VOR Radio Navigation, I
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 transmitting station on the ground and the receiver in the aircraft with indicator, either mechanical or on a multi function EFIS display.
The objective is to show the pilot on which spoke, of the wheel, he/she is flying and in which direction. It thus provides course guidance, automatic wind correction and magnetic headings to fly on.
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. We start with things every pilot should know about using and operating the VOR.
A VOR station sends out radio signals (beams) like the spoke of a wheel in the VHF frequency band. There are 360 of those spokes (radials). They are numbered from 0 to 359, one for every degree and called FROM the VOR. The VOR is oriented in such a way that the 0 radial is pointing to the magnetic north pole.
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 even if reception is not perfect. If its not transmitting or sends the word 'test', don't use it. Its either out of order or being serviced, but check your NOTAMs to verify VOR availability.
VOR stations can also be used to transmit other data like meteorological or even ATC calls to aircraft with COM failures.
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. The reception range of a VOR is line of sight (like all other VHF and higher frequencies).
Not beyond the horizon
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 technical reasons: the signal is transmitted in a horizontal plane and not vertical.
To learn more about the range of a VOR read our article about aircraft antennas.
Some aircraft have a so called dipole layed out horizontally in the (composite, only (not carbon)) fuselage. COM signals are radiated vertically thus in all directions (omni-directional) hence the vertical positioned antenna's on the back and belly of the aircraft.
VOR and ILS use
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 is easily received by this antenna. You will need a so called splitter for two receivers to work on one antenna. The signal will be split in two with the same impedance but the strength 3 dB lower (1/2), this might not be ideal.
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 is hinged at the top and can move to the right or to the left, this 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.
On our next page we describe how you can use this radio navigation device and prevent from becoming lost.