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Very High Frequency Omnidirectional Range Station (VOR)^[1] is a type of short-range radio navigation system for aircraft, enabling aircraft with a receiving unit to determine its position and stay on course by receiving radio signals transmitted by a network of fixed ground radio beacons. It uses frequencies in the very high frequency (VHF) band from 108.00 to 117.95 MHz. Developed in the United States beginning in 1937 and deployed by 1946, VOR became the standard air navigational system in the world,^[2][3] used by both commercial and general aviation, until supplanted by satellite navigation systems such as GPS in the early 21st century. As such, VOR stations are being gradually decommissioned.^[4][5] 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.^[6] 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.^[7] In 2015, the UK planned to reduce the number of stations from 44 to 19 by 2020.^[4]

A VOR ground station uses a specialized antenna system to transmit both an amplitude modulated and a frequency modulated signal. Both modulations are done with a 30 Hz signal, but the phase is different. The phase of one of the modulation signals is dependent on the direction of transmission, while the phase of the other modulation signal is not, in order to serve as a reference. The receiver will demodulate both signals, and measure the phase difference. The phase difference is indicative of the bearing from the VOR station to the receiver relative to magnetic north. This line of position is called the VOR "radial".

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

VOR stations are fairly short range: the signals are line-of-sight between transmitter and receiver and are useful for up to 200 nautical miles (370 kilometres). Each station broadcasts a VHF radio composite signal including the mentioned navigation and reference signal, station's identifier and voice, if so equipped. The station's identifier is typically a three-letter string in Morse code. The voice signal, if used, is usually the station name, in-flight recorded advisories, or live flight service broadcasts.

A VORTAC is a radio-based navigational aid for 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 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.^[8]

Description

History

Developed from earlier Visual Aural Radio 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 ft or 5,500 m) 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 centred 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^[9] 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 90 m (300 ft), 2 sigma at 2 NM from a pair of VOR beacons;^[10] as compared to the accuracy of unaugmented Global Positioning System (GPS) which is less than 13 meters, 95%.^[10]

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 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 MHz (with 50 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 MHz which have an even 100 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 MHz pass band with an odd 100 kHz first digit after the decimal point (108.10, 108.15, 108.30, 108.35, and so on) are reserved for ILS.^[11]

The VOR encodes azimuth (direction from the station) as the 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 Hz reference signal is frequency modulated (FM) on a 9,960 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 Hz components are detected and then compared to determine the phase angle between them.

The VOR signal also contains a modulated continuous wave (MCW) 7 wpm Morse code station identifier, and usually contains an amplitude modulated (AM) voice channel.

This information is then fed over an analog or digital interface to one of four common types of indicators:

1. A typical light-airplane VOR indicator, sometimes called an "omni-bearing indicator" or OBI^[12] 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 centred 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 ILS signals.
2. 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.
3. 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.
4. An 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.

In many cases, VOR stations have co-located distance measuring equipment (DME) or military Tactical Air Navigation (TACAN) – 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^[11] 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 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 130 nautical miles (240 kilometres) 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).^[13]

Additionally, two new service volumes – "VOR low" and "VOR high" – 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.^[14]

VORs, airways and the en route structure

VOR and the older NDB stations were traditionally used as intersections along 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

This section needs to be updated. Please help update this article to reflect recent events or newly available information. (December 2020)

As of 2008^[update], space-based Global Navigation Satellite Systems (GNSS) such as the Global Positioning System (GPS) are increasingly replacing VOR and other ground-based systems.^[16] In 2016, GNSS was mandated as the primary needs of navigation for IFR aircraft in Australia.^[5]

GNSS systems have a lower transmitter cost per customer and provide distance and altitude data. Future satellite navigation systems, such as the European Union Galileo, and GPS 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 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]}

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]} The VOR signal has the advantage of static mapping to local terrain.^{[clarification needed]}

The US FAA plans^[17] by 2020 to decommission roughly half of the 967^[18] 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]} On July 27, 2016, a final policy statement was released^[19] 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.^[20][21] Similar efforts are underway in Australia,^[22] 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.^[4][7] However, there are long-term plans to decommission DME, TACAN and NDBs.

Technical specification

The VOR signal encodes a morse code identifier, optional voice, and a pair of navigation tones. The radial azimuth is equal to the phase angle between the lagging and leading navigation tone.

Constants

Variables

CVOR

The conventional signal encodes the station identifier, i(t), optional voice a(t), navigation reference signal in c(t), and the isotropic (i.e. omnidirectional) component. The reference signal is encoded on an F3 subcarrier (colour). The navigation variable signal is encoded by mechanically or electrically rotating a directional, g(A,t), antenna to produce A3 modulation (grey-scale). Receivers (paired colour and grey-scale trace) in different directions from the station paint a different alignment of F3 and A3 demodulated signal.

$${\displaystyle {\begin{array}{rcl}e(A,t)&=&\cos(2\pi F_{c}t)(1+c(t)+g(A,t))\\c(t)&=&M_{i}\cos(2\pi F_{i}t)~i(t)\\&+&M_{a}~a(t)\\&+&M_{d}\cos(2\pi \int _{0}^{t}(F_{s}+F_{d}\cos(2\pi F_{n}t))dt)\\g(A,t)&=&M_{n}\cos(2\pi F_{n}t-A)\\\end{array}}}$$

DVOR

The doppler signal encodes the station identifier, i(t), optional voice, a(t), navigation variable signal in c(t), and the isotropic (i.e. omnidirectional) component. The navigation variable signal is A3 modulated (greyscale). The navigation reference signal is delayed, t₊, t₋, by electrically revolving a pair of transmitters. The cyclic doppler blue shift, and corresponding doppler red shift, as a transmitter closes on and recedes from the receiver results in F3 modulation (colour). The pairing of transmitters offset equally high and low of the isotropic carrier frequency produce the upper and lower sidebands. Closing and receding equally on opposite sides of the same circle around the isotropic transmitter produce F3 subcarrier modulation, g(A,t).