Principle of operation
The principle of VOR is bearing measurement by ‘phase comparison’. A VOR transmitter sends two separate signals which can be compared to provide the directional information. No special receiver antenna is required in the aircraft.
Station transmits an omnidirectional horizontally polarised continuous wave signal on its allocated frequency between 108.0 and 117.95 MHz. The emission code is A9W. That signal is amplitude modulated by a 9960Hz sub-carrier which itself is frequency modulated at 30Hz. This omnidirectional radiation has a circle as its horizontal polar diagram. At a given range from the transmitter, an aircraft’s receiver will detect the same phase on all bearings around it.
Variable or directional signal
This is also transmitted on the station frequency, by an aerial which rotates either physically or electronically 30 times per second (30 Hz). The signal is less strong than the omnidirectional signal, so that together the signals appear to be a single carrier wave, amplitude modulated at 30 Hz to a depth of 30%. The horizontal polar diagram of this resultant signal is called a Limacon.
An aircraft in a certain position will receive this rotating pattern as a signal with a 30 Hz amplitude modulation. The phase of this received AM signal will vary as the position of the receiver around the circle of rotation. This phase can be compared with the phase of the FM omnidirectional signal, and displayed to the pilot. The transmitter is arranged so that the two signals are in phase when the receiver is in the direction of magnetic north from the transmitter. As the bearing from the transmitter changes, so does the phase he variable signal. The phase difference from the reference signal equates to the magnetic bearing of the aircraft from the station.
The carrier wave has a keyed AM audio frequency signal to provide station identification at least once every 10 seconds, as recommended by ICAO.
Cone of confusion
ICAO recommends that the signals are transmitted at least up to 40° in elevation. In practice, modern equipment is capable of radiating signals up to 60° or even 80 above the horizon. That still leaves a gap overhead the station, in the form of a cone where no planned radiation takes place. However, while passing through this zone the receiver comes influence of weak ‘overspill’ (signals transmitted in incorrect directions, but usually very weak), causing confusion to the indications in the airborne equipment, which may change irregularly.
The conventional VOR ground antenna can take several forms, but one of the most common is effectively a cylindrical cover enclosing a horizontal rotating dipole. Slots in the cylinder combined with the rotating dipole produce the limacon shaped rotating polar diagram. It is normally mounted above a room containing the power supply.
Every VOR station has a monitor unit located in the radiation field, near the transmitter. This monitor receives the signal and compares it with known parameters. It will switch off the transmitter, or at least remove the identification and navigation signal, if any of the following take place:
•Bearing information is wrong by more than 1°
•Reduced signal strength (15% below normal)
•Failure of the monitor itself
Each station has a standby transmitter which will take over responsibility Or transmission if the monitor switches off the main transmitter. This take equires a certain amount of time, and during that time the signal can be received by airborne equipment. However, until the signal from the standoy transmitter has stabilised and the monitor checks that it is correct, no identification signal is transmitted and the signal must not be used for navigation – considerable errors may be present.
The omnibearing selector (OBS)
The magnetic bearing from the station is called the ‘radial’. There are an infinite number of radials, of course, but pilots normally refer to a radial as being of a particular whole number of degrees, so one can say there are 360 possible radials. Most VOR stations are positioned along airways, and an airway centreline is defined as a particular radial from one station until it meets the reciprocal radial from the next station along the airway. The changeover comes at the halfway point. Because of this, the original display was the so-called omnibearing indicator or OBI. The pilot has to select the desired radial manually, so the unit is often called the ‘omnibearing selector’ or OBS. The two expressions are frequently interchanged. To be pedantic, the OBS is only part of the unit, which includes a TO/FROM indicator and a LEFT/RIGHT Course Deviation Indicator (CDI) also. The pilot selects the radial along which he wishes to fly, or its reciprocal, on the OBS as an intended track. The TO/FROM indicator, in the form of white triangles, tells him whether his selected track will take him closer to the station or further away (TO meaning closer). The vertical deviation (LEFT/RIGHT indicator) bar tells him whether the desired track is to his left or his right.
It is sometimes difficult to interpret the OBI information, It must be remembered that everything depends on the selected track. As we have said, if that track will take the aircraft closer to the station, then TO will appear in the TO/FROM indicator. If the track will take the aircraft away from the station, the FROM triangle will appear. If the track will take the aircraft at an angle of between 80 and 100″, then there will be no indication of either TO or FROM.
Course deviation indicator
The course deviation indicator (CDI) bar shows the pilot where his selected track lies, either to his left or his right. When the actual radial or its reciprocal lies outside 10° of the selected track, the bar will be positioned against a stop on the relevant side. As the aircraft moves closer to the selected track, the bar moves towards the centre until the aircraft is on the selected track. Dots on the indicator show how many degrees away from the selected track you are.
The important fact about the deviation indicator is that it will only tell the truth when the aircraft is trying to do what the TO/FROM indicator is indicating. In other words, if the pilot is trying to fly towards the station, he must set a desired track on the OBS which falls inside the TO area. If he is trying to follow a radial away from the station, he must select a track which falls inside the FROM area. If his selected track falls in the wrong area, the deviation indicator will indicate in the reverse sense.
A warning flag, usually red, will appear on the face of the instrument if any of the following is detected:
•no power or low power to the aircraft equipment
•failure of the aircrafť’s equipment •failure of the ground station equipment •failure of the indicator •weak signals from either the reference or variable signal
The flag will also appear during tuning.
VOR shares the frequency band from 108.00 to 111.97 MHz with ILS signals. The frequency band 112.00 to 117.97 MHz is allocated only to VOR stations.
Factors affecting range
The higher the transmitter power, the greater the range the signal can be received with adequate strength.
Station elevation and aircraft altitude
Because VOR transmissions are in the VHF frequency band, the theoretical maximum range depends on line-of-sight distance, as discussed earlier.
The transmitted signal is subject to errors, but for 95% of the time it must be at least better than +-3°. The errors due to the airborne equipment and interpretation are similar, but when all errors are combined, the accuracy of the indication will be within +- 5% for that 95% of the time.