The Decca Navigator

The Decca Navigation System, although now redundant, used a hyperbolic radio navigation system that utilized at least three shore-based transmitter stations consisting of one master and two slave stations (chains), ideally located at the vertices of a triangle (master at the centre), usually about 100km apart.

The stations would emit radio band signals that could be measured for a ‘phase’ difference against the master and one of the other slaves. The phase difference could then be presented on a display (decometer) similar to a clock.

As the vessel (or aircraft) changed position, so would the phase difference change. The information taken from the decometers could then be plotted on an appropriate chart overlaid with the hyperbolic position lines of each station.

Where the position lines crossed would indicate the vessels position.

The slaves, hyperbolic lanes and decometers were given colours to aid identification. Usually these would be Red, Green and Purple. A ‘Lane indicator’ gauge, via a sector pointer and vernier pointer assembly would indicate which lane was being read at the time and give a rough indication of the reading. The appropriate lane’s decometer could then be read to obtain a more accurate position line, this included a zone indication to plot the position line in the correct lanes zone. The lane indicator gauge would alternate between the slaves to indicate which was being read at the time at the time. On the more modern gauges the lane indicator would illuminate alternately, green red or purple to show which slave was active.

The Zones would be identified by the letters A to J.

The lanes would have been identified as follows:

Red lanes numbers 0-23.

Green lanes numbered 30-47

Purple lanes numbered 50-79

Hopefully the diagrams below will explain a little better.

Standard chart for Scotland.

Chart overlaid with hyperbolic position lines. Note master station mid Scotland (black triangle), Green slave station NE of Dundee (Green triangle), Purple slave station Northern Island (Purple triangle) and the Red slave station near Stornoway (Red triangle). Also note ships position between Ireland and Scotland.

As an example, imagine the decometer gauges indicated as follows :

So, with those readings taken from the decometer gauges we can plot the position of the ship.

Purple: Zone G. 63.3

Red: Zone B. 10.9

Plotted on the chart it would look a bit like this (zoomed in).

As you can see the zone indication is important so that a navigator would know which lanes to plot the position line in.

Although similar systems were already in their infancy, the Decca Navigator system can be attributed to William J. O’Brian in the United States around 1936-39. The idea was handed to the Decca Record Company in the UK around 1939 (via a mutual friend) and developed from there. After many trials and tests, it was successfully used to aid Mine sweepers in clearing safe channels in the run-up to D-Day 1944.This success was largely attributable to its accuracy and simplicity of use. It was used around the UK up until the year 2000 after which satellite (GPS) and more advanced systems took over.

I hope you have gleaned a basic understanding of the Decca Navigation System. It goes without saying that for the real enthusiast, there is a plethora of information on the internet written by people with a much deeper knowledge than me. However, I only ever strive to present a broad-brush view for interests’ sake. Also, for me, this is a trip down memory lane as I used Decca in anger many times during my time at sea. It was however, so many moons ago that satellite navigation was in its infancy and not even installed on most of the ships I sailed on. I would have used a Decca set similar to the one below. This picture has been taken from the internet. The set (a MK 12) is showing its age, not least because the lane colours have faded so badly.