These days we use radar to help us park our cars, but during the early years of the second world war it was white hot technology and a closely guarded military secret. First used to detect aircraft in 1935, within a few years it had helped win the Battle of Britain and sink the Bismarck. It was so secret that work on it was forbidden even to physicists of genius who had fled the Nazis. (In the event, this freed up two such émigrés, Rudolf Peierls and Otto Frisch, to prove the viability of the atom bomb and thus kick-start what became the Manhattan Project.) Intelligence about what the enemy was up to with radar had a price above rubies.

In particular, during the autumn of 1941 the RAF was desperate to find ways to degrade German flak and night-fighter defences and the radars that controlled them. With America not yet in the war, the Soviet Union fighting for its survival at the gates of Moscow and Hitler dominant across Europe, the bomber offensive was almost the only weapon the Allies could use to hit back at Germany. The results, however, were meagre and the costs terrible. On one raid against Berlin in November 1941, fewer than half of the 169 planes ever found the target, and 21 machines and their crews never made it home........

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