General

The VHF omnidirectional range (VOR) is a ground-based, short-distance navigation aid (NAVAID) which provides continuous azimuth information in the form of 360 usable radials to or from a station. It is the basis for the very high frequency (VHF) airway structure. It is also used for VOR non-precision instrument approaches.

A VOR ground station uses a phased antenna array to send a highly directional signal that rotates clockwise horizontally (as seen from above) 30 times a second. It also sends a 30 Hz reference signal on a subcarrier timed to be in phase with the directional antenna as the latter passes magnetic north. This reference signal is the same in all directions. The phase difference between the reference signal and the signal amplitude is the bearing from the VOR station to the receiver relative to magnetic north. This line of position is called the VOR “radial”. The intersection of radials from two different VOR stations can be used to fix the position of the aircraft, as in earlier radio direction finding (RDF) systems.

VOR stations are fairly short range: the signals are line-of-sight between transmitter and receiver and are useful for up to 200 miles. Each station broadcasts a VHF radio composite signal including the navigation signal, station’s identifier and voice, if so equipped. The navigation signal allows the airborne receiving equipment to determine a bearing from the station to the aircraft (direction from the VOR station in relation to Magnetic North). The station’s identifier is typically a three-letter string in Morse code. The voice signal, if used, is usually the station name, in-flight recorded advisories, or live flight service broadcasts.