Early history of the Decca Navigator system

The i.f. phase comparison hyperbolic position-fixing technique that became the Decca Navigator was invented in 1937 in the USA. Tests there in 1941, sponsored by the Decca Company, demonstrated an unprecedented position-line accuracy. In the UK the Air Ministry had reservations about the technique but the Admiralty saw it as the possible basis of a navigation system for use on the projected invasion of the French coast. After further development and seagoing trials under the codename QM, the system played a key part (overlooked by historians) in operation Overlord/Neptune of 1944. QM was later employed for hydrographic surveying in the Scheldt estuary. When the system was released for commercial use in 1945 as the Decca Navigator, the design of a lane-identification facility was presenting problems. An intermediate solution (`Mk. V¿) was developed, followed in the 1950s by the `Multipulse¿ technique used today.