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CALIBRATION OF BLIND BOMBING SYSTEMS.
The Oboe blind bombing system operated on the same basic principles as radar, in so far as it determined range by measuring the time interval between the transmission of radio frequency pulses and their amplified return sent back from the aircraft under control. However, in view of the fact that the objective of the system required that the range of the aircraft be measured to an accuracy of a few yards, it was of paramount importance that detailed knowledge of all aspects contributing to the time interval were established with considerable precision.

The Path Finder aircraft, which were to be controlled by the Oboe system, had to fly at a height which ensured that there was direct radio wave propagation between it and the two ground stations involved in its control. To achieve this at ranges between 200 and 300 miles from the ground stations, the aircraft needed to fly between 28,000 and 32,000 feet, or just at the bottom of the stratosphere, when over the target area - which was no mean feat for any aircraft in 1942/3.

In late 1942, when the Oboe system was under development, it became apparent that there was not enough known of the speed of electro-magnetic wave propagation within the atmosphere of the earth. For normal radar, the figure of 186,400 statute miles per second was quite accurate enough for all existing applications. However, in the atmosphere the temperature, partial pressure of the dry air and the water vapour content normally decrease with increasing altitude and, therefore, the index of refraction decreased with altitude. The velocity of propagation of electro-magnetic waves varies inversely with the index of refraction of the medium; hence the radio waves move slightly less rapidly in the lower atmosphere than in space or the upper atmosphere.

The Oboe propagation link to the aircraft spanned these variations and, therefore, detailed knowledge was vital as, for the system to achieve its objective, the measurement in range was required to an accuracy of better than one part in 5,000. To this end, in the spring of 1943, a special mobile calibration unit (MCU) was set up specifically to establish the actual variations and the operational corrections necessary.

At the outset the MCU, equipped with optical sights and the best Army gun laying equipments available, was set up on the high ground overlooking Margam Sands. The object was to track Oboe controlled aircraft and their bomb falls, as accurately as possible, on practice bombing runs over the Margam bombing range just off the coast of South Wales. In the beginning, the accuracy of the track flown and the timing of bomb release was poor and the resultant impact errors were considerable. However, as the propagation velocity corrections (up to one part in 1,000) were established and a number of other errors corrected the bomb impact point was usually within yards of the intended target. From the measurements by the MCU of aircraft altitude, speed and track together with exact time of bomb release, it became possible to predict the impact point within a few yards and, as a result, the dropping of dummy bombs became unnecessary.

This may sound as though this was an easy task which should have been accomplished within a few days. However, it must be remembered that in early 1943 there was only the Mosquito bomber aircraft which could fly at 32,000 feet and, even when stripped of all weapons, would take forty minutes to climb to this altitude and then only when its engines were in very good heart. Having got up there, it only had the endurance of a short while and, therefore, could only do a very few bombing runs before fuel shortage meant it had to return to base. The weather and lack of good visibility certainly did not help.

Having established the necessary system and equipment accuracies plus the overall measurement technique the MCU moved to various alternative sites so that checks on and practice of operational equipment could be carried out without the need to drop dummy bombs.

Eventually the Unit moved to a site near Islip from where it was possible to monitor runs over both Ot Moor bombing range and the City of Oxford. At this location, it was possible to use dummy bombs on the range and actual selected buildings, in and around Oxford, as aiming points and then predict impact points for pretend drops.

The technical advisers of the United States Eighth Air Force became aware of special capabilities of the MCU and requested their help in checking the performance of the American X-band airborne radar used for blind bombing, by the B29s. As a result, some thousands of high level bombing runs were flown over Oxford by the American Super Fortresses and their performance predicted by the MCU.

Not surprisingly, the Unit was disbanded shortly after VE Day.

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