Equipment used for determining the direction from which a received signal is being radiated. Direction finders serve a variety of applications. Direction finders can be used for navigation purposes to determine the heading of a vehicle relative to a transmitter at a known location, and to determine the location of the vehicle when the range to that transmitter is known, as in the case of VOR/DME instrumentation. Direction finding may also be used to determine vehicle heading for guidance along a desired route toward a given destination.

Surveillance uses of direction finders include discovering and locating covert transmitters used for espionage, locating the emitter in search-and-rescue missions, and tracking crewless objects in scientific missions. A direction finder may be implemented either as a ground station used to track a moving source or on a moving vehicle. Bearing information obtained by the ground station can also be passed to a moving vehicle that is equipped with a transmitter that can be tracked by the ground station. As mentioned above, a direction finder may also be found on a moving vehicle, such as an aircraft or a ship, to determine the vehicle's heading relative to a transmitter or the transmitter's bearing relative to the known track of the vehicle.