Editing VHF omnidirectional range (section)

===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.