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Oboe AMES Type 9000

    and target range coincided, i.e. it was precisely on track.3 The dot/dash signal was transmitted by width-modulation of the primary interrogating pulse.

    The sensitivity of the system was such that a deviation of plus or minus 17 yards from the circular arc would cause either dots or dashes to be sent, thereby enabling the pilot to steer along an invisible track some 35 yards wide in the sky above the target; there was no beam as such [see sketch, bottom].

    The other ground station, code-named MOUSE, signalled the Mosquito as it passed a number of 'milestones' along the arc until it reached the release point - the intersection of the Cat and Mouse ranges - when the bomb release signal (five dots and a dash) was given automatically. Like the Cat station, the target was set by a strobe on a CRT with a delayed, magnified time base. Unlike the Cat, the returned pulse moved along the trace as the Mosquito approached the target region.

    The precise release point was influenced by many factors e.g. time of bomb fall, trail distance4 (a function of bomb ballistics and airspeed), meteorological data, the velocity and heading of the Mosquito just prior to release plus instrumental corrections.5 These were taken into account by the Mouse computer (aptly named the Micestro!), the release point being continuously and automatically corrected to ensure that the predicted impact point of the bomb (or Target Indicator) was within the target zone.

    There were several Oboe stations around the east and south coasts of England (from Cleadon in Durham to Sennen in Cornwall), any of which could be nominated to perform a Cat or Mouse function depending on the target location. Early stations (MkI) used modified CHL equipment working on 200 MHz using pulse space modulation for signalling. Later stations (MkIII) worked in S-band (10cm wavelength) and used pulse width modulation as already stated.

It is easy to understand Bruce Neal's fascination for Oboe, because the elegance of its principles plus the ingenuity and precision of its circuits reflected perfectly his own passion for first-rate electronic design.

Oboe, perhaps to a greater extent even than Gee, advanced the techniques of stable and cunning electronic timing circuits. To give just one example, at Mouse stations, the Micestro's 'walking strobe' monitored and measured the controlled aircraft's velocity to within half a mile per hour. Another step towards system accuracy was serious consideration of the tiny variation in the velocity of radio waves at different altitudes. It was concluded that a mean figure of 186,234 miles per second was appropriate for the path between a ground station and an aircraft flying at 30,000 ft (186,240 miles per second is generally assumed for ordinary radar work).

In retrospect, wartime Oboe operations, in which solitary aircraft were carefully guided to a precise point in the sky up to 300 miles away by a dedicated ground-based team, may be viewed as a portent of things to come in the space age. Houston's control of the Apollo moon missions was on a vastly greater scale, but parallels are not hard to see.

To achieve a precise bombing run, a single aircraft needed to be under Oboe control for ten minutes, and thus the absolute theoretical maximum number of runs per hour, from any given pair of Oboe stations, was six. In practice, a

 

3  Not to be confused with the dot/dash signals of the Lorenz blind-approach system developed in Germany and widely used by airlines before the war. Indication was similar, but meaning, principles and purpose quite different.

4 Trail distance: at the moment of release, aircraft and bomb are travelling together, but the bomb, as it falls, is retarded by air resistance.

5 All factors contributing to bombing accuracy were embraced by the work of AWAS - the Air Warfare Analysis Section of the Air Ministry.


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Updated 12/03/2002

Constructed by Dick Barrett

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ęCopyright 2000 - 2002 Dick Barrett

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