VHF omnidirectional range


A very high frequency omnidirectional range station is a type of short-range VHF radio navigation system for aircraft, enabling aircraft with a VOR receiver to determine the azimuth, referenced to magnetic north, between the aircraft to/from fixed VOR ground radio beacons. VOR and the first DME system 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 began in 1949 by the U.S. CAA. ICAO standardized VOR and DME in 1950 in ICAO Annex, Edition 1. Frequencies for the use of VOR are standardized in the very high frequency band between 108.00 and 117.95 MHz. To improve azimuth accuracy of VOR even under difficult siting conditions, Doppler VOR was developed in the 1960s. VOR is according to ICAO rules a primary means navigation system for commercial and general aviation, VOR are gradually decommissioned and replaced by DME-DME RNAV 7.2.3 and satellite based navigation systems such as 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. 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. In 2015, the UK started to reduce the number of stations from 44 to 19 which it completed in 2020.
A VOR beacon radiates via two or more antennas an amplitude modulated signal and a frequency modulated subcarrier. By comparing the fixed 30 Hz reference signal with the rotating azimuth 30 Hz signal the azimuth from an aircraft to a VOR is detected. 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". 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 Hz signal in space, by continuously switching the signal of about 25 antenna pairs that form a circle around the center 30 Hz reference antenna.
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 systems.
VOR stations are short range navigation aids limited to the radio-line-of-sight between transmitter and receiver in an aircraft. Depending on the site elevation of the VOR and altitude of the aircraft Designated Operational Coverages of at max. about Att.C, Fig.C-13 can be achieved. The prerequisite is that the 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 VOR station broadcasts a VHF radio composite signal, including the mentioned navigation and reference signal, and a station's identifier and optional additional voice. 3.3.5 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 ATIS. 3.3.6
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 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 contract was awarded 28 December 1981.

Description

History

In 1937, David G. C. Luck filed a patent for a rotating radio beacon on behalf of RCA Corporation, which was issued in 1940. He followed that with two additional patents in 1940 for omnidirectional radio range. Previous radio beacons confined a pilot to a definite course, without information as to how far off course he may be. Luck described his radio range beacon as a radio lighthouse, "All this works like a lighthouse that sends out two kinds of light, one a beam that sweeps around steadily and the other a flash sent out in all directions just as the beam points north. Time the interval from the flash until the beam sweeps over you, and you know your exact direction from the lighthouse."
Developed from earlier Visual Aural Radio Range systems, the development of VOR was part of a U.S. civil/military program for aeronautical navigation aids. In 1949 VOR for the azimuth/bearing of an aircraft to/from a VOR installation and UHF DME and the first ICAO Distance Measuring Equipment standard, were put in operation by the U.S. CAA. In 1950 ICAO standardized VOR and DME in Annex 10 ed.1.
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. DVOR was gradually implemented and became the major radio navigation system in 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 radiobeacons.
A worldwide land-based network of "air highways", known in the US as Victor airways and "jet routes", 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 or a horizontal situation 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 approach procedures; however, receiver and data update costs 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 around terrain features and coastlines. Phase encoding suffers less interference from thunderstorms.
VOR signals offer a predictable accuracy of, 2 sigma at 2 NM from a pair of VOR beacons; as compared to the accuracy of unaugumented Global Positioning System which is less than 13 meters, 95%.
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 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 ; this is in the very high frequency range. The first 4 MHz is shared with the instrument landing system 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 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 are reserved for ILS.
The VOR encodes azimuth 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, the 30 Hz reference signal is frequency modulated 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, which use a circular array of typically 48 omnidirectional 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 modulated signal is transmitted from a separate omnidirectional antenna. The roles of amplitude and frequency modulation are 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 7 wpm Morse code station identifier, and usually contains an amplitude modulated 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 is shown in the illustration at the top of this entry. It consists of a knob to rotate an "Omni Bearing Selector", the OBS scale around the outside of the instrument, and a vertical course deviation indicator or 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" 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 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 course to the station. An RMI may present information from more than one VOR or ADF receiver simultaneously.
  3. A horizontal situation indicator, 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 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. 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 Germany
In many cases, VOR stations have co-located distance measuring equipment or military Tactical Air Navigation – 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 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.