TRE Journal home page

T. R. E. JOURNAL
ISSUED BY THE
TELECOMMUNICATIONS RESEARCH ESTABLISHMENT
MALVERN
October 1945

EDITORIAL
(and index)

In this issue there are a number of articles on the blind bombing device called Oboe. This is one of the most important radar devices employed in the war and one which makes considerable appeal to the imagination since it enabled ground crews to control, by means of aural signals to the pilot, the flight of aircraft 250 miles away, and to instruct them when to release their bombs with such accuracy that a radial error of not more than 300 yards was obtainable under average conditions.

While Oboe ground stations were used to control Mosquito bombers, by far the most important contribution of this device consisted in the control of pathfinder Mosquitos dropping target indicators for the main follow-up force of heavy bombers. It was also used to control a single aircraft leading formations of aircraft of the same type which released their bombs at the same time as the leading aircraft. The Americans used Marauders and the RAF used Mosquitos for formation flying of this kind. The bomb density obtained from Mosquito formations led in this way was 2½ times greater than that obtained by any other method.

Very extensive use of this device was made by Bomber Command: in December 1944 Oboe was used on 55% of Bomber Command's raids and the total number of sorties of Oboe directed aircraft (i.e. carrying Oboe airborne equipment) in 1944 was 6,500.

The major use of Oboe was for strategical bombing and it was this device which was mainly responsible for the devastation of the Ruhr. It was also used for attacking V-bomb launching sites and aerodromes, and after D-day, was used for tactical bombing of marshalling yards, road and rail junctions, bridges and dumps. At the end of the war it was used for dropping food to-the Dutch.

On D-day Oboe was employed to drop approximately 500 tons on each of 10 targets, a total of 5,200 tons. These targets consisted of German Coastal batteries and it was reported that this treatment was effective in reducing their opposition to the landings to a very low level. During these operations there was 10/10ths cloud over at least 5 of the targets so that without some accurate blind bombing aid the neutralization of these targets would have been impossible.

Recent figures for accurate bombing from 24,000 ft. with a 50 degree angle of cut are:‑

Mark I and IIF - Low bursting target indicators - 212 yards
Mark I and IIF - High bursting target indicators - 277 yards
Mark IIM and III - High bursting target indicators - 337 yards.

Under adverse conditions the accuracy is naturally worse, for instance, bombing from 32,000 ft. with a 26 degree angle cut gave a radial error of 500 yards. Latterly a better performance was obtained under similar conditions: in March and April 1945, bombing from 30,000 ft. with an angle of cut of 30 degrees, a radial error of 320 yards was obtained.

The above figures relate directly to the accuracy of dropping of the Oboe controlled aircraft, the final result is, of course, determined by the error of the follow-up force which was found to increase considerably in the case of a heavily defended target.

Typical figures for the follow-up force are:‑

Lightly defended target - 460 yards radial error
Heavily defended target - 1478 yards radial error

The introduction of controlled Oboe marking, in which a master bomber directs the follow-up force by telling them which marker or group of markers to bomb, appears to reduce the error considerably in the case of heavily defended targets, while making little difference in the case of lightly defended targets. Typical figures for controller Oboe marking are:‑

Lighty defended target - 479 yards radial error
Heavily defended target - 721 yards radial error

Broadly two types of Oboe were developed, one working on the 1½ meter band and one on the S-band. Development of the S-band equipment was started in anticipation of jamming being introduced by the Germans on the 1½ meter band. This anticipation was fully realised; but by means of anti-jamming measures, the Mark I equipment on the longer wavelength was kept operational for almost exactly two years, and except for the last three months of that period, its average operational serviceability was high.

The Mark II system which operated on the S-band was originally limited to frou ground stations, providing for simultaneous operation of two aircraft only; one on each of two radio frequency channels. These were referred to as Mark IIF stations and used P.R.F. Phase Modulation for sending signals to the aircraft. Later a total of seven S-band stations were built using pulse-width modulation, and these, by using different P.R.Fs, provided a number of sub-channels on each radio frequency so that each ground station could control simultaneously as many as eight aircraft. The number of aircraft simultaneously controlled in practice was however never greater than four. These stations were referred to as Mark III ground stations.

In addition a total of 24 mobile trailers working on the S-band were built, 20 of them using width modulation and referred to as Mark IIM trailers and four of them (emergency) working on the same band but using space (P.R.F. Phase) modulation. The latter were referred to as Mark II SM trailers. After D-day these were all deployed on the continent to extend the range of Oboe coverage. Before the German war was over there were four convoys always effectively operational with two others being used as "leap-frog" convoys. Each of these convoys contained four trailers so that there were always available four Oboe control channels each channel being represented by one trailer at each of four ground station locations. The term "control channel" is here intended to indicate any channel over which an aircraft can be controlled, whether selectivity is obtained by choice of radio frequency or by choice of P.R.F.

The rapid way in which these convoys were deployed and their close following on the heels of the retreating Germans ­ they were usually not more than about 50 miles behind the front line - is a great credit to those who were responsible. By this means it was finally possible to control aircraft leading a raid on Berlin itself. An attempt was made to control aircraft over Berchtesgarden, in spite of the fact that mountains were known to be in the line of sight, but unfortunately this gamble failed.

Two squadrons of Mosquito aircraft totalling about 60 aircraft constituted the Oboe path-finder force in England and operated from airfields in that country. The Americans had first 30 and later 40 Marauders equipped with Oboe airborne equipment and these aircraft flew under control from the ground stations described above.

It will be appreciated that the planning of operations, the choice of targets and the calculation of target ranges and ground station adjustments, in addition to the briefing of air and ground crews, required very careful organisation and necessitated reliable communications. On the continent communications were maintained by wireless telegraph which was not always reliable and which was capable of being tapped by the enemy. It seems essential that in future devices of this kind some form of secret communication should be provided between ground stations as an integral part of the equipment. Alternatively, attempts should be made to bring "H" systems of bombing up to a standard of accuracy comparable with Oboe.
E.K.S.

Please note: the original of this document is typewritten. I OCRd the whole of the document to produce a text file and then used that to create these web pages. You may find errors in the OCRing and also in the original document for which my apologies - I corrected quite a lot of typos in the original. There are lots of Greek symbols in some places, hopefully I have found all these, along with the integrals and square roots. In some cases the maths are too complicated to show in html and so I have used the original scanned pages to display these.
Colin Hinson.