Direction finding (
DF), or
radio direction finding (
RDF), is the measurement of the direction from which a received signal was transmitted. This can refer to
radio or other forms of wireless communication, including
radar signals detection and monitoring (ELINT/ESM). By combining the direction information from two or more suitably spaced
receivers (or a single mobile receiver), the source of a transmission may be located via
triangulation. Radio direction finding is used in the navigation of ships and aircraft, to locate emergency transmitters for
search and rescue, for tracking wildlife, and to locate illegal or interfering transmitters. RDF was important in combating German threats during both the
WW-II Battle of Britain and the long running
Battle of the Atlantic. In the former, the Air Ministry also used RDF to locate its own fighter groups and vector them to detected Germain raids.