Frequencies
21. VOR shares the frequency band from 108.00 to 111.97 MHz with ILS signals. VOR signals in this band use all the frequencies with 'even' first decimals, for example 108.20, 108.25, 110.65. (ILS uses frequencies with 'odd' first decimals.) The frequency band 112.00 to 117.97 MHz is allocated only to VOR stations.
Range
22. Most VOR transmissions, being VHF, can be received at 'line-of-sight' ranges, calculated as in equation (2.2). However, because of the limited number of frequencies available, interference from other stations can be a problem. Sky waves from other stations have been known to interfere in certain circumstances. Listed in the FLlP, each VOR station has a published 'Designated Operational Coverage', or DOC, within which the signal strength is guaranteed to be enough to avoid interference.
This DOC is a cylinder of stated range from the station, up to a stated altitude, as shown in Fig 6-8. Do not use a VOR
outside its DOC. Fig 6-8: DOC
Factors Affecting Range
23. Transmission Power. The higher the transmitter power, the greater the range the signal can be received with adequate strength. En-route VORs with a power output of 200 W achieve ranges of around 200 nm. Terminal VORs (TVORs) normally transmit at 5O W, giving less range. The
published DOC normally gives an indication of the transmission power available.
24. Station Elevation and Aircraft Altitude. Because VOR transmissions are in the VHF frequency band, the theoretical maximum range depends on line-of-sight distance in practice however it is slightly better due to atmospheric refraction. To calculate theoretical ranges for various altitudes, the VHF range equation is given by:
D = 1.25 √HT + 1.25 √HR
From a station close to sea level, the formula can be simplified for practical purposes to:
D = 12 √F (6.1)
Where F is the aircraft's flight level. Each flight level is a 100 foot 'step' of pressure altitude (related to the standard atmosphere).
Accuracy
25. 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 95% of the time.
Factors Affecting Accuracy
26. Beacon Alignment. The agency operating the VOR station is responsible for ensuring that the 000° radial is aligned with magnetic North. Of course, variation changes continually.
27. Site Error. Uneven terrain or physical obstacles in the vicinity of a VOR transmitter affect its directional propagation. Stringent requirements are laid down by ICAO regarding site contours and the presence of structures, trees, wire fences etc. Even the overgrown grasses affect the signals. This is sometimes called 'VOR course displacement error'. As mentioned earlier, the station monitor checks that this is kept to within ±1°.
28. Propagation Error. After the signals leave the transmission site, they can still be reflected by the terrain over which they pass, and any obstructions in the path to the aircraft. These reflected waves further reduce the accuracy of the received signal.
29. Airborne Equipment Error. Manufacturing inaccuracies produce small differences between the received signal and the actual displayed radial.
30. Pilotage Error. This is not a factor affecting the display, but when considering total
accuracies of VOR signals, the difficulty in holding the aircraft on the desired radial is included in the calculations.
Using the Equipment
31. The important fact about the OBI indications is that they are totally independent of the aircraft's heading. Only the OBS track is important. The VOR equipment can be used in a number of ways as described in the succeeding paragraphs.
32. Deriving a Position Line. To find a position line from the OBI, the pilot turns the OBS until the TO / FROM indicator shows FROM, and the deviation bar is in the centre. The resulting track shown in the OBS is the radial, or the magnetic bearing of the aircraft from the station. The reciprocal is the QDM to the station. Most equipment do not give instant readings and time & care are needed.
33. Homing. To home to the station, turn the OBS until the TO / FROM indicator shows TO, and the deviation bar is in the centre. The track shown in the OBS is the QDR. Turn on to that QDR, and allow for drift if you can. If your drift allowance is incorrect, the deviation bar will move to one side.
Turn towards the bar by a sensible number of degrees, and hold that heading. Realign the CDI with your current track. If the bar moves again, turn towards it again through half your original change, and re-align it again. Continue to change heading by progressively smaller amounts and re-align the bar until the TO / FROM indicator either changes or fails to indicate.
34. Track Following. When flying on airways, or making a VOR procedural let-down to an airfield, you will usually have to follow specific radials to or from a VOR station. To follow a selected track to or from the station, you must first arrive on that track. Select the intended track on the OBS, and look at the OBI. Confirm that the TO / FROM indicator shows what you expect! If the deviation bar is against the stops, select a heading which is towards the deviation bar but about 60° away from the desired track, as shown in Fig 6-9.
35. Once established on a heading which will take you towards the desired track (270° in Fig 6-9), calculate a heading which takes account of the wind to track along the desired radial. At the same time watch the deviation bar.
10° before you reach the track, the bar will start to move. If it moves quickly, you are close to the station and should turn on to your calculated heading immediately. If the bar moves slowly, you are far away and should wait until the bar is within about 2° of the centre.
Fig 6-9: Finding the Track
36. If you know your radius of turn and your range from the station, you can make a mathematical calculation to give a time to turn exactly on to the radial. Using the 'one-in-60' rule will tell you how many miles each degree of deviation equals. However, a little practice at watching the speed of bar movement and turning accordingly is equally as good and much less effort.
37. Once established on your calculated heading, maintain that, provided the bar is within half of its full deflection, and watch the bar. If it moves away from the centre, turn 5 to 10° towards it.
Maintain that and wait. If the bar does not start to move towards the centre, double your original heading change. When it does return to the centre, turn back half your original change and hold that.
Continue to 'bracket' the radial until you are satisfied you have allowed for the correct wind effect. As you close into the station, the bar will become increasingly sensitive. Do not attempt to follow the bar once you can actually see it moving.
Other Displays
38. The VOR signals can be displayed in many forms, including hand-held combined communication / navigation radios. Modern airborne navigation computers can take the signals and use them to compare with other aids to find accurate positions without calculations from the pilot.
Apart from this area navigation (R-Nav) facility, many light aircraft displays use the remote magnetic indicator (RMI). A more modern version of a combination of the OBI and RMI, the horizontal situation indicator (HSI), is in use in larger general aviation aircraft and some airliners.