A short-range air navigation aid, which provides azimuth aid by visual means of cockpit instruments. A VOR system provides properly equipped aircraft with bearing information relative to the VOR station and magnetic north. The VOR system is used for landing, terminal, and en route guidance. It also gives virtually static-free regular weather broadcasts, special flight instructions, and voice and code station identification. The VOR service operates in the very high frequency (VHF) band between 108 and 118 MHz, sharing alternate channels with the localizer in the instrument landing system. Typically, VOR stations are co-located with a distance measuring equipment (DME) system or a tactical are navigation (TACAN) system. The combined systems are referred to as VOR/DME or VORTAC stations and provide both azimuth and distance information. See also Distance-measuring equipment; Instrument landing system (ILS).
The VOR operates on the principle that the phase difference between two signals can be employed as a means of determining azimuth if one of the signals maintains a fixed phase through 360°, so it can be used as a reference, while the other is made to vary as a direct function of azimuth. The phase difference between these two signals will then equal the azimuth of the aircraft. In practice, two demodulated 30-Hz signals are used. These are called the reference-phase and variable-phase signals. See also Electronic navigation systems.
There are two types of VOR equipment. One employs a four-loop antenna array and is called conventional VOR. The other, Doppler VOR, having a 50-loop or larger antenna array located around a single-loop carrier antenna, is based on the Doppler principle and is designed for installation at locations which present especially difficult siting problems due to multipath conditions. Although the two types differ in design, from the standpoint of air navigation they function in essentially the same manner and can be received by the same equipment. Both types are used to define route intersections and most domestic airways. See also Doppler effect; Doppler VOR.
As the United States national airspace is increasingly congested, serious consideration is being given to replacing or supplementing the VOR system with satellite-based navigation. It is believed that satellites will enable more flexible routing and that satellite positioning can be effectively integrated into air-traffic surveillance systems. However, VOR is expected to remain in place at least until the integrity of satellite-based services improves to the level of VOR services and until the costs of transitioning to satellite services are reduced to an acceptable level. See also Satellite navigation systems.
