Decca Navigator

The Decca Navigator was a hyperbolic radio navigation system that fixed position by phase comparison of low-frequency continuous-wave signals between 70 and 130 kHz. Each chain had a master and usually three secondary transmitters (Red, Green, Purple); the phase difference between station pairs placed the receiver on a hyperbolic line of position, and the intersection of two such lines gave the fix. First used to mark cleared channels before the 1944 Normandy landings, it served coastal navigation and hydrographic positioning for decades. The European chains shut down in spring 2000 and the last Japanese chain in 2001, superseded by GPS.