short for VHF omnidirectional radio range, is a type of radio navigation system for aircraft. A VOR ground station broadcasts a VHF radio composite signal including the station's identifier, voice (if equipped), and navigation signal. The identifier is morse code. The voice signal is usually station name, in-flight recorded advisories, or live flight service broadcasts. The navigation signal allows the airborne receiving equipment to determine a magnetic bearing from the station to the aircraft (direction from the VOR station in relation to the Earth's magnetic North at the time of installation). VOR stations in areas of magnetic compass unreliability are oriented with respect to True North. This line of position is called the "radial" from the VOR. The intersection of two radials from different VOR stations on a chart provides the position of the aircraft.
Developed from earlier Visual-Aural Range (VAR) systems, the VOR was designed to provide 360 courses to and from the station, selectable by the pilot. Early vacuum tube transmitters with mechanically-rotated antennas were widely installed in the 1950s, and began to be replaced with fully solid-state units in the early 1960s. They became the major radio navigation system in the 1960s, when they took over from the older radio beacon and four-course (low/medium frequency range) system. Some of the older range stations survived, with the four-course directional features removed, as non-directional low or medium frequency radiobeacons (NDBs).
A worldwide land-based network of "air highways", known in the US as Victor airways (below 18,000 feet) and "jetways" (at and above 18,000 feet), was set up linking VORs. An aircraft can follow a specific path from station to station by tuning the successive stations on the VOR receiver, and then either following the desired course on a Radio Magnetic Indicator, or setting it on a Course Deviation Indicator (CDI, shown below) or a Horizontal Situation Indicator (HSI, a more sophisticated version of the VOR indicator) and keeping a course pointer centered on the display.
Presently, due to advances in technology, many airports are replacing VOR and NDB approaches with RNAV (GPS) approach procedures; however, receiver and data update costs are still significant enough that many small general aviation aircraft are not equipped with a GPS certified for primary navigation or approaches.
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