VOR stands for Very High Frequency Omnidirectional Range. It is a radio transmitter capable of sending signals resembling the spokes of a bike wheel. Basically the system consists of two parts: the ground station and the receiver in the aircraft with indicator, either mechanical or on a nice multi function display. Main goal is to show the pilot on which spoke, of the wheel, he is flying and in which direction. It thus provides course guidance, automatic wind correction and magnetic headings.

On this page we discuss how easily such a device is operated and not how it works in depth technically. There is enough information on that subject to be found in aviation books and on the Internet.

VOR details you should know

VOR Rose

A VOR station sends out radio signals (beams) like the spoke of a wheel. There are 360 of those spokes (radials). They are numbered from 0 (360) to 359, one for every degree and FROM the VOR. The VOR is oriented in such a way that the 0 radial is pointing to the magnetic north pole. Click on the picture to the right to see a larger, more clearer one. The VOR also sends its own identification in Morse code (pull the squelch and turn up the volume). Why Morse? Its easier to pickup than spoken word and positively identifies the VOR. If its quiet or sends the word 'test', don't use it. Its out of order or being serviced.
VORs can also be used to transmit other data like meteo or even ATC calls.

The radio signals transmitted by the VOR (range 108 - 117.95 MHz) are picked up by a horizontal oriented antenna and this antenna is usually located on the vertical fin on the aircraft. Reception of a VOR is line of sight (like all other VHF and higher frequencies). This means that if the VOR is located on the other side of the horizon (ie. you cannot 'see' it) there will be no reception either, usually. This varies with altitude and VOR transmitting power of course. There is also a "cone of silence" directly above the VOR, this is due to mechanical and radio reasons: the signal is transmitted in a horizontal plane and not vertical.

Some aircraft have a so called dipole layed out horizontally in the (composite, only) fuselage. COM signals are radiated vertically thus in all directions (thought you should know) hence the vertical antennae on the back and belly of the aircraft.

The same antenna can be used by the localizer receiver for the ILS (Instrument Landing System). These ILS signals are transmitted at a frequency three times higher than the VOR frequency and the VOR antenna can pick them up too. It is the 3rd harmonic frequency and easily received by this antenna (ok, somewhat technical). You will need a so called splitter for two receivers to work on one antenna.

These signals are then fed to a receiver and decoded for an indicator to display. This indicator has a white pointer which has been centered in the display. It can move to the right or to the left, depends on which radial (spoke) you are. The selected (with a knob, OBS) radial is indicated on top and the opposite one at the bottom (+/-180 degrees). The indicator also has a flag which says TO or FROM.

Operating a VOR receiver/indicator

First set the correct frequency, identify (Morse) the station and be within receiving range (depends on your altitude and type of VOR used). You need to determine that the station is operating and has a reliable signal.

Flying toward a VOR
This is easy: rotate the radial dial with the OBS knob until the pointer is centered and the flag says TO. Easy huh? The top radial says the direction to fly TO the station and the bottom (reciprocal radial) indicates the radial you are on. Now turn the aircraft toward the VOR and fly that heading. The fun part is that if you keep the radial centered by turning the aircraft left or right (and following the needle), you are automatically correcting for any wind.

Keep an eye on the flag indicator and notice when the TO/FROM flag goes to blank. At this point you are overhead of the station in a cone of silence (no radio signals from the VOR can be heard there), continue the same heading and the flag flips over to a FROM indication. Saying that you are have passed the station flying away FROM the VOR. The radial you are on is indicated on top.

VOR position check
Two VOR stations can be used to determine one's position. Required is that you are within receiving range of both VORs. With one VOR receiver the procedure is as follows:

• Set the frequency of VOR one and turn the OBS for a FROM indication (needle centered) and note the radial (under the mark)
• Draw a radial line on the sectional from that VOR station
• Now set the frequency of VOR two and turn the OBS for a FROM indication and note the radial (under the mark)
• Draw a radial line from the second VOR on the same chart
• Where the two lines intersect is your position

This one receiver procedure works great in slow (100Kts) aircraft. But in the faster types it really pays to have two receivers, this is much quicker. The Bendix King 125 COMM/NAV radio has an active and standby frequency with a TO knob and can be used very quickly to indicate which radial the aircraft is on.

VOR receiver check
The FAA operates a VOR test facility (VOT) and this station transmits one radial with a 0° FROM and 180° TO indication on the VOR indicator if its properly calibrated. Accuracy is +/- 4° on the ground and +/- 6° flying. Make sure to note in the aircraft logs when you used the test facility and the error indicated, if any.
Note: These test should be done at certified airborne or ground checkpoints.

VOR and distance measuring

A VOR only transmits a radial, a line if you will. But nothing to indicate where you are on that line. For this you can use a second VOR and where the lines intersect is your position. To ease this process, DME was invented. DME stands for Distance Measuring Equipment. This system uses a interrogator in the aircraft and a transponder on the ground at the VOR. The aircraft starts the transmission and the groundstation replies. The time this took place is noted and the airborne unit calculates the distance and even ground speed to the DME station for display on the unit.

Use of DME
DME shows the relative ground speed toward the station and not the ground speed of the aircraft, that happens only when flying toward or from the station.
DME distance is slant range and this is the result of horizontal distance and altitude of the aircraft. Slant range becomes altitude when flying exactly overhead of the DME.

DME is used with IFR procedures, ILS approaches, as a distance tool for reporting, flying a constant arc (at a certain distance/DME). VFR pilots can use a DME too, if the aircraft has the proper equipment installed.