The Oboe navigational system was first introduced by the Royal Air Force in December 1942. A control station in Britain broadcast a radar beam in the direction of the target, and another beam tracked an Oboe-equipped Pathfinder bomber. A person in the control station could then guide the aircraft directly to the target. The main problem with Oboe was that the curvature of the earth restricted the range of all landbased signals and so the system could not be used against certain parts of Nazi Germany such as Berlin and Hamburg.

 

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(1) Arthur Harris, Bomber Command (1947)

Oboe developed out of the methods used by the R.A.F. to "bend" and interfere with the beams used as navigational aids by the German bombers during the blitz. It was then discovered that an aircraft could fly along a certain beam and that its position on that beam could be calculated by measuring the distance of the aircraft, by means of radar, from a given point. A primitive version of the Oboe system was actually used during the Command's attacks on the Scharnhorst and Gneisenau in Brest, but it proved unreliable; the equipment was liable to break down and we had not yet got the right aircraft to use it. But it seemed likely that when these difficulties were overcome, Oboe would be far more accurate than any other then existing navigational aid; later tests showed that this was in fact the case. The system depended on the re-radiation from the aircraft of radar signals sent out to it, and use was made of the fact that it is far more easy to find the distance than the precise bearing of an object by radar. There are two ground stations. One controls the aircraft by signalling to it by a system of dots and dashes whenever it deviates from a given course; this course is a part of the circumference of a circle passing through the target, the centre of the circle being the ground station itself. Meanwhile a second ground station measures at intervals how far the aircraft has got along this arc of a circle; from these measurements the position and speed of the aircraft is calculated at the ground station and not in the aircraft-an important advantage. When the aircraft is in (he exact position at which the bombs should be dropped a signal is sent, and the bombs are released. For the bomber's crew, the whole thing is pretty well automatic.

The main drawback to this system is that the aircraft must follow a steady course, without deviation, for a considerable distance until it reaches the target. This makes the aircraft extremely vulnerable. At the same time the range of the system is limited by the height at which the aircraft can fly, because the Oboe transmissions follow a straight line and therefore, owing to the curvature of the earth, must be received at an increasingly greater height above the ground as the aircraft's distance from the ground station increases.

 

(2) Donald Bennett was appointed commander of Pathfinders on 5th July 1942. In his autobiography, Pathfinders, he recorded how Arthur Harris was against the creation of this new force.

My appointment to command Pathfinders was on 5th July 1942, and I immediately began work on every aspect of the problem. My ideas had already been fairly well formed by my constant discussions with various individuals in contact with the bomber operations, and as the result of my experience as a squadron commander. The divisions of the problems were fairly clear. The human element was undoubtedly the most important, and the selection and training of crews was my most vital consideration. Second was the development and production of the very best navigational equipment available. Third, I had to provide the means of illuminating and/or marking the target in such a way that the main force crews could identify it in spite of all the decoys and dummies and the diversions that the enemy might provide.

Of these problems, the one on which I got moving most quickly related to the equipment. Navigational facilities in R.A.F. bombers in those days consisted roughly of a compass, a sextant, an astro compass and a few other minor instruments. The standard of navigation was elementary in the extreme. Just before I had left 10 Squadron, a new device known as Gee had been introduced. This was very hush-hush at the time, but details of it have since been published. It consisted of a pulse phasing radar system with receiving equipment in the aircraft, displaying signals on a cathode ray tube which could be aligned and measured. The resultant figures gave the position on a hyperbolic grid with reasonable accuracy. It had been hoped that this system would give sufficient accuracy in navigation to make blind bombing of the Ruhr practical. Unfortunately this was not quite so, but its value as a navigational aid was tremendous. In particular its use in coming back directly to home aerodromes was a tremendous help, and saved many stupid flying accidents such as had occurred in the earlier days. Gee was the first radar device used on bombers, and was most valuable. The hope for Pathfinders, however, lay in two other devices. The first of these was an airborne radar ground reflection system known as H2S. The second was subsequently given the code name Oboe.

The principle of H2S was simply that it transmitted a directional beam of high energy impulses outwards and downwards towards the ground. This beam rotated with its aerial at the rate of about once per second, and reflections received from the ground of its own impulses were accepted back by the aerial and fed into a receiver, which showed its results on a cathode ray tube which had a radial scanning line from its centre to its circumference rotating in phase with the aerial. As a signal was received, it showed up as a bright spot on the screen, and the retentively thereof painted a map on the screen similar to the built-up areas and water in the area in range below the aircraft. H2S had various scales, so that the longer range aspects could be appreciated from navigational points of, view, whilst the bombing run could be done on the shortest range of all.

 

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