Indicator unit for H2S Mk IIc radar installation, c 1950.

UNKNOWN - APRIL 18: Radar (radio detection and ranging) revolutionised British offensive and defensive capability in World War II, playing a vital role in locating incoming German bomber formations during the Battle of Britain in 1940. It works by bouncing powerful, ultra-short radio waves (microwaves) off distant objects. By measuring the total time taken for the beam to travel to the target and back, it is possible to calculate how far away it is. The bearing of the target is obviously the direction from which the reflected beam is received. As well as its military applications, radar is used to determine relative positions of ships and aircraft to avoid collisions, and in meteorology to locate precipitation. (Photo by SSPL/Getty Images)