VORVOR stands for VHF Omni-directional Range, which is a type of ground-based electronic aid to navigation for aircraft. 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. A VOR receiver in the aircraft provides the pilot with a means to determine the bearing in degrees from the VOR to the current position of the aircraft, or the bearing from the aircraft to the VOR. The bearing 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 two signals at a known difference in frequency to encode direction. A master signal sends out an omnidirectional pulse, while a secondary is electronically shifted in phase as it is rotated mechanically. By comparing the phase of the two signals, the angle between them can be easily calculated. 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. DME provides the pilot with the aircraft's distance from the ground station. By knowing both the distance and bearing from the station, the aircraft's position can be plotted on an aeronautical chart.