Range and Direction Finding (RDF) was the initial technique and hardware in Great Britain that eventually came to be called 'radar.'
Since the earliest days of radio (wireless), the signals had been used in direction finding on land, sea, and in the air. This was accomplished by rotating the antenna at a receiving station and noting the direction of rotation that gave the maximum signal, thus showing the direction to the transmitting station. The abbreviation DF was often applied to this technique, as well as the receiving equipment. Since radio was being used, the abbreviation RDF came to be commonly used for Radio Direction Finder equipment.
Radio direction finding was a passive technique – the receiving station simply detected the signal from the emitting station. If two receiving stations separated by some known distance were used, the method of triangulation might be applied – with restrictions – to give an approximation of the distance to the transmitting station.
Starting in the mid-1930s, active techniques were developed in a number of nations to give precise measures of range (distance) from the transmitter to a given reflecting target. These techniques were developed in great secrecy, and various cover names were applied. (See History of radar)
In Great Britain, the development of this active technology was concentrated in a technical group under the Air Ministry and initially working at Orford Ness, an isolated location on the North Sea coast. Albert Percival Rowe, a physicist from the Air Ministry, was stationed on site, and in August 1936, suggested that RDF – an initialism for Range and Direction Finding – be used for reference. This was a good ‘cover’ for the work, since to an outsider it would seem to be the original Radio Direction Finder.[1]
In the United States, the cover name was RADAR, an acronym for RAdio Detection And Ranging. This was coined by S. M. Tucker and F. R. Furth of the U.S. Navy, and in November 1940, this designation was officially ordered to be used for all unclassified references to this technology.[2] By early 1941, the work on such systems in Great Britain and America was basically combined, and the name 'radar' quickly replaced RDF.