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 navigaion in coastal waters.