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The Decca Navigator System was a hyperbolic radio navigation system that allowed ships and aircraft to determine their position by using radio signals from a dedicated system of static radio transmitters. The system used phase comparison between pairs of low frequency signals between 70 and 129 kHz, as opposed to pulse timing systems like Gee and LORAN. This made it much easier to design receivers using 1940s electronics, and operation was simplified by giving a direct readout of Decca coordinates without the complexity of a cathode-ray tube and highly skilled operator.
The system was invented and developed by Decca in the UK. It was first deployed by the Royal Navy during World War II for the vital task of clearing the minefields to enable the D-Day landings. The Allied forces needed an accurate system not known to the Germans and thus free of jamming. After the war, it came off the secret list and was commercially developed by the Decca Company and deployed around UK and later used in many areas around the world. At its peak there were about 180 transmitting stations using "chains" of three or four transmitters each to allow position fixing by plotting intersecting electronic lines. 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 some 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 to 2000. Decca Navigator, along with Loran and similar systems, was eventually replaced by the GPS in 2000, when that became available for public use.