VOR
VHF Omni-directional Range, which is a type of ground-based electronic aid to navigation for aircraft. Flight routes can be planned using VORs as beacons. Each VOR operates on a radio frequency assigned to it between 108.0 MHz (Megahertz) and 117.95 MHz, which is in the VHF (very high frequency) range. The channel width is 50kHz. Because of using the VHF signal, VOR requires a line of sight to function. The maximum operating radius is 160 km.
A VOR receiver in the aircraft provides the pilot with a means to determine the radial the aircraft is on. Radial is defined as the magnetic course from the VOR station to the aircraft. The pilot may also set his VOR display to show the reciprocal, which is the magnetic course from the aircraft to the VOR station. The radial (or reciprocal) is referenced to magnetic north, which differs from true north by a number called the magnetic variation, which varies depending on one's location around the world and is available on aeronautical charts and in directories.
VOR systems use a phase relationship between two signals to encode direction. One signal is AM (amplitude modulated) and the other is FM (frequency modulated). The transmitting antenna is directional, and rotates at 30 revolutions per second. This creates the AM signal. The FM signal is encoded on a 9960 Hz sideband, and is modulated with a 30 Hz signal. The FM, or reference signal is synchronized to the rotating mechanism such that the FM (reference) signal peaks at the same instant the rotating signal passes through magnetic north. A receiver in the aircraft separates the two signals by decoding them with AM and FM demodulators, respectively. By comparing the phase of the two signals, the angle between them can be easily calculated. Note that the transmitter need not be (and is usually not) physically rotating - most VOR transmitters use a phased array of antennae such that the system is "rotated" electronically.
Many VORs have another navigation aid called DME (distance measuring equipment) at the same location. The combination may be called a VOR-DME or VORTAC, depending on the agency operating the DME. (A VORTAC, for example, is a civilian VOR co-located with a military TACAN navigation system. Both systems share the distance system, known as the DME.) DME provides the pilot with the aircraft's distance from the ground station. By knowing both the distance and radial from the station, the aircraft's position can be plotted on an aeronautical chart.
VOR also stands for vestibulo-ocular reflex, an important eye movement that has provided much insight into cerebellum-dependent motor learning.
