VHF omnidirectional range

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This article is about the radio navigation aid, see vestibulo-ocular reflex for the important eye movement that has provided insight into cerebellum-dependent motor learning.

Image:D-VOR PEK.JPG VOR, short for VHF Omni-directional Radio Range, is a type of radio navigation system for aircraft. VORs broadcast a VHF radio signal encoding both the identity of the station and the angle to it, telling the pilot in what direction he lies from the VOR station, referred to as the radial. Comparing two such measures on a chart allows for a fix. In many cases the VOR stations have a colocated DME or Distance Measuring Equipment to provide distance measurement allowing for a one-station fix.

VORs became the major radio navigation system in the 1960s, when they took over from the older radio beacon system. The older system retroactively became known as non-directional beacons, or NDBs. VOR's major advantage is that the radio signal provides a to/from bearing to the beacon, allowing pilots to follow a line in the sky more easily than with an NDB. A major network of "air highways", known in the US as Victor Airways, were set up linking the VORs and airports. On any particular part of the journey the airway would say to fly at a specific angle from a particular station, in which case the pilot simply tunes in the station on the VOR receiver, dials that angle into the indicator, and then keeps a pointer centered in a display.

VORs also provided considerably greater accuracy and reliability than NDBs due to a combination of factors in their construction. But these same factors also make VOR transmitters and receivers rather more expensive to install and maintain. In addition VORs have limited maximum ranges of between 25–130 nautical miles (46–240 km), which means that an extensive network of stations needs to be used to provide reasonable coverage along main air routes. The VOR network is a major cost in operating the current air navigation standards.

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How the VOR works

Each VOR operates on a radio frequency assigned to it between 108.0 megahertz (MHz) and 117.95 MHz, which is in the VHF (very high frequency) range. The channel width is 50 kHz. VHF was selected because it travels only in straight lines, resisting bending due to atmospheric effects, thereby making angle measurements accurate. However, this also means that the signals do not operate "over the horizon"; VOR is line-of-sight only, limiting the maximum operating radius to 130 nmi (240 km).

VOR systems use the phase relationship between two 30 Hz signals to encode direction. The main "carrier" signal is a simple AM tone broadcasting the identity of the station in Morse code. The second 30 Hz signal is FM modulated on a 9960 Hz subcarrier. The combined signal is fed to an array of four omnidirectional antennas, which rotates the signal at 30 times a second. Note that the antennas need not be physically rotating—all VOR beacons use a phased antenna array such that the signal is "rotated" electronically.

When the signal is received in the aircraft, the FM signal is decoded from the subcarrier and the frequency extracted. The two 30 Hz signals are then compared to extract the phase difference between them. The phase difference is equal to the angle of the antenna at the instant the signal was sent, thereby encoding the direction to the station as the narrow beam washed over the receiver.

The phase difference is then mixed with a constant phase produced locally. This has the effect of changing the angle. The result is then sent to an amplifier, the output of which drives the signal pointers on a compass card. By changing the locally produced phase, using a knob known as the Omni-Bearing Selector, or OBS, the pilot can zero out the angle to a station. For instance, if the pilot wishes to fly at 90 degrees to a station, the OBS mixes in a −90 phase, thereby making the indicator needle read zero (centered) when the plane is flying at 90 degrees to the station.

Image:VORTAC TGO Aichtal Germany 01.JPG Many VORs have another navigation aid called DME (distance measuring equipment) at the same location. The combination may be called a VOR-DME or VORTAC, depending on the agency operating the facility; a VORTAC is a civilian VOR co-located with a military TACAN navigation system. Both VOR-DME and TACAN 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 ground distance). At lower altitudes and/or at a respectable distance from the DME, the difference is negligible, and so 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 are low power for regional navigation and others are high power for high altitude long range navigation.

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Using the VOR

Image:Vor.gif A typical mechanical VOR display consists of a compass dial (usually called a compass card) surrounding a vertical needle and a To/From indicator. Outside the compass dial is a knob called the Omni Bearing Selector (OBS) that rotates the compass dial. All angles are referenced to magnetic north, to allow VOR and compass angles to be easily compared. Magnetic north differs from true north by a number called the magnetic variation, which varies depending on one's location around the world and is available on aeronautical charts and in directories.

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 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. 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 on the 270 degree radial. The needle will generally swing all the way to one side as the aircraft passes over the vicinity of the VOR station but will recenter once the aircraft has flown a short distance beyond the station.

In the illustration above, notice that the compass ring is set at 254 degrees, 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. However, it is probably safe to assume that the aircraft is flying a course of 254 degrees, has overflown the VOR station and is now flying away from it.

Following a single course with a VOR is much easier than with a NDB. With an NDB only the direction to the station is known, not the radial on which the aircraft lies. This may sound like the same thing, but the key difference is that in order to overfly an NDB the indicator must be centered in the display, the exact location of the aircraft in reference to that station is unknown. In order to find the radial, the NDB pointer must be centered and then referenced to the compass. In addition, as the aircraft approaches the NDB any crosswind will cause the aircraft to drift to one side of the desired course. As the pilot recenters the indicator the aircraft will follow a curved path to the NDB and overfly it from a direction far from the one he started the approach from.

When the aircraft passes overhead a VOR station, it enters the cone of confusion, an imaginary inverted cone, where it cannot correctly identify its radial (and distance for DME). Once the aircraft has passed through this area, the VOR will indicate the "From" radial that is now being flown; the pilot continues to navigate by keeping the pointer centered in the display. With an NDB the pointer will suddenly "flip over" as the station is passed, and in order to continue flying the same direction the pilot has to reverse all corrections. This is often very difficult.

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.

Navigating along lines between stations, as opposed to over them, also remains a difficult problem for either system. In this case the radials change as the aircraft moves, and the only reasonable way to do this manually is to plot the course and sample fixes along it before flight. Errors in navigation can be very difficult to correct, requiring a fix and then comparing that to one of the sample fixes plotted earlier.

Electronics can solve this problem, and Area Navigation (RNAV) systems makes such tasks almost foolproof. An RNAV system is an analog computer that is attached to several VOR receivers and can use both VOR and DME data in order to continually calculate a fix. Flight paths can be selected as the pilot wishes, and the electronics will continually calculate the direction needed to stay on that path, just as if the pilot was flying a VOR radial.

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VORs and aerial highways

Image:VOR on sectional.gif VOR stations are 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. There are also navigational reference points where two radials from different VOR stations intersect. These intersections may or may not lie along mapped airways.

Most instrument-rated aircraft have 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.

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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 shutdown if the station error exceeds 1.0°.Template:Ref

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Future

Like many other forms of aircraft radio navigation currently used, it is likely that VOR systems will be replaced by some form of space based navigational system such as GPS. VOR is specifically in jeopardy because of the need for numerous stations to cover a large area. The Wide Area Augmentation System appears to be more cost effective since it can cover most of North America and it is sufficiently accurate for en route usage. The Local Area Augmentation System is planned to use the same VHF frequency band for its correction message. This would require some existing VOR facilities to be shutdown or there could be interference issues. Template:Ref

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See also

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References

| author=Department of Transportation and Department of Defense
| year=March 25, 2002
| url=http://www.navcen.uscg.gov/pubs/frp2001/FRS2001.pdf
| title=2001 Federal Radionavigation Systems
| format=PDF
| work=
| publisher=
| accessdate=November 27
| accessyear=2005}}

| author=Department of Transportation and Department of Defense
| year=March 25, 2002
| url=http://www.navcen.uscg.gov/pubs/frp2001/FRP2001.pdf
| title=2001 Federal Radionavigation Plan
| format=PDF
| work=
| publisher=
| accessdate=November 27
| accessyear=2005}}
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External links

da:VHF Omnidirectional Range de:Funkfeuer fr:Visual omni range it:VHF Omnidirectional Range ja:超短波全方向式無線標識 nl:VHF_Omnidirectional_Range_station no:VHF-peileutrustning zh:甚高频全向信标

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