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VHF omnidirectional range
VOR, short for VHF Omni-directional Range, is a type of radio navigation system for aircraft. VOR's 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 also 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-direction beacons, or NDBs. VOR's major advantage is that the radio signal provides more information, allowing pilots to follow a line in the sky more easily than with an NDB. A major network of "air highways", known as 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 radio, 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 broadcasters and receivers rather expensive. In addition VORs have a limited range of about 160km, which means that an extensive network of stations needs to be used to provide reasonable coverage. The VOR network is a major cost in operating the current air navigation standards.
VORs are quickly being ignored in favour the much more user-friendly GPS system, and it is generally thought that they will be turned off around 2010. Oddly the airways have become so important to air traffic control that aircraft are forced to follow them even when they are using GPS for navigation.
How the VOR works
Each VOR operates on a radio frequency assigned to it between 108.0 MHz (Megahertz) and 117.95 MHz, which is in the VHF (very high frequency) range. The channel width is 50kHz. 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 operating radius to 160 km.
VOR systems use a phase relationship between two signals to encode direction. The main "carrier" signal is a simple AM tone broadcasting the identity of the station in morse code. The second signal is sent on a 9960 Hz sideband, and is modulated with a 30 Hz signal. Both are sent from the highly directional antenna, rotating the signal 30 times a second. Note that the transmitter need not be physically rotating - most VOR transmitters use a phased array of antennae such that the signal is "rotated" electronically.
When the signal is received in the aircraft, the FM signal is decoded from the sideband and the frequency extracted. The two signals are then paired, resulting in a phase difference between the two signals. The phase difference between the two signals is 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.
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 DME. A VORTAC, for example, 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.
Using the VOR
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 swing all the way to one side as the aircraft passes over 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 knowm, 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 the flight director makes such tasks almost foolproof. A flight director is an analog computer that is attached to several radios and can use measurements from both VORs and NDBs, using the angle information 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. The only thing lacking in such a system is a known ground location -- flight directors know directions, not locations -- but this can be easily calculated by listening to one of the stations to identify it, and then finding it on a chart. Of course this task has also been made considerably easier with the introduction of computers. Modern flight directors do know the identity and location of the stations, and can display it all on a moving map.
VORs and aerial highways
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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