In recent years a great deal of research has been carried out in the twin fields of ship and aircraft navigational systems. The areas investigated have been closely related, but since 1945 the work has always been conducted independently, with virtually no cross-reference between the two industries. During the war years, however, this was not so and intensive research at that time resulted in some remarkable technical advances which related to both fields. The aids to navigation which then emerged have remained until recently absolutely fundamental and in marine navigation they have formed a platform of reliability, particularly the Decca Navigator system and radar. In aviation, airline competition on the transatlantic routes and the need for absolute safety has maintained a continuing process of evolution in the design of navigation and control systems. One such aid to emerge has been DME and, with the research and development complete, this is now a standard device for both military and civil operators throughout the world. A chain of ground beacons has been set up to cover the major world air routes and there are a considerable number of these beacons in the United Kingdom.