Decca Navigator System

The Decca Navigation System is an early precursor to positioning systems like the American GPS system and the planned European Galileo system. It uses radio signals from three transmitters to fix a position. The receiver fixed a position by measuring the Phase shift of signals received from the three transmitters. One consequence was that positions were not unique. The system is deployed extensively in the North Sea and was used by helicopters operating to Oil Platforms. As Loran C, its primary use was for ship navigation in coastal waters.