The Decca Navigator system was first deployed by the Royal Navy during World War II and began transmitting on the day before the D-Day invasion force landed. The Decca Navigator receiver, pictured below, allowed ships and aircraft to determine their position by receiving radio signals from fixed navigational beacons. After the war the Decca Navigator was extensively developed around the world until eventually being superseded by GPS  technology.

A personal record of the opening phase of D-Day by Lieutenant-Commander Oliver Dawkins, R.N. V.R.

On D-Day the purpose of the radio position-fixing service was first to guide the leaders of the mine sweeping flotillas on the exacting task of clearing the prescribed paths through the enemy minefields: later the leading landing craft of the force were to pass with the aid of their own Decca Receivers through the narrow swept channels to arrive precisely at the required points on the coast line. Lt.-Cdr. Oliver Dawkins, RN. VR., was a specialist navigating officer in the Senior Officer's ship of a flotilla of minesweepers. The following are extracts from a personal record.

'The enemy knows we are coming'

'The enemy knows that we intend to invade, and can surmise within certain limits where we shall land. The unknown factor is the exact time and date.'
There was not much comfort in these words spoken by a senior officer at the briefing for one of the many exercises we were conducting  during April and May of 1944. I had been appointed Flotilla Navigator to the senior ship of a new flotilla of fleet sweepers. The ship was completed and commissioned in 1943, and was fitted with every possible device for sweeping moored and ground mines. Built at Belfast these Algerine class ships had a complement of 8 officers and 110 men. Also attached to us for operational duties were four dan-Iaying trawlers and two mine sweeping M.Ls. Our primary function during the assault was to clear and mark a narrow passage through a known enemy minefield for the first wave of landing craft and bombarding cruisers. Small spar buoys were laid by the trawlers astern and the motor launches searched ahead of the leading'sweeper.