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Oral History of Defence Electronics
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Cooke-Yarborough then moved onto the design of an automatic radar range finder for a gyroscopic gun sight on a Hurricane commissioned by the Royal Aircraft Establishment at Farnborough. This would have been the first fully automated airborne radar in use, but the project soon fell through and was replaced by an order for a warning device for fighters. The problem was to create an early warning device for the Army's fighter aircraft who attacked convoy lines along roads. These planes were frequently intercepted from behind with little warning; this device would alert the pilot when a plane was coming in from behind. Cooke-Yarborough decided that the principle behind the range finder could be adapted and placed in the tail end of the plane and would work just as well. His design used a radar in the tail to give an audio signal when a plane was closing in on the rear of the fighter. Although the device worked in trials the Army did not use it. However the system was again adapted for use in bombers.

The restriction this time was on the parts available to Cooke-Yarborough and the team. Due to the need to have this device in all new bombers, and slow speed of manufactured parts, the unit was restricted to basic, common parts. This was eventually overcome and the Monica was born. Monica would send out a pulse from the tail of the bomber and wait for a return signal. If no signal was returned then no plane was within the radar's range; if a bleep was heard over the intercom system in the bomber, then something was coming in from behind. As the rate of the bleeps increased so the distance between the enemy and the bomber decreased.

Cooke-Yarborough became very involved in the whole development process of the Monica, from construction to the debriefing of actual pilots who had used the Monica system in battle conditions, with mixed results. One problem that the team were worried about was that of the radar pulse acting as a homing beacon for the enemy, however the TRE Maths division dismissed this due to the fact that for an enemy plane to home in on the beacon it would already be within range anyway. After the war the Maths division were to be proved wrong when it came to be known that the Germans had done just that!

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