A continuous-wave hyperbolic navigation system of high accuracy. It was first conceived in the USA in 1937 and further developed in the UK by the Decca Company with the Admiralty's Signal Establishment. It was first used operationally for the navigation of minesweepers and landing craft in the Normandy landings in 1944, but is now defunct. The equipment was on hire, not for sale, from the Decca Navigator Company. Decca had a maximum range of something like 400 kilometres (250 mls.) by night and about 640 kilometres (400 mls.) by day. Its accuracy close to the base line was of the order of 90 metres (295 ft) and at 160 kilometres (100 mls.) between 1.6 kilometres (1 m.) and 0.8 kilometres (0.5 m.) A Decca chain consisted of a land-based master transmitter and three slave transmitters, designated red, green and purple, locked in a precise phase relationship with the master station and set up in a triangular pattern situated between 135 kilometres (85 mls.) and 385 kilometres (240 mls.) from the master.
A vessel fitted with Decca would carry the receiver and three decometers (red, green, and purple) and the appropriate nautical charts overprinted with the Decca lattice displaying the hyperbolic position lines in the colours of the slave stations. The phase differences between waves emitted simultaneously from the master and one of the slave stations were displayed on decometers where a couple of clocklike pointers gave the lane number and the lane fraction. This enabled the navigator to identify on the lattice chart the appropriate hyperbolic position lines, and their point of intersection would be the ship's position. Three slave stations were used, as the angle of intersection might not in all circumstances be suitable.
The coverage included the coastal waters of NW Europe and parts of Africa, the Persian Gulf, India, Australia, and Japan. In addition to merchant shipping it was widely used for fishing and for surveys. The system was the most accurate of the hyperbolic systems and enjoyed half a century of full operational life, longer than any other radio navigational aid except what is now known as Loran-C. Its demise was at least in part due to the growing promise of satellite navigation.
Subjects: Maritime History.