Decca Navigator System

The Decca Navigator System was a hyperbolic radio navigation system which allowed ships and aircraft to determine their position by receiving radio signals from fixed navigational beacons. The system used phase comparison of low frequencies from 70 to 129 kHz, as opposed to pulse timing systems like Gee and LORAN. This make it much easier to implement the receivers using 1940s electronics.

The system was invented in the US, but development was carried out by Decca in the UK. It was first deployed by the Royal Navy during World War II when the Allied forces needed a system which could be used to achieve accurate landings and was not known to the Germans and thus free of jamming. After the war it was extensively developed around the UK and later used in many areas around the world. Decca's primary use was for ship navigation in coastal waters, offering much better accuracy than the competing LORAN system. Fishing vessels were major post-war users, but it was also used on aircraft, including a very early (1949) application of moving map displays. The system was deployed extensively in the North Sea and was used by helicopters operating to oil platforms.

The opening of the more accurate Loran-C system to civilian use in 1974 offered stiff competition, but Decca was well established by this time and continued operations into the 1990s. Decca was eventually replaced, along with Loran and other similar systems, by the GPS during the 1990s. The Decca system in Europe was shut down in the spring of 2000, and the last worldwide chain, in Japan, in 2001.

The Decca Navigator System consisted of a number of land-based radio beacons organised into chains. Each chain consisted of a master station and three (occasionally two) slave stations, termed Red, Green and Purple. Ideally, the slaves would be positioned at the vertices of an equilateral triangle with the master at the centre. The baseline length, that is, the master-slave distance, was typically 60–120 nautical miles (110–220 km).

...
Wikipedia