The determination of a ship's position by means of radio observations from fixed shore transmitters has now been practised for a considerable period of time, but up to the outbreak of the last war the accuracy obtained from the conventional direction finding systems then in use was only sufficient to provide a general estimate of position suitable for off-shore navigation in waters where the ship's position need be known only to within a mile or two. It was not until, under the pressure of wartime requirements, the modern range of radio aids to marine navigation were produced that means were provided by which the mariner was enabled to determine his position with an accuracy comparable with that of the best visual fixing but with the great advantage that these observations could be made in all conditions of visibility and weather. At the end of the last war there were three systems of outstanding merit available: Gee, Loran and Decca. All three systems, as is well known, employ a combination of master and slave stations, the transmissions of which provide radio position lines capable of accurate calculation and plotting on navigational charts.