Using the VHF omnidirectional range in a flight plan

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What is a VHF omnidirectional range used in a flight plan?

VOR which is short for VHF omnidirectional SL4000 radio ( range, is a type of short range radio navigation system for aircraft in a flight plan, enabling aircraft to determine their position and stay on course by receiving radio signals transmitted by a network of fixed ground radio beacons, with a receiver unit.

They use radio frequencies in the very high frequency (VHF) band from 108 to 117.95 MHz. Developed in the US beginning in 1937 and deployed in 1946, VOR is the standard air navigational system in the world used by bother commercial and general aviation. There are about 3000 VOR stations around the world.

History and background
Developed from earlier Visual-Aural Range (VAR) systems, the VOR was designed to provide 360 courses 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 of the 1960s when they took over from the older radio beacon and four course system. Some of the older range stations survived, with the four course directional features removed, as non directional low or medium frequency radio beacons.

Features used for a flight plan
VORs signals provide considerably greater accuracy and reliability than NDBs due to a combination of factors. VHF radio is less vulnerable to diffraction or course bending around terrain features and coastlines. Phase encoding suffers less interference from thunderstorms.

VOR signals offer a predictable accuracy of 90 meters, 2 sigma at 2nm from a pair of VOR beacons; as compared to the accuracy of unaugmented GPS which is less than 13 meters. VOR stations rely on the ‘line of sight’ because they operate in VHF band so if the transmitting antenna cannot be seen on a perfectly clear day from the receiving antenna, a useful signal cannot be received.

This limits VOR (and DME) range to the horizon or closer if mountains intervene. Although the modern solid state transmitting equipment requires much less maintenance than the older units, an extensive network of stations, needed to provide reasonable coverage along main air routes, is a significant cost in operating current airway systems in a flight plan.


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