Decca Navigator System

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The Decca Navigator System was a hyperbolic low frequency radio navigation system (also known as multilateration) that was first deployed during World War II when the Allied forces needed a system which could be used to achieve accurate landings. As was the case with Loran C, its primary use was for ship navigation in coastal waters.

Fishing vessels were major post-war users, but it was also used on 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. After being shut down in the spring of 2000, it has been superseded by systems such as the American GPS and the planned European GALILEO positioning system.

It was deployed in the United Kingdom after World War II and later used in many areas around the world. Decca employees used to joke that DECCA was an acronym for Dedicated Englishmen Causing Chaos Abroad.


Principles of Operation


The Decca Navigator System consisted of a number of land-based stations organised into chains. Each chain consisted of a Master station and three (occasionally two) Slave stations, termed Red, Green and Purple. Ideally, the Slaves would be positioned at the vertices of an equilateral triangle with the Master at the centre. The baseline length, i.e. the Master-Slave distance, was typically 60~120 nautical miles. Each station transmitted a continuous wave signal that, by comparing the phase difference of the signals from the Master and one of the Slaves, resulted in a set of hyperbolic lines of position called a pattern. As there were three Slaves there were three patterns, termed Red, Green and Purple. The patterns were drawn on nautical charts as a set of hyperbolic lines in the appropriate colour. Receivers identified which hyperbola they were on and a position could be plotted at the intersection of the hyperbola from different patterns, usually by using the pair with the angle of cut closest to orthogonal as possible.

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