Editing VHF omnidirectional range (section)

{{short description|Aviation navigation system}}
{{more citations needed|date=September 2018}}
[[Image:D-VOR PEK.JPG|thumb|DVOR (Doppler VOR) ground station, collocated with [[Distance measuring equipment|DME]].]]
[[File:Vor.gif|thumb|200px|On-board VOR display with [[Course Deviation Indicator|CDI]]]]
[[File:MCTVOR.jpg|thumb|MCT DVOR, [[Manchester Airport]], United Kingdom.]]
'''Very High Frequency Omnidirectional Range Station''' ('''VOR''')<ref name="FAAVOR">{{cite web |title=Ground-Based Navigation - Very High Frequency Omni-Directional Range (VOR) |url=https://www.ecfr.gov/current/title-14/chapter-I/subchapter-A/part-1/section-1.2 |website=www.ecfr.gov |publisher=[[Federal Aviation Administration]] |access-date=2021-11-29 |language=en-us |archive-date=2022-05-22 |archive-url=https://web.archive.org/web/20220522144437/https://www.ecfr.gov/current/title-14/chapter-I/subchapter-A/part-1/section-1.2 |url-status=live }}</ref> is a type of short-range VHF [[radio navigation]] system for [[aircraft]], enabling aircraft with a VOR receiver to determine the azimuth (also radial), referenced to magnetic north, between the aircraft to/from fixed VOR ground [[transmitter|radio beacons]].  VOR <ref name=":0">{{Cite book |title=Report 540-2, Vol.I, Final Report on Evaluation of Omni-Bearing-Distance System of Air Navigationm, by W. R. Rambo, J. S. Prichard, D. P. Duffy, R. C. Wheeler, A. E. Dusseau, Jr., and S. Goldstein, October.1950.}}</ref> and the first DME<sub>(1950)</sub><ref name=":1">{{Cite book |title=TDR-114 (Technical Development Report No. 114), UHF Distance Meassuring Equipment for Air Navigation, By. R. C. Borden, C. C. Trout and E. C. Williams Electronics Division, June 1950.}}</ref> system (referenced to 1950 since different from today's DME/N)  to provide the slant range distance, were developed in the [[United States]] as part of a U.S. civil/military program for Aeronautical Navigation Aids in 1945. Deployment of VOR and DME<sub>(1950)</sub> began in 1949 by the U.S. CAA (Civil Aeronautics Administration). ICAO standardized VOR and DME<sub>(1950)</sub> in 1950 in ICAO Annex ed.1.<ref name=":2">{{Cite book |title=ICAO, International Standards and Recommended Practices, Aeronautical Telecommunications, Annex 10, ed. 1, Mai.1950.}}</ref> Frequencies for the use of VOR are standardized in the [[very high frequency]] (VHF) band between 108.00 and 117.95&nbsp;[[MHz]]<ref name=":3">{{Cite book |title=ICAO, International Standards and Recommended Practices, Vol. I Radio Navigation Aids, Annex 10, ed.8, July.2023 https://elibrary.icao.int/reader/299828/&returnUrl%3DaHR0cHM6Ly9lbGlicmFyeS5pY2FvLmludC9wcm9kdWN0LzI5OTgyOA%3D%3D?productType=ebook.}}</ref> <sup>Chapter 3, Table A</sup>. To improve azimuth accuracy of VOR even under difficult siting conditions, Doppler VOR (DVOR) was developed in the 1960s. VOR is according to [[International Civil Aviation Organization|ICAO]] rules a ''primary means navigation system'' for commercial and general aviation,<ref name="Kispo">[http://www.kispo.net/fsportugal/p8-2e.htm VOR VHF omnidirectional Range] {{Webarchive|url=https://web.archive.org/web/20170424174013/http://www.kispo.net/fsportugal/p8-2e.htm|date=2017-04-24}}, Aviation Tutorial – Radio Navaids, kispo.net</ref><ref>{{cite book |last1=Kayton |first1=Myron |url=https://books.google.com/books?id=1KLTUWLz8jcC&pg=PA122 |title=Avionics navigation systems, 2nd Ed. |last2=Fried |first2=Walter R. |publisher=John Wiley & Sons |year=1997 |isbn=0-471-54795-6 |edition=2nd |location=USA |pages=122}}</ref> (D)VOR are gradually decommissioned<ref name="NATS">{{Cite web |date=29 May 2015 |title=Has GPS killed off the VOR? |url=https://nats.aero/blog/2015/05/has-gps-killed-off-the-vor/ |url-status=live |archive-url=https://web.archive.org/web/20210724151207/https://nats.aero/blog/2015/05/has-gps-killed-off-the-vor/ |archive-date=24 July 2021 |access-date=19 September 2021}}</ref><ref name="Australian Aviation">{{Cite web |date=26 May 2016 |title=Airservices to begin turning off ground-based navaids from May 26 |url=https://australianaviation.com.au/2016/05/airservices-to-begin-turning-off-ground-based-navaids-from-may-26/ |url-status=live |archive-url=https://web.archive.org/web/20211205133416/https://australianaviation.com.au/2016/05/airservices-to-begin-turning-off-ground-based-navaids-from-may-26/ |archive-date=5 December 2021 |access-date=19 September 2021}}</ref> and replaced by DME-DME [[Area navigation|RNAV]] (area navigation)  <sup><ref name=":3" /> 7.2.3</sup> and satellite based navigation systems such as [[Global Positioning System|GPS]] in the early 21st century. In 2000 there were about 3,000 VOR stations operating around the world, including 1,033 in the US, but by 2013 the number in the US had been reduced to 967.<ref name="SatNav 2011">{{Cite web |url=https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/library/satnav/media/SatNav%20News_Spring_2011_Final.pdf |title=Archived copy |access-date=2019-09-19 |archive-date=2020-10-29 |archive-url=https://web.archive.org/web/20201029080712/https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/library/satnav/media/SatNav |url-status=dead }}</ref> The United States is decommissioning approximately half of its VOR stations and other legacy navigation aids as part of a move to [[performance-based navigation]], while still retaining a "Minimum Operational Network" of VOR stations as a backup to GPS.<ref name="2017 Federal Radionavigation Plan">{{Cite web|url = https://rosap.ntl.bts.gov/view/dot/32801|title = 2017 Federal Radionavigation Plan|year = 2017|last1 = Mattis|first1 = James N.|last2 = Chao|first2 = Elaine L.|last3 = Duke|first3 = Elaine C.|access-date = 2021-09-19|archive-date = 2022-01-06|archive-url = https://web.archive.org/web/20220106143149/https://rosap.ntl.bts.gov/view/dot/32801|url-status = live}}</ref> In 2015, the UK planned to reduce the number of stations from 44 to 19 by 2020.<ref name="NATS" />

A VOR beacon radiates via two or more antennas an [[Amplitude modulation|amplitude modulated]] signal and a [[Frequency modulation|frequency modulated]] [[subcarrier]]. By comparing the fixed 30&nbsp;Hz reference signal with the rotating azimuth 30&nbsp;Hz signal the azimuth from an aircraft to a (D)VOR is detected. The phase difference is indicative of the [[Bearing (angle)|bearing]] from the (D)VOR station to the receiver relative to magnetic north. This [[position line|line of position]] is called the VOR "radial". While providing the same signal over the air at the VOR receiver antennas. DVOR is based on the Doppler shift to modulate the azimuth dependent 30&nbsp;Hz signal in space, by continuously switching the signal of about 25 antenna pairs that form a circle around the center 30&nbsp;Hz reference antenna.

The intersection of radials from two different VOR stations can be used to [[position fixing|fix]] the position of the aircraft, as in earlier [[radio direction finding]] (RDF) systems.

VOR stations are short range navigation aids limited to the radio-[[sightline|line-of-sight]] (RLOS) between transmitter and receiver in an aircraft. Depending on the site elevation of the VOR and altitude of the aircraft Designated Operational Coverages (DOC) of at max. about {{convert|200|nmi|km|abbr=off}} <ref name=":3" /> <sup>Att.C, Fig.C-13</sup> can be achieved. The prerequesite is that the [[Effective radiated power|EIRP]] provides in spite of losses, e.g. due to propagation and antenna pattern lobing, for a sufficiently strong signal at the aircraft VOR antenna that it can be processed successfully by the VOR receiver. Each (D)VOR station broadcasts a [[VHF]] radio composite signal, including the mentioned navigation and reference signal, and a station's identifier and optional additional voice.<ref name=":3" /> <sup>3.3.5</sup> The station's identifier is typically a three-letter string in [[Morse code]]. While defined in Annex 10 voice channel is seldomly used today, e.g. for recorded advisories like [[Automatic terminal information service|ATIS]].<ref name=":3" /> <sup>3.3.6</sup>

A '''VORTAC''' is a radio-based [[navigational aid]] for [[pilot (aircraft)|aircraft pilots]] consisting of a co-located VHF omnidirectional range and a [[tactical air navigation system]] (TACAN) beacon. Both types of beacons provide pilots [[azimuth]] information, but the VOR system is generally used by civil aircraft and the TACAN system by military aircraft. However, the TACAN [[Distance measuring equipment (aviation)|distance measuring equipment]] is also used for civil purposes because civil DME equipment is built to match the military DME specifications. Most VOR installations in the United States are VORTACs. The system was designed and developed by the Cardion Corporation. The Research, Development, Test, and Evaluation (RDT&E) contract was awarded 28 December 1981.<ref>[https://babel.hathitrust.org/cgi/pt?id=mdp.39015082343461;view=1up;seq=464 Department of Defense appropriations for 1983] {{Webarchive|url=https://web.archive.org/web/20211220023902/https://babel.hathitrust.org/cgi/pt?id=mdp.39015082343461;view=1up;seq=464 |date=2021-12-20 }}, pt. 2, p. 460.</ref>