VHF omnidirectional range
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- This article is about the radio navigation aid, see VOR for other uses.
VOR, short for VHF Omni-directional Radio Range, is a type of radio navigation system for aircraft. VORs broadcast a VHF radio composite signal including the station's morse-code identifier (and sometimes a voice identifier), and data that allows the airborne receiving equipment to derive the magnetic bearing from the station to the aircraft (direction from the VOR station in relation to the earth's magnetic North). This line of position is called the "radial" in VOR parlance. The intersection of two radials from different VOR stations on a chart allows for a "fix" or specific position of the aircraft.
Developed from earlier Visual-Aural 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).
The VOR's major advantage is that the radio signal provides a reliable line (radial) to or from the station which can be selected and easily followed by the pilot. A nationwide network of "air highways", known in the US as Victor (for VHF) Airways (below 18,000 feet) and "jet routes" (at and above 18,000 feet), was set up linking the VORs and airports. An aircraft could follow a specific path from station to station by tuning the successive stations on the VOR receiver, and then either following the desired course on a Radio Magnetic Indicator, or setting it on a conventional VOR indicator (shown below) or a Horizontal Situation Indicator (HSI, a more sophisticated version of the VOR indicator) and keeping a course pointer centered on the display.
VORs also provided considerably greater accuracy and reliability than NDBs due to a combination of factors in their construction -- specifically, less course bending around terrain features and coastlines, and less interference from thunderstorms. Although VOR transmitters were more expensive to install and maintain (as was the airborne equipment, initially), today VOR has almost entirely replaced the low/medium frequency ranges and beacons in civilian aviation . . . and is now in the process of being supplanted by the Global Positioning System (GPS). Because of their VHF frequency, VOR stations rely on "line of sight" -- if the transmitting antenna could not be seen on a perfectly clear day from the receiving antenna, a useful signal would not be received. This limits VOR (and DME) range to the horizon -- or closer if mountains intervene. This means that an extensive network of stations is needed to provide reasonable coverage along main air routes. The VOR network is a significant cost in operating the current airway system, although the modern solid state transmitting equipment requires much less maintenance than the older units.
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[edit] How VORs work
VORs are assigned radio channels between 108.0 MHz (megahertz) and 117.95 MHz (with 50-kHz spacing); this is in the VHF (very high frequency) range.
The VOR system uses the phase relationship between a reference-phase and a rotating-phase signal to encode direction. The carrier signal is omni-directional and contains the amplitude modulated (AM) station Morse code or voice identifier. The reference 30 Hz signal is frequency modulated (FM) on a 9960 Hz sub-carrier. A second 30 Hz signal is derived from the electronic rotation of a directional antenna array 30 times a second. Although older antennas were mechanically rotated, current installations are scanned electronically to achieve the same result with no moving parts. When the signal is received in the aircraft, the FM signal is decoded from the sub carrier and the frequency extracted. The two 30 Hz signals are then compared to determine the phase angle between them. The phase angle is equal to the direction from the station to the airplane, in degrees from local magnetic north, and is called the "radial."
This information is then fed to one of three common types of indicators:
- The typical light-airplane VOR indicator is shown in the accompanying illustration. It consists of a knob to rotate an "Omni Bearing Selector" (OBS), and the OBS scale around the outside of the instrument, used to set the desired course. A "course deviation indicator" (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.
- A horizontal Situation Indicator (HSI) 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.
- A Radio Magnetic Indicator (RMI) was developed previous to the HSI, and features a course arrow superimposed on a rotating card which 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 degrees different) course to the station.
In many cases the VOR stations have colocated DME (Distance Measuring Equipment) or military TACAN (TACtical Air Navigation -- which includes both the distance feature, DME, 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 with co-located DME only is called a VOR-DME. A VOR radial with DME distance allows a one-station position fix. Both VOR-DMEs and TACANs share the same DME system.
DME provides the pilot with the aircraft's "slant" distance from the ground station (i.e. the direct distance, not the distance along the ground from a point directly below the aircraft (which can be calculated using the Pythagorean theorem and the aircraft's altitude)); except very close to the station, the difference between direct and slant distance is negligible. By knowing both the distance and radial from the station, the aircraft's position can be plotted on an aeronautical chart from a single station.
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 (nm) or more. Although it is popularly thought that there is a standard difference in power output between T-VORs and other stations, in fact the stations' power output is set to provide adequate signal strength in the specific site's service volume.
[edit] Using a VOR
If the pilot wants to approach the VOR station from due east he will have to fly due west to reach the station. The pilot will use the OBS to rotate the compass dial until the number 27 (270 degrees) aligns with the pointer (called the Primary Index) at the top of the dial. When the aircraft intercepts the 90-degree radial (due east of the VOR station) the needle will be centered and the To/From indicator will show "To". Notice that the pilot set the VOR to indicate the reciprocal; the aircraft will follow the 90-degree radial while the VOR indicates that the course "to" the VOR station is 270 degrees. This is called "proceeding inbound on the 090 radial." The pilot needs only to keep the needle centered to follow the course to the VOR station. If the needle drifts off-center he turns toward the needle until it is centered again. After the aircraft passes over the VOR station the To/From indicator will indicate "From" and the aircraft is then proceeding outbound on the 270 degree radial. The CDI needle may oscillate or go to full scale in the "cone of confusion" directly over the station but will recenter once the aircraft has flown a short distance beyond the station.
In the illustration above, notice that the heading ring is set with 254 degrees at the primary index, the needle is centered and the To/From indicator is showing "From" (FR). The VOR is indicating that the aircraft is on the 254 degree radial, west-southwest "from" the VOR station. If the To/From indicator were showing "To" it would mean the aircraft was on the 74-degree radial and the course "to" the VOR station was 254 degrees. Note that there is absolutely no indication of what direction the aircraft is flying. The aircraft could be flying due north and this snapshot of the VOR could be the moment when it crossed the 254 degree radial.
Taking a position fix with a VOR is no easier than with an NDB however. In both cases two stations must be tuned in and their directions found and plotted on a chart. The VOR does offer somewhat better accuracy in this case due to the nature of the signals, but offsets this slightly by the need to rotate the OBS in order to find the direction to the station.
[edit] VORs, Airways and the Enroute 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. As you fly in a commercial airliner you 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. 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 upon GPS, more and more airways have been defined by such points, removing the need for some of the expensive ground-based VORs. A recent development is that, in some airspace, the need for such points to be defined with reference to VOR ground stations has been removed. This has led to predictions that VORs will be obsolete within a decade or so.
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 Jetroutes).
Most aircraft equipped for instrument flight 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 toward one VOR station while watching the second receiver to see when he crosses a certain radial from another VOR station.
[edit] Accuracy
The predictable accuracy of the VOR system is ±1.4°. However, test data indicates that 99.94% of the time a VOR system has less than ±0.35° of error. VOR systems are internally monitored so that it will shut down if the station error exceeds 1.0°.[1]
ARINC 711-10 January 30, 2002 states that receiver accuracy should be within 0.4 degrees with a statistical probability of 95% under various conditions. Any receiver compliant to this standard should meet or exceed these tolerances.
[edit] Future
Like many other forms of aircraft radio navigation currently used, it is likely that some form of space-based navigational system such as Global Positioning System (GPS) will replace VOR systems. VOR is specifically in jeopardy because of the need for numerous stations to cover a large area. The satellite-based GPS is capable of reliably locating an aircraft's position within about 100 feet horizontally. Augmented by "Wide Area Augmentation System" (WAAS) currently being deployed in the U.S., the error is reduced to a cube about 10 feet on a side. This allows precision instrument approaches (with lateral and vertical guidance) with landing weather minima nearly as low as the Category I Instrument Landing System -- but no ground-based equipment except for a relatively few units that determine the WAAS correction signals relayed through satellites to user aircraft. Further refinements include "Local Area Augmentation System" (LAAS) which will probably allow Category III approaches (practically speaking, landings in "zero-zero" weather -- again, with minimal requirement for ground stations. LAAS is planned to use the same VHF frequency band for its correction message. This might require some existing VOR facilities to be shut down or shifted to different frequencies to avoid interference issues.[2]
[edit] See also
- Direction finding
- Instrument flight rules (IFR)
- Instrument Landing System (ILS)
- Non-directional beacon (NDB)
- Distance Measuring Equipment (DME)
- Global Positioning System (GPS)
- Wide Area Augmentation System (WAAS)
[edit] References
- ^ Department of Transportation and Department of Defense (March 25, 2002). 2001 Federal Radionavigation Systems (PDF). Retrieved on November 27, 2005.
- ^ Department of Transportation and Department of Defense (March 25, 2002). 2001 Federal Radionavigation Plan (PDF). Retrieved on November 27, 2005.
