Editing Decca Navigator System (section)

=== Overview ===
[[Image:Crude loran diagram.PNG|thumb|right|''The Decca Navigator principle.'' <br> The phase difference between the signals received from stations A (Master) and B (Secondary) is constant along each hyperbolic curve. The foci of the hyperbolas are at the transmitting stations, A and B.]]

The Decca Navigator System consisted of individual groups of land-based radio transmitters organised into ''chains'' of three or four stations. Each chain consisted of a [[master station]] and three (occasionally two) secondary stations, termed Red, Green and Purple. Ideally, the secondaries would be positioned at the vertices of an equilateral triangle with the master at the centre. The baseline length, that is, the master–secondary distance, was typically {{convert|60-120|nmi|km}}.

Each station transmitted a continuous wave signal; comparing the relative [[phase (waves)|phases]] of the signals from the master and one of the secondaries produced a relative phase measure that was presented on a clock-like display. The phase difference was caused by the relative distance between the stations as seen by the receiver. As the receiver moves these distances change and those changes are represented by the movement of the hands on the displays.

If one selects a particular phase difference, say 30 degrees, and plots all the locations where that phase difference occurs, the result is a set of [[hyperbola|hyperbolic]] ''[[lines of position]]'' called a ''pattern''. As there were three secondaries there were three patterns, also termed Red, Green and Purple. The patterns were drawn on [[nautical chart]]s as a set of hyperbolic lines in the appropriate colour.

Navigators determined their location by reading the phase difference from two or more of the patterns from the displays. They could then look at the chart to find where the two closest charted hyperbolas crossed. The accuracy of this measurement was improved by choosing the set of two patterns that resulted in the lines crossing at as close to a right angle as possible.