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Oboe is a system of ground controlled blind bombing utilising the ability to measure ranges by radar methods to a high degree of accuracy.
The earliest record of adapting range measurement for this purpose dates from November 1940 when it was proposed to use normal CHL equipment with the addition of a fast time base and a crystal controlled calibrator. A number of flights over prominent features of landscape were made during which an observer signalled to the CHL station when he was vertically above the object, assessing this with the aid of a vertical bomb sight. From these flights it was estimated that an accuracy of 80 yards could be achieved.
A further proposal dated January 1941 to combine such accurate range measurement with azimuthal control of an aircraft was the first step towards blind bombing. It was claimed that a CHL radar station by virtue of the horizontal "split" could detect very small deviations in azimuth from a fixed radial track and it was proposed to modulate a remote transmitter in accordance with the off bearing indication obtained by the "split" method. The type of modulation to be used was to be the same as that used on Blind Approach systems in which the distance off track was to be indicated by the depth of modulation of dots or dashes according to which side of the mean track the aircraft had erred.
A unit capable of controlling the modulation depth was already available for use with the Link Trainer Dummy Lorentz Landing system, and it was proposed that an experienced controller should manipulate the control in accordance with the CHL "split" presentation. It was found that a regular deviation of 6' could be detected which would correspond to a deviation of 90 - 100 yds at 30 miles range. Normal range methods as indicated above would be applied for determining the release point along the track with an accuracy of 80 yards up to 70 miles.
This system of azimuthal control was called the "Howler Chaser" and the note emitted by the modulated CW transmitter was considered by one member of the experimental group to sound like an Oboe. This name has stuck and is now attributed to a completely different system of control. Only one flight controlled by the Howler Chaser is recorded, and the aircraft reached a point one mile from the planned point.
A requirement rose in February 1941 for the bombing of German beam transmitters on the Cherbourg peninsular. It was proposed that a certain Squadron, highly skilled in beam flying, should fly aircraft along the German beam in the direction of the transmitter and that range measurement should be carried out from a CHL station as described above, a release signal being conveyed to the aircraft on an HF channel. The range of the target was 60 miles and in order to be certain of obtaining reliable pulse returns from an aircraft at that range it was decided that the aircraft should carry an IFF set from which the sweeping mechanism was disconnected and which would be tuned to the CHL frequency. In order to determine the delay of the pulse through the IFF set, range measurements on a fixed IFF set were carried out over a distance of 30 miles. The technique was, however, fraught with difficulties, and troubles were experienced with both the IFF set and the HF link. Two sorties on which both radio links functioned satisfactorily were finally carried out but there was positive evidence that the target had not been rendered unserviceable.
In May 1941, a new Group was formed at TRE to pursue this application and the name "Oboe" persisted despite the fact that the technique underwent considerable changes. The first change made was that, because the two R.F. links rendered the system twice as prone to jamming as when only one R.F. link is used, a system for communication over a pulsed transmission was developed.
The inaccuracy of azimuth measurement in comparison with range measurement was recognised and proposals made to fix the aircraft by means of range measurement from two ground stations. The limitations of range on UHF and the nature of the geography of England with respect to Germany impose the limitation that the angle subtended by two ground stations located on the East Coast at a target in W. Germany is less than a right angle and that inaccuracies in fixing are therefore greater along the direction parallel to the line joining the ground stations than along a line at right angles to it. These inaccuracies could, it was thought, be taken up to some extent by dropping a stick of bombs from an aircraft flying in a direction approximately parallel to the base line. A simple method of approximating to this direction of flight was, of course, to fly the aircraft at constant range from one station and to use the other station to determine the release point. It is along these lines that the system has been developed.
One of the strongest criticisms raised against the system was that in order for the mean direction of attack to be accurately assessed and maintained by the pilot, and the ground speed to be accurately measured at the ground station, it was necessary to fly the aircraft straight and level for a period of a few minutes. It was considered that under these conditions and at the height at which operational aircraft could then fly, the losses would prove uneconomic.
Another serious criticism levelled against the Oboe system was on the ground of the low traffic handling capacity The straight and level run in would take some few minutes and one pair of ground stations could control only one aircraft at a time (i.e. every 10 minutes). Such a slow stream of aircraft, even if trebled by means of erecting three pairs of ground stations on different spot frequencies, would not unload any great weight of bombs in a single night, so it was planned that by use of up to twenty pulse recurrence frequencies on the same radio frequency, with a pulse recurrence frequency selector installed in each aircraft, the traffic rate could be increased very appreciably.
However three very important developments occurred in late 1941 and early 1942 which completely modified the requirements demanded of the Oboe system.
The first important fact was that during a series of operations carried out in December 1941 in which Oboe range measuring methods were used in conjunction with a very fine "split" Lorenz type of beam, over 30 Stirling aircraft flew singly at between 15,000' and 17,000' for a distance of 20 miles over the most heavily defended portion of the Continent at constant height and on a perfectly straight path without loss. This fact to some extent confounded the critics and more sympathy was enlisted for the system.
The second fact was that about the time Oboe became available for operational use an effective Target Indicator bomb had been developed, suitable for target marking. This meant that Oboe might be used with a comparatively small force of aircraft to drop the T.I's which would be bombed by a large concentration of heavy bombers. As the T.I's are of small weight, a light and therefore fast aircraft could be used. This coupled with the third fact - the availability of the Mosquito - proved to be of enormous advantage to the system and Oboe was installed in Mosquito aircraft which, because of their speed and operational height, (300 m.p.h. at 30,000') reduced the importance of the criticism of the straight and level run in. In fact the losses due to enemy action have proved to be less than 0.25%.
When the new Oboe Group was formed it was in two Sections. One Section was to develop the scheme on S-band, likely to be less subject to jamming; but since at that time centimetre technique was still in a comparatively early stage of development, it was realised that this would be a long term project. Consequently it was intended that the other section should develop a system on the 200 Mc/s band, adapting units and technique already available. Thus 200 Mc/s Oboe was to be developed quickly and trials held in order that the difficulties and limitations might be discovered sufficiently early to enable the S-band Oboe to benefit by this experience. It was never intended that the 200 Mc/s system should ever be used operationally since it was fully realised that such a frequency could be very easily jammed and that the life of such a system would probably be very short. The report on the trials of this system when published in April 1942 raised such interest and was considered to meet such an urgent requirement that it was decided to proceed with the installation of this 200 Mc/s equipment known as Oboe Mark I. Two pairs of ground stations were sited on the East Coast of England and one Squadron of Mosquito aircraft in P.F.F. Group was fitted. Operations commenced on the 20th December 1942, and despite enemy attempts later on at interference, the Mark I system continued operationally for two years, although in some areas more successfully than in others.
The development of the S-band system was considerably retarded by the necessity of carrying out the Mark I installation and maintaining these installations; all the ground station equipment and the early airborne sets were made by hand at TRE or converted from existing equipment. Consequently the final S-band system as designed in TRE was not put into operation until Spring 1944. In the meantime, however, a certain amount of cross breeding between the systems had occurred and a system was introduced operationally in October 1943 which combined the principles of the Mark I system with RF equipment on the S-band.
The remaining fundamental development has been the introduction of the. Mark III system in April l944 which utilises the technique outlined above whereby a number of aircraft - four up to the present - may be controlled simultaneously on the same radio frequency by using different pulse recurrence frequencies with each aircraft. It was originally intended to use one common RF system at each ground station serving the four display units, but it has become necessary to install individual RF units and aerial for each display so that aircraft may be controlled in different target areas at the same time.
The proposal to extend the range of the system by flying an additional aircraft between each ground station and the target to relay the pulses in both outward and inward links has not yet been fully developed, although a few operations were successfully carried out in October 1943 in which a relay aircraft was employed between one ground station and the target. These operations utilised existing Mark I equipment (200 Mc/s band) but further work on this frequency involving relay aircraft was terminated under instructions from HQ. Development of a similar system on the S-band continues, however.
This introductory chapter has dealt only summarily with techniques which may not be generally familiar to the reader. Subsequent articles therefore deal in some detail with the principles involved which are peculiar to this application. The main principles are:‑
The chapters do not however fall precisely under these headings since some have been written to cover a particular Mark of the system.
Fundamentals of the System
The Oboe system as at present operated is one in which an aircraft is controlled by range measurements from two ground radar stations. These two ground stations transmit pulses on the same radio frequency but on different pulse recurrence frequencies and the aircraft carries a pulse repeater to provide adequate signal strength of the return pulse at the ground stations over great distances. The controlled aircraft flies at constant range measured by normal radar methods from one station (the "Cat" or "Tracking" station) such that the track will take it directly over the target (see Fig.1). At the other ground station (the "Mouse" or "Releasing" station) located some considerable distance from the "Cat", the aircraft's range and the component of ground speed along the line joining ground station to aircraft are measured; from these measurements, in conjunction with a knowledge of the ballistic data of the bomb and the prearranged height and airspeed of the aircraft, the point at which the aircraft must
release its bomb is determined and a signal given to the aircraft accordingly.
.
Figure 1. Oboe Principle
Signals (a) to the pilot to
assist him to keep on track
(b) to the bomb aimer to indicate the moment of release
are transmitted on the same RF
channel as that used for range measurement.
Range measurements are made against one mile interval calibration marker pulses provided from a crystal oscillator running at a frequency of 93.117 Kc/s, a figure chosen in conformity with the average velocity of propagation at a mean height of 15000'. By means of dividers, distinctive marker pulses are provided at intervals corresponding to 5 and 25 miles. The setting up of the equipment to an order approaching 0.01 miles is achieved by providing a very fast auxiliary time base which can be triggered by any one of the 1 mile calibration pulses, and which will give a magnification such that the distance between two adjacent one mile pips is about 6". Tenth mile calibration pips are also available. A strobe is set to target range and the residual range of the aircraft signal from this is automatically measured. At the Cat station a voltage proportional to residual range is fed to the modulator which controls (a) the phase of the pulses in the Mark I and IIF systems (b) the width of the pulses in the Mark IIM and III systems. The pulses are demodulated in the aircraft and the audio frequency fed to the pilot's headphones. The aural indication is of the dot dash form as used in the Lorentz BA system and the pilot endeavours to maintain his aircraft on track such that he is receiving the equisignal tone. The beam as defined by the dots and dashes, unlike the BA system, is of constant width and is arranged such that 50% depth of modulation corresponds to about 88 yards off track.
At the Mouse station measurements of the velocity of the aircraft are made either direct from the cathode ray tube by timing, or automatically from available voltages corresponding to velocity and residual range; a signal is then given over the phase or width modulated pulse communication channel to the navigator to indicate the moment at which the bombs must be released.
The pulse recurrence frequencies are derived by means of a succession of dividing stages from the 93.117 Kc/s crystal and are therefore accurately maintained.
The aircraft equipment consists fundamentally of a receiver and transmitter acting in the manner of a transponder. In addition, signals are fed from the receiver to a filter unit which can take either of two forms as described in later chapters. The purpose of this unit, which comprises two sections, is to demodulate the phasing or the width of the pulses and pass the morse information at audio frequency to the pilot and navigator separately.
Operational Technique
It has been pointed out that one of the limitations of the Oboe system is that it can control only comparatively few aircraft simultaneously. This has not proved severe in practice, however, since the most important application has been for the purpose of target marking; and re-marking at intervals of three minutes, which is normally ample, is easily achieved.
A channel is defined by (a) the RF frequency (of which three spot frequencies f.g.h. are available) and (b) the pair of PRF's (of which four pairs a.b.c.d. are available). Thus we have the possibility of 12 channels (four on each of the three spot radio frequencies) providing sufficient pairs of ground display units are available. In practice not more than five have been used viz.
fa, fb, fc, ga, ha.
Five aircraft may therefore be controlled simultaneously. In view of the fact that each aircraft is under the control of the ground stations for about ten minutes, markers may be dropped at the rate of 5 in 10 minutes and this rate may be maintained provided successive waves of aircraft are available. Aircraft equipment unserviceability problems will reduce this rate but even if the reduction factor is 2, the marking rate is still adequate.
In order that the aircraft may remain under ground station control for as short a tine as possible a rendezvous area (the waiting point) is defined and the aircraft navigated by other means (Loran, Gee, DR etc.) to this area, with all but the Transmitter HT of the Oboe equipment switched on.
At a pre-arranged time the call sign of the first aircraft
on each particular channel is sent out on the Oboe pulse communication, channel. The appropriate aircraft then switches on its HT and repeats pulses back to both ground stations. The tracking station immediately sends out the 100% dot or dash modulation according to which side of the "beam" the aircraft is. The aircraft then proceeds in a direction roughly at right angles to the 'bean" and when it is 10 and 5 miles from the beam, the tracking station interrupts the dots or dashes to
send Y and X respectively.
When the aircraft is about 200 yards from the centre of the bean the depth of dot or dash modulation will begin to decrease, and as the aircraft overshoots the beam, the depth of modulation of dashes or dots will again build up. In quite a short time however, about two minutes the aircraft will
Figure 2. Oboe Target Approach have turned through
90° and settled down on track. Throughout, the position of the aircraft is plotted at the ground stations and as the aircraft passes through pre-arranged positions, the letters A, B, C, D are keyed appropriately. These "fixes" are most useful to the aircrew in timing the operation, a factor of the highest importance where "marking" is being carried out, They also indicate to the air crew when to open the bomb doors in order that the aircraft may have settled to a constant height and airspeed before the release signal is given. After "D" the pilot gives even greater attention to maintaining, as far as possible, a steady and accurate track.
The release signal follows a rhythmical pattern with which the navigator is familiar. Apparatus is available whereby the bombs may be released automatically by the executive signal from the ground station, provided the navigator has operated the master switch in accordance with a warning signal from the ground station a few seconds before the release; but this has not been used operationally. After the bombs have been released the Oboe equipment is switched off (in some cases this is carried out automatically from the operation of the bomb release) and the aircraft returns to base using navigational aids other than Oboe. When one aircraft has carried out his run, released the bombs, and switched off, the ground stations turn their aerials in the direction of the waiting point and send out the call sign of the next aircraft and repeat the operation as described.
For certain important operations there is a reserve aircraft available in the area of the waiting point; if for any reason an aircraft does not switch on within so many minutes of being called, the ground stations will call up the reserve and control him instead. It is interesting to note that any aircraft may listen to signals being transmitted to its forerunner and can form a general picture of its behaviour. Operations have been saved in some cases because the crew of a subsequent aircraft has realised from the signals that all is not well with its forerunner, and has been able to navigate to a position from which it might take over from the aircraft experiencing troubles without very much loss in time.
Operation Trinity
Reference has been made above to a series of operations carried out in December 1941, in which a number of Stirling
aircraft, under the control of a ground radar station, took part in raids on a very important target.
The target in question was Brest Harbour in which were located at that time the German ships - the Scharnhorst and the Gneisenau. Normal attacks by visual methods were found to be considerably hampered by the artificial fog shrouding the target and it was suggested that some means of blind bombing should be tried. It was recommended that attacking aircraft should approach the target along a very fine beam of the B.A. type and that measurements of range and ground speed should be carried out by radar methods. As the Oboe Group had by this time carried out a number of experiments on accurate range determination, it was obvious that the requirement should be dealt with by them.
The beam transmitter was developed by 80 Wing and was known as the "Baillie Beam". The frequency was of the order of 50 - 60 Mc/s and the aerial system was such that the beam was much finer than the normal B.A. beam. Because of the narrow width, side lobes of considerable strength existed and it was necessary for the pilot to identify the main lobe from the compass. This usually presented little difficulty. The beam transmitter was erected at Helston in Cornwall and the aerial beam was capable of being rotated electrically by phase adjustment. Very careful survey of the site was made and the bearing of the equi-signal line from the transmitter carefully monitored remotely throughout all operations.
Range measurement was carried out as during the Oboe experiments. An IFF set with its frequency sweeping mechanism removed was fitted in the aircraft and was interrogated by an Oboe ground station. This station consisted of a CHL station modified slightly in frequency and fitted with a crystal control calibrator and an extra fast time base. The p.r.f. was derived from the calibrator crystal by a succession of dividing stages. Such a station was already available at West Prawle and was being used for Oboe experimental purposes, and it was decided to use this in view of the fact that the range of the target from there was only 130 miles. Communication with the aircraft for the purposes of identification and giving the release signal was effected by means of keying the ground station transmitter on and off. The resultant interruptions in the interrogation of the IFF set could be detected in 'phones suitably inserted in the circuit.
Trials of the technique were carried out over a camera obscura and a GL set located at S. Cerney near Cirencester. The Helston beam was appropriately orientated and it was found from analysis of the tracks as recorded by the GL that the effective beamwidth over Brest would be of the order of a quarter of a mile. The accuracy with which the bomb release signal was given by the ground station was found to be somewhat better than this; this was assessed by communicating the release signal to the GL cabin over a land line.
Operations were commenced on 7th December 1941, when five Stirling aircraft from East Anglia were despatched. The aircraft, in addition to the normal crew, carried a second pilot who was expert in beam flying and who took control of the aircraft in flight, and a special wireless operator to manipulate the modified IFF set - the "Broody Hen". The aircraft flew from East Anglia to Boscombe Down on the latter station's BA beam, and from there to Cornwall on the St. Eval beam. There they crossed the peninsula and picked up the Baillie beam at Helston and flew from there in a straight line to the target at a height of between 15,000 and 17,000 ft. Between Boscombe Down and St. Eval, the aircraft was able to pick up transmission from a similar ground station to West Prawle, situated at Swanage and so effect satisfactory tuning. Of the five aircraft operating on the first night only one coped satisfactorily although two more were reported to have bombed visually. The aircraft were separated by 20 minute intervals.
Considerable interference was experienced by the aircraft and it was difficult to detect the note resultant upon the response of the IFF set to the Oboe station, because of responses to other CHL and GCI stations in the vicinity. For this reason it was necessary to close down such stations for the duration of the operation.
A further improvement was effected by introducing an LC filter circuit into the output of the 'phones which was tuned to the p.r.f. of the Oboe station.
Further operations were carried out throughout the month and out of 33 sorties, 13 were completely successful in that the aircraft unloaded its bombs in accordance with the signal from the ground station. Operations were then suspended with the intention of developing an improved aircraft transponder, but the project was never revived.
Some ten months later information received from Intelligence sources indicated that some excellent results had been achieved. At least one direct hit was credited. Another bomb exploded between the wall of the dock and the side of the ship, and repeated damage was done to dock installations.
It is important to note that the Oboe scheme benefited considerably from this operation for the following reasons:‑
The Future of Oboe
It is thought that it might be profitable to review the academic achievements of such a device as Oboe in order to appreciate the problems of blind bombing which still persist.
An aircraft travelling at 300 m.p.h. at a height of 30,000' releases its bombs some 3¼ miles before it reaches the target. In order to do this accurately the aircraft must be at the correct height and air speed and on the correct heading, and an accurate knowledge of the drift angle and the ground speed must be available. Even when these conditions are satisfied the bomb will only hit the target if its ballistic properties are known. This implies a guarantee that all bombs of a particular type are identical. Recent trials with a 500 lb bomb revealed that of three, one had a time of fall 20% greater than the alleged value and two a time of fall 50% greater. When it is realised that most ballistic data is guaranteed only up to 15,000' the difficulties of increasing the accuracy of blind bombing may be judged.
In order to illustrate the required accuracy of the above variants, the following facts are quoted.
At 30000' and 300 m.p.h. and using a 250 lb bomb an error
in ground speed determination of 10 m.p.h. results in a
bomb error of 220 yards.
An error in height of 500' results in 130 yards error.
An error in airspeed of 10 m.p.h. results in 3 yards
error.
An error in heading of 1° results in 95 yards error.
An error in drift angle of 10 in 15° results in 60 yards
error.
In addition there are a number of second order errors also involved together with the unknown errors results from an imperfect bomb.
Results recently obtained from trials in this country give the following figures.
Height feet |
Angle of Cut | Average radial error yds |
Radius of 50% zone yds |
| 6000 | 53° |
45 |
- |
| 12000 | 53° |
80 |
75 |
| 25000 | 53° |
115 |
100 |
| 30000 | 23° |
150 |
180 |
It is apparent therefore that considerable emphasis must be placed on such factors as the determination of accurate winds at the time and place at which the information is required and upon the necessity of obtaining reliable ballistic data.
The results quoted also show that Oboe is a precision instrument and calls for a somewhat complicated set up. For this reason it has no general application to ordinary navigational problems such as are met in civil aviation for which purpose too, Oboe accuracy is superfluous. Its application to peace time activities must therefore remain rather academic but it may well be that some system of the order of accuracy which Oboe has achieved might well be used for such purposes as:‑