles per second.

Oboe could only be used for a very limited number of aircraft which were used to normally release coloured marker flares used by the PFF (Path Finder Force.) Food and weapon canisters could also be dropped to Resistance fighter groups and these items could land within 30 yards of the marked aiming point. Oboe’s accuracy was also used to destroy most of the German V1 and V2 weapon sites. The mathematics involved in setting up the CAT and MOUSE stations for an operation included the working out of the correct (to a few yards) path measurements which had to take into consideration the curvature of the Earth. Flat map measurements could well be different from the actual flying distance.

My old friend, the late Colin May G3KMP (former Hastings Club Member) was involved in this exercise when the Training Programme for Navigators was scrapped towards the end of the War. He was, being a good mathematician switched over to Oboe calculations.

Following D-Day, Oboe stations were set up on the Continent and their locations advanced in step with the advancing Allied Armies. The range of Oboe was 300 miles from the CAT and MOUSE stations so initially the Ruhr area was just within bombing range. Even then the target accuracy was within 100 yards. I flew as a passenger in one of our Halifaxes to look down at what we had done to the Ruhr and especially at the once mighty Krupps steel works at Essen.  There had been direct hits on most of the factories and power plants.  When the Allies advanced well into West Germany Oboe was set up to finalise the bombing of Berlin.

By May 1944 the enemy knew how Oboe operated and they constructed mobile jammers and these were designated equipments ABG23 and ABG24 (ABG being the initials of Anti-Bumerang Geraet, or what we would describe as Anti-Boomerang Equipment.) an airborne anti-Oboe tracking radio was developed which was code named Naxos. On May 24th 1944 General-Leutnant von Axthelm issued a paper titled ‘Combating Boomerang Aircraft.’ It stated that to date their ‘Mosquito Hunters’ had little success and that the best defence was Flak. Actually before Axthelms paper was prepared only eight Mosquito aircraft had been shot down; four by Bf110’s, two to He219’s and one to a Fw109.

Ground marking with dropped flares was code named Parramatta by the RAF and sky marking using flares on parachutes was called Wanganui. If the flares were dropped with the help of Oboe the word ‘Musical’ was used in front of the odd (Maori ?) words just mentioned.  I bet those words could have confused the German code-breakers.

Next time, meet ‘Rebecca.’

John Heys (G3BDQ) – Vital Spark February 2008.

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