" By the time of the Munich Crisis in 1938 a plan for emergency broadcasting had been evolved, and by the beginning of September 1939 equipment for implementing the plan was ready. Low-powered transmitters were dotted all over the country and a number of "portable" transmitters were mounted on lorries. During an air raid alert instructions would go out to the fixed transmitters - the pre-war ones that the Luftwaffe had pinpointed on their maps of Britain- either to switch off altogether or to reduce power, and broadcasting would then continue on various combinations of the new and old transmitters. Reception on the ground would hardly be affected, but aircraft "homing" would be impossible. Throughout the war this system operated effectively, and the BBC never helped the Luftwaffe home on their targets.

From The Last Ditch by David Lampe

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