VOR Radio Navigation, II
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 mechanically or on a multi function EFIS display or radio display (Icom).
The objective is to show the pilot on which spoke, of the wheel, he/she is flying and in which general direction (to / from). It thus provides course guidance, automatic wind correction and magnetic headings to fly on.
Operating a VOR receiver/indicator
First set the correct frequency in the NAV radio, identify (with Morse) the station and make sure to be within receiving range (depends on your altitude and type of VOR you are using). You need to determine that the station is operating and has a reliable signal before you can use it.
Flying toward a VOR
VOR navigation is easy: rotate the radial dial with the OBS knob until the pointer is centered and the flag says TO. 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. And as VORs are aligned with magnetic North you also follow a magnetic track toward the VOR.
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 with a FROM indication.
VOR position check
Two VOR stations can be used to determine one's position. The only requirement is that you are within receiving range of both VORs. Should you have only one (1) 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.
VORs and Distance Measuring Equipment
A VOR only transmits a radial, a line if you will from the station outward. But there's nothing to indicate where you are on that line. The only thing you have is a line of position (LOP).
To resolve this you can use a second VOR and where both lines intersect you have your position. To ease this process, DME was invented in the late 50s in Australia and it operated in the 200 MHz band. Today, DME transponders transmit on a channel in the 962 to 1150 MHz range and receive on frequencies between 962 to 1213 MHz (L band with 252 channels).
DME stands for Distance Measuring Equipment. This system uses an interrogator in the aircraft and a transponder on the ground at the VOR. The aircraft starts the transmission and the ground station replies. The time these pulses took is noted and the airborne unit calculates the distance and after a couple of measurements, ground speed relative to the DME station is displayed on the unit.
Some DME interrogators are able to address five stations sequentially, if they are within range.
DME shows the relative ground speed toward the station and not the actual ground speed of the aircraft, that happens only when flying toward or from the station. DME distance is called 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, and as a distance tool for position reporting, flying a constant arc (at a certain distance/DME). VFR rated pilots can use the DME system too, if the aircraft has the proper equipment installed.