Radio direction finding is a technique for obtaining the direction of a radio transmitter. Radio waves can travel very long distances, over the horizon, by following the curvature of the Earth or as a result of refraction within the ionosphere. This makes radio direction finding a particularly good navigation system for aircraft. The oldest form of radio direction finding navigational aid (navaid) is the non-directional beacon (NDB). The principle of NDB navigation is uncomplicated and consists of two parts. An NDB station on the ground transmits radio signals in all azimuth 360° directions and an automatic direction finder (ADF) on the aircraft receives and processes the electromagnetic radio signal. Direction of the NDB station is displayed on an azimuth compass card and needle instrument in the cockpit. The primary direction finding instrument is the relative bearing indicator (RBI), which shows the direction of the beacon relative to the nose of the aircraft.
In this Figure, a single NDB station and the RBI instrument are used to obtain a fix and, thus, locate aircraft position on the aeronautical chart. This technique requires the aircraft to be above a distinctive line feature on the ground, in this case a forest boundary.
The NDB is one of the first navaids still in use in aviation because of its simplicity, low running cost and acceptable accuracy. In fact, the NDB ground equipment is a simple AM radio station and the aircraft receiver little more complex than an AM radio. NDBs are technically obsolete and modern navigation more reliant on the Global Positioning System. However, there are innumerable NDB facilities around the world and traditional ADF navigation remains invaluable backup for aircraft operating under Instrument Flight Rules.
Adapted from 'Radio Nav: NDB Navaid and ADF Avionics', published by Marques Aviation Ltd, 2010.