Decca Navigation System

The Decca Navigation System is an early precursor to GPS. 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.