Editing VHF omnidirectional range (section)

==Description==

===History===
Developed from earlier [[Visual Aural Radio Range]] (VAR) systems. The VOR development was part of a U.S. civil/military program for Aeronautical Navigation Aids.<ref name=":0" /> In 1949 VOR for the azimuth/bearing of an aircraft to/from a VOR installation and UHF DME<sub>(1950)</sub><ref name=":1" /> and the first ICAO Distance Measuring Equipment standard,<ref name=":2" /> were put in operation by the U.S. CAA (Civil Aeronautics Administration). In 1950  ICAO standardized VOR and DME<sub>(1950)</sub> in Annex 10 ed.1.<ref name=":2" />

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 (electronics)|solid-state]] units in the early 1960s. DVOR were gradually implemented They became the major radio navigation system in the 1960s, when they took over from the older radio beacon and [[Low frequency radio range|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 ([[Non-directional beacon|NDB]]s).
{{Gallery
|height=75
|width=75
|align=right
|title=Aeronautical chart VOR pictograms
|File:Pictogram VOR.svg|VOR
|File:VOR-DME.svg|VOR/DME
|File:Pictogram VORTAC.svg|VORTAC
}}
A worldwide land-based network of "air highways", known in the US as [[Victor airways]] (below {{convert|18000|ft|m|abbr=on|disp=or}}) and "jet routes" (at and above 18,000 feet), was set up linking VORs. An aircraft can follow a specific path from station to station by tuning into 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) or a [[horizontal situation indicator]] (HSI, a more sophisticated version of the VOR indicator) and keeping a course pointer centered on the display.

As of 2005, due to advances in technology, many airports are replacing VOR and NDB approaches with RNAV (GNSS) approach procedures; however, receiver and data update costs<ref name="AOPA2005">{{cite web | author=Airplane Owners and Pilots Association | date=March 23, 2005 | url=http://www.aopa.org/whatsnew/air_traffic/gps_databases.html | title=Inexpensive GPS Databases | work=AOPA Online | publisher=Airplane Owners and Pilots Association | access-date=December 5, 2009 | archive-date=June 21, 2010 | archive-url=https://web.archive.org/web/20100621152318/http://www.aopa.org/whatsnew/air_traffic/gps_databases.html | url-status=live }}</ref> are still significant enough that many small general aviation aircraft are not equipped with GNSS equipment certified for primary navigation or approaches.

===Features===
VOR signals provide considerably greater accuracy and reliability than NDBs due to a combination of factors. Most significant is that VOR provides a bearing from the station to the aircraft which does not vary with wind or orientation of the aircraft. VHF radio is less vulnerable to diffraction (course bending) around terrain features and coastlines. Phase encoding suffers less interference from thunderstorms.

VOR signals offer a predictable accuracy of {{convert|90|m|ft|abbr=on}}, 2 sigma at 2&nbsp;NM from a pair of VOR beacons;<ref name="FRS2001">{{cite web | author=Department of Transportation and Department of Defense | date=March 25, 2002 | url=http://www.navcen.uscg.gov/pdf/frp/frp2001/FRS2001.pdf | title=2001 Federal Radionavigation Systems | access-date=November 27, 2005 | archive-date=June 14, 2011 | archive-url=https://web.archive.org/web/20110614015708/http://www.navcen.uscg.gov/pdf/frp/frp2001/FRS2001.pdf | url-status=dead }}</ref> as compared to the accuracy of unaugumented [[GPS|Global Positioning System]] (GPS) which is less than 13 meters, 95%.<ref name="FRS2001"/>

VOR stations, being VHF, operate on "line of sight". This means that if, on a perfectly clear day, you cannot see the transmitter from the receiver antenna, or vice versa, the signal will be either imperceptible or unusable. This limits VOR (and [[Distance Measuring Equipment|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.

Typically, a VOR station's identifier represents a nearby town, city or airport. For example, the VOR station located on the grounds of [[John F. Kennedy International Airport]] has the identifier JFK.

===Operation===
VORs are assigned radio channels between 108.0 [[MHz]] and 117.95&nbsp;MHz (with 50&nbsp;kHz spacing); this is in the ''very high frequency'' (VHF) range. The first 4 [[MHz]] is shared with the [[instrument landing system]] (ILS) band. In the United States, frequencies within the pass band of 108.00 to 111.95&nbsp;MHz which have an even 100&nbsp;kHz first digit after the decimal point (108.00, 108.05, 108.20, 108.25, and so on) are reserved for VOR frequencies while frequencies within the 108.00 to 111.95&nbsp;MHz pass band with an odd 100&nbsp;kHz first digit after the decimal point (108.10, 108.15, 108.30, 108.35, and so on) are reserved for ILS.<ref name="ntia.doc.gov">{{Cite web |url=https://www.ntia.gov/sites/default/files/publications/4_2021_edition_rev._2022.pdf |author=NTIA |title=Red Book. Chapter 4 - Frequency Allocations |date=January 2022 |access-date=2023-09-30 |archive-date=2023-10-21 |archive-url=https://web.archive.org/web/20231021125139/https://www.ntia.gov/sites/default/files/publications/4_2021_edition_rev._2022.pdf |url-status=live }}</ref>

[[File:VOR principle.gif|border|200px|right]]
The VOR encodes [[azimuth]] (direction from the station) as the [[phase (waves)|phase]] relationship between a reference signal and a variable signal. One of them is amplitude modulated, and one is frequency modulated. On conventional VORs (CVOR), the 30&nbsp;Hz reference signal is [[Frequency modulation|frequency modulated]] (FM) on a 9,960&nbsp;Hz [[subcarrier]]. On these VORs, the amplitude modulation is achieved by rotating a slightly directional antenna exactly in phase with the reference signal at 30 revolutions per second. Modern installations are Doppler VORs (DVOR), which use a circular array of typically 48 omni-directional antennas and no moving parts. The active antenna is moved around the circular array electronically to create a doppler effect, resulting in frequency modulation. The amplitude modulation is created by making the transmission power of antennas at e.g. the north position lower than at the south position. The role of amplitude and frequency modulation is thus swapped in this type of VOR. Decoding in the receiving aircraft happens in the same way for both types of VORs: the AM and FM 30&nbsp;Hz components are [[detector (radio)|detected]] and then compared to determine the phase angle between them.

The VOR signal also contains a [[modulated continuous wave]] (MCW) 7&nbsp;wpm Morse code station identifier, and usually contains an [[amplitude modulation|amplitude modulated]] (AM) voice channel.

This information is then fed over an analog or digital interface to one of four common types of indicators:
#A typical light-airplane VOR indicator, sometimes called an "omni-bearing indicator" or OBI<ref>CASA. [http://www.casa.gov.au/wcmswr/_assets/main/pilots/download/vor.pdf Operational Notes on VHF Omni Range (VOR)] {{Webarchive|url=https://web.archive.org/web/20140212091724/http://www.casa.gov.au/wcmswr/_assets/main/pilots/download/vor.pdf |date=2014-02-12 }}</ref> is shown in the illustration at the top of this entry. It consists of a knob to rotate an "Omni Bearing Selector" (OBS), the OBS scale around the outside of the instrument, and a vertical [[course deviation indicator]] or (CDI) pointer. The OBS is used to set the desired course, and the CDI is centered when the aircraft is on the selected course, or gives left/right steering commands to return to the course. An "ambiguity" (TO-FROM) indicator shows whether following the selected course would take the aircraft to, or away from the station. The indicator may also include a [[glideslope]] pointer for use when receiving full [[Instrument Landing System|ILS]] signals.
#A [[radio magnetic indicator]] (RMI) features a course arrow superimposed on a rotating card that shows the aircraft's current heading at the top of the dial. The "tail" of the course arrow points at the current radial from the station and the "head" of the arrow points at the reciprocal (180° different) course to the station. An RMI may present information from more than one VOR or ADF receiver simultaneously.
#A [[horizontal situation indicator]] (HSI), developed subsequently to the RMI, is considerably more expensive and complex than a standard VOR indicator but combines heading information with the navigation display in a much more user-friendly format, approximating a simplified moving map.
#An [[RNAV|area navigation]] (RNAV) system is an onboard computer with display and may include an up-to-date navigation database. At least one VOR/DME station is required for the computer to plot aircraft position on a moving map or to display course deviation and distance relative to a waypoint (virtual VOR station). RNAV type systems have also been made to use two VORs or two DMEs to define a waypoint; these are typically referred to by other names such as "distance computing equipment" for the dual-VOR type or "DME-DME" for the type using more than one DME signal.

[[Image:VORTAC TGO Aichtal Germany 01.JPG|thumb|D-VORTAC TGO (TANGO) Germany]]
In many cases, VOR stations have co-located [[distance measuring equipment]] (DME) or military Tactical Air Navigation ([[TACAN]])&nbsp;– the latter includes both the DME distance feature and a separate TACAN azimuth feature that provides military pilots data similar to the civilian VOR. A co-located VOR and TACAN beacon is called a [[VORTAC]]. A VOR co-located only with DME is called a VOR-DME. A VOR radial with a DME distance allows a one-station position fix. Both VOR-DMEs and TACANs share the same DME system.

VORTACs and VOR-DMEs use a standardized scheme of VOR frequency to TACAN/DME channel pairing<ref name="ntia.doc.gov"/> so that a specific VOR frequency is always paired with a specific co-located TACAN or DME channel. On civilian equipment, the VHF frequency is tuned and the appropriate TACAN/DME channel is automatically selected.

While the operating principles are different, VORs share some characteristics with the [[localizer]] portion of [[Instrument Landing System|ILS]] and the same antenna, receiving equipment and indicator is used in the cockpit for both. When a VOR station is selected, the OBS is functional and allows the pilot to select the desired radial to use for navigation. When a localizer frequency is selected, the OBS is not functional and the indicator is driven by a localizer converter, typically built into the receiver or indicator.

===Service volumes===
A VOR station serves a volume of airspace called its Service Volume. Some VORs have a relatively small geographic area protected from interference by other stations on the same frequency—called "terminal" or T-VORs. Other stations may have protection out to {{convert|130|nmi|km|abbr=off}} or more. It is popularly thought that there is a standard difference in power output between T-VORs and other stations, but in fact the stations' power output is set to provide adequate signal strength in the specific site's service volume.

In the United States, there are three standard service volumes (SSV): terminal, low, and high (standard service volumes do not apply to published [[instrument flight rules]] (IFR) routes).<ref>FAA Aeronautical Information Manual 1-1-8 (c)</ref>

Additionally, two new service volumes&nbsp;– "VOR low" and "VOR high"&nbsp;– were added in 2021, providing expanded coverage above 5,000 feet AGL. This allows aircraft to continue to receive off-route VOR signals despite the reduced number of VOR ground stations provided by the VOR Minimum Operational Network.<ref>{{Cite web|date=2 December 2021|title=Aeronautical Information Manual §1-1-8(c)(2)|url=https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap1_section_1.html#aim0101.html.8|website=Federal Aviation Administration|access-date=13 January 2022|archive-date=2 January 2022|archive-url=https://web.archive.org/web/20220102143740/https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap1_section_1.html#aim0101.html.8|url-status=live}}</ref>

{| class="wikitable" summary="Table of standard service volume classes"
|+ US standard service volumes (from FAA AIM<ref name="Online AIM">{{cite web | author=Federal Aviation Administration | date=April 3, 2014 | url=https://www.faa.gov/air_traffic/publications/media/aim.pdf | title=Aeronautical Information Manual | publisher=FAA | access-date=Jun 29, 2015 | archive-date=December 2, 2017 | archive-url=https://web.archive.org/web/20171202184724/https://www.faa.gov/air_traffic/publications/media/aim.pdf | url-status=live }}</ref>)
|-
! SSV class designator !! Dimensions
|-
| T (terminal) || From 1,000 feet above ground level (AGL) up to and including 12,000 feet AGL at radial distances out to 25 NM.
|-
| L (low altitude) || From 1,000 feet AGL up to and including 18,000 feet AGL at radial distances out to 40 NM.
|-
| H (high altitude) || From 1,000 feet AGL up to and including 14,500 feet AGL at radial distances out to 40 NM. From 14,500 AGL up to and including 18,000 feet at radial distances out to 100&nbsp;NM. From 18,000 feet AGL up to and including 45,000 feet AGL at radial distances out to 130 NM. From 45,000 feet AGL up to and including 60,000 feet at radial distances out to 100 NM.
|-
|VL (VOR Low)
|From 1,000 feet ATH up to but not including 5,000 feet ATH at radial distances out to 40 NM. From 5,000 feet ATH up to but not including 18,000 feet ATH at radial distances out to 70 NM.
|-
|VH (VOR High)
|From 1,000 feet ATH up to but not including 5,000 feet ATH at radial distances out to 40 NM. From 5,000 feet ATH up to but not including 14,500 feet ATH at radial distances out to 70 NM. From 14,500 ATH up to and including 60,000 feet at radial distances out to 100 NM. From 18,000 feet ATH up to and including 45,000 feet ATH at radial distances out to 130 NM.
|}

===VORs, airways and the en route structure===
[[Image:VOR on sectional.gif|thumb|The Avenal VORTAC (at 35.646999,-119.978996) shown on a sectional aeronautical chart. Notice the light blue Victor Airways radiating from the VORTAC. (click to enlarge)]]
VOR and the older NDB stations were traditionally used as intersections along [[airway (aviation)|airways]]. A typical airway will hop from station to station in straight lines. When flying in a commercial [[airliner]], an observer will notice that the aircraft flies in straight lines occasionally broken by a turn to a new course. These turns are often made as the aircraft passes over a VOR station or at an intersection in the air defined by one or more VORs.
Navigational reference points can also be defined by the point at which two radials from different VOR stations intersect, or by a VOR radial and a DME distance. This is the basic form of [[RNAV]] and allows navigation to points located away from VOR stations. As RNAV systems have become more common, in particular those based on [[GPS]], more and more airways have been defined by such points, removing the need for some of the expensive ground-based VORs.

In many countries there are two separate systems of airway at lower and higher levels: the lower ''Airways'' (known in the US as ''Victor Airways'') and ''Upper Air Routes'' (known in the US as ''Jet routes'').

Most aircraft equipped for instrument flight (IFR) have at least two VOR receivers. As well as providing a backup to the primary receiver, the second receiver allows the pilot to easily follow a radial to or from one VOR station while watching the second receiver to see when a certain radial from another VOR station is crossed, allowing the aircraft's exact position at that moment to be determined, and giving the pilot the option of changing to the new radial if they wish.

===Future===
[[Image:Table Rock VOR.jpg|thumb|right|VORTAC located on [[Upper and Lower Table Rock|Upper Table Rock]] in [[Jackson County, Oregon|Jackson County]], [[Oregon]]]]
{{update section|date=December 2020}}
{{As of|2008}}, space-based [[Global Navigation Satellite System]]s (GNSS) such as the Global Positioning System ([[GPS]]) are increasingly replacing VOR and other ground-based systems.<ref>{{cite web | author=Department of Defense, Department of Homeland Security and Department of Transportation | date=January 2009 | url=http://www.navcen.uscg.gov/pdf/frp/frp2008/2008_Federal_Radionavigation_Plan.pdf | title=2008 Federal Radionavigation Plan | access-date=June 10, 2009 | archive-date=January 26, 2017 | archive-url=https://web.archive.org/web/20170126142016/http://www.navcen.uscg.gov/pdf/frp/frp2008/2008_Federal_Radionavigation_Plan.pdf | url-status=live }}</ref> In 2016, GNSS was mandated as the primary needs of navigation for IFR aircraft in Australia.<ref name="Australian Aviation"/>

GNSS systems have a lower transmitter cost per customer and provide distance and altitude data. Future satellite navigation systems, such as the [[Galileo (satellite navigation)|European Union Galileo]], and GPS [[Local Area Augmentation System|augmentation]] systems are developing techniques to eventually equal or exceed VOR accuracy. However, low VOR receiver cost, broad installed base and commonality of receiver equipment with [[Instrument landing system|ILS]] are likely to extend VOR dominance in aircraft until space receiver cost falls to a comparable level. As of 2008 in the United States, GPS-based approaches outnumbered VOR-based approaches but VOR-equipped IFR aircraft outnumber GPS-equipped IFR aircraft.{{Citation needed|date=January 2011}}

There is some concern that [[GNSS]] navigation is subject to interference or sabotage, leading in many countries to the retention of VOR stations for use as a backup.{{citation needed|date=September 2021}} The VOR signal has the advantage of static mapping to local terrain.{{clarify|date=September 2021}}

The US FAA plans<ref>{{Cite web |url=https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/mobileAll/VOR_MON.pdf |archive-url=https://web.archive.org/web/20141006132148/http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/mobileAll/VOR_MON.pdf |archive-date=2014-10-06 |url-status=dead |title=VOR Minimum Operational Network Information Paper |author=[[FAA]]}}</ref> by 2020 to decommission roughly half of the 967<ref name="SatNav 2012">{{Cite magazine |url=https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/library/satnav/media/SatNavNews_Winter2012.pdf |title=SatNav News  |access-date=2019-09-19 |archive-date=2020-10-19 |archive-url=https://web.archive.org/web/20201019150252/https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/library/satnav/media/SatNavNews_Winter2012.pdf |volume=42 |url-status=dead }}</ref> VOR stations in the US, retaining a "Minimum Operational Network" to provide coverage to all aircraft more than 5,000 feet above the ground. Most of the decommissioned stations will be east of the [[Rocky Mountains]], where there is more overlap in coverage between them.{{citation needed|date=December 2015}} On July 27, 2016, a final policy statement was released<ref>{{cite web|url=https://www.federalregister.gov/documents/2016/07/26/2016-17579/provision-of-navigation-services-for-the-next-generation-air-transportation-system-nextgen|title=Provision of Navigation Services for the Next Generation Air Transportation System (NextGen) Transition to Performance-Based Navigation (PBN) (Plan for Establishing a VOR Minimum Operational Network)|date=26 July 2016|access-date=29 December 2016|archive-date=29 December 2016|archive-url=https://web.archive.org/web/20161229100815/https://www.federalregister.gov/documents/2016/07/26/2016-17579/provision-of-navigation-services-for-the-next-generation-air-transportation-system-nextgen|url-status=live}}</ref> specifying stations to be decommissioned by 2025. A total of 74 stations are to be decommissioned in Phase 1 (2016–2020), and 234 more stations are scheduled to be taken out of service in Phase 2 (2021–2025).

In the UK, 19 VOR transmitters are to be kept operational until at least 2020. Those at Cranfield and Dean Cross were decommissioned in 2014, with the remaining 25 to be assessed between 2015 and 2020.<ref>{{cite letter|url=http://www.caa.co.uk/docs/7/20090813NATMACConsultativeVOR.pdf|title=RATIONALISATION OF THE UNITED KINGDOM'S VOR GROUND-BASED INFRASTRUCTURE |recipient=All NATMAC Representatives|author=[[Civil Aviation Authority (United Kingdom)|CAA]] ||access-date=2014-10-01|archive-date=2014-10-06|archive-url=https://web.archive.org/web/20141006104448/http://www.caa.co.uk/docs/7/20090813NATMACConsultativeVOR.pdf|url-status=dead}}</ref><ref>{{cite book | publisher=CAA | title=Clued Up, Autumn/Winter 2014}}</ref> Similar efforts are underway in Australia,<ref>{{cite web|url=http://www.casa.gov.au/scripts/nc.dll?WCMS:STANDARD::pc=PC_101178|title=CNS-ATM Navigation frequently asked questions|first=Industry|last=permissions|date=15 November 2012|website=www.casa.gov.au|access-date=1 October 2014|archive-date=19 August 2014|archive-url=https://web.archive.org/web/20140819040014/http://casa.gov.au/scripts/nc.dll?WCMS:STANDARD::pc=PC_101178|url-status=dead}}</ref> and elsewhere.

In the UK and the United States, DME transmitters are planned to be retained in the near future even after co-located VORs are decommissioned.<ref name="NATS" /><ref name="2017 Federal Radionavigation Plan"/> However, there are long-term plans to decommission DME, TACAN and NDBs.