The
Decca Navigation System is an early precursor to positioning systems like the American
GPS system and the planned European
Galileo positioning 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 was the case with
Loran C,
its primary use was for ship navigation in coastal waters.
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