Decca Navigator

WW2 changed everything as the Royal Navy needed to position its ships to within about 20 yds, even at night or in thick fog. In 1941 a radio method of accurate position fixing was proposed by the Decca Company in which a phase tracking receiver on the ship could determine its position relative to a chain of four special land based transmitters. The system was conceived by an American, William J O'Brien.

It was used initially during the invasion of Europe in 1944 and may be described as a medium range hyperbolic system operating between 70 and 130 kHz. The frequency of all the transmitters in each chain are locked together in a coherent way giving rise to hyperbolic lines of position. There were over fifty eight chains covering the UK, mainland Europe and other parts of the world.

The receiver presents the lane identifier and the relative phase of four channels on the 'Decometers' shown in the display unit above. The navigator obtains a fix by referring these values to a special chart on which a hyperbolic lattice is overprinted; a quick and simple operation.

After the war the system came into civilian use and it was adopted quickly by the fishing industry, it revolutionised every aspect of navigation. The above example shows the indicator unit of the Decca Navigator equipment as widely fitted to fishing vessels. Although now superseded by satellite navigation, it gave 50 years of good service to navigators around the long coastline of the UK and mainland Europe; it closed down in March 2000 leaving a fine mark on our maritime history of technology. More on the history of the Decca navigation system can be found at www.jproc.ca/hyperbolic/decca.html and in the Proceedings of the Royal Institute of Navigation, see www.rin.org.uk .

The picture below shows the complicated Mk 12 receiver opened as it would have been for adjustment or repair. The three circuit modules are hinged at the bottom to allow access by the service engineer, the left hand module has been lowered revealing the four receiver channels. The two other modules contain the timing and phase comparison circuits.

Wheelhouse Project - Decca Navigator

Radio direction finding had been widely used at sea from the time of the sinking of the Titanic in 1912, although even with a skilled operator its accuracy was limited.

Radio Direction Finder loop aerials are affected by their surroundings, 'night effect' changed the angle at which the signal arrived, and refraction at the coastline all gave problems. It also depended on the operator's ability to measure the angle the signal arrives at. Thus a bearing was inherently less accurate as distance increased.