X-Gerat

In World War II, German radio bombing system used against Britain 1940, eventually countered by electronic jamming.

A transmitter sent out a narrow beam over the target, along which the bombers flew. Three other beams were directed across the guide beam; one at 50 km/31 mi from the target to warn pilots, the second at 20 km/12 mi, and the third at 5 km/3 mi. The signals from the second and third beams were fed into a calculator and gave an accurate measurement of the bomber's speed, from which the calculator worked out the optimum bomb release point and told the bomb-aimer when to drop.