Aviation Tutorial - Radio Navaids
VOR VHF Omnidirectional Range
The VHF Omnidirectional Range (VOR) is the most widely used navaids around the world, to support the medium and short distance routes on the actual ATS route structure. It's main characteristic is that using his directional emission signal can provide to airmen accurated information about the bearing of the VOR station the aircraft is crossing. There are 360 possible bearings and we call each one RADIAL.

The working principle of the VOR is based on the elapsed time between the reception of two signals emitted by the VOR station, the first is emitted omnidirectionally and called the reference signal, the second is emitted directionally and is called the variable signal. The elapsed time of the reception of these two signal is computed by the VOR reception device and give the pilots the precise bearing they are crossing from the station. The process of the emission of the reference signal and the 360 variable signal happens 30 times in each second.
To understand how VOR emitting process works. Imagine that instead of 30 times per second this process was 6 minutes (360 seconds) long, the reference signal is emitted at same time the variable signal related to bearing 0, then second by second one variable signal will be emitted until bearing 359 (359 seconds after the reference signal). Aboard the aircraft the elapsed time between the arrival of the two signals is computed and the VOR reception unit, through a display informs the pilot of the his actual position related to the selected VOR station. I our case if the difference between the reference signal and the variable signal was 173 seconds we would be crossing RADIAL 173 of the selected VOR. You saw this in a larger scale, one complete turn in 360 seconds, in real lift things happen just 10800 times faster.
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The VOR stations use the VHF band between 108.00MHz and the 117.95MHz frequencies using 160 channels. Around the world there are more than 3.000 VOR stations! The VOR station is identify by  two or three letters code and this code is transmitted phonetically and using Morse code allowing pilots to identify the code of the tuned VOR station to verify if they are picking the right one.
The range of the VOR coverage varies with it's type, there are VOR's operating only as support to approach and departure procedures with limited range 25nm and until 12.000ft, to those o support also enroute facilities that could reach 200nm. The high of the flying aircraft is also an important factor, in general the higher you fly the longest will be the VOR range.

Usually the VOR facilities are located in plane places and as free of obstacles as possible, they are composed by an emitter and by a monitoring equipment.
Aboard the aircraft the VOR equipment is composed by one antenna, V shaped and usually located on the tail vertical plan or over the fuselage, by a VHF reception equipment and by a display unit called navigation indicator on the aircraft instruments panel.
The navigation indicator has been improved since the early stages of the VOR until our days and we can find it usually on one of the three following representations.
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The Radial Selector (OBS) also known by Bearing Selector is the selector where we choose the radial of the VOR that we want to work with. On the first display the green arrow will turn until it points to the chosen radial (on the picture above would be RADIAL 180, the second and third displays the circular bearing scale will rotate until the selected radial is positioned on along the vertical axis of the display (on the picture above the middle display indicates RADIAL 180 and the rightern display is selecting RADIAL 165).
The Deviation Indicator (CDI) show us the our lateral deviation regarding our selected RADIAL. If we are on the selected RADIAL the indicator will be centred, if we are flying to the left of the chosen RADIAL the needle will be to the right and the opposite occurs if we are flying to the right of the selected RADIAL. The Deviation Scale indicates until 10 RADIALs of deviation for each side of the chosen RADIAL, after that value the Indicator will be always at his maximum deflexion position.

The pilot flying using the VOR equipment must understand the concepts of RADIAL and of TO/FROM. RADIAL (RAD) is the word o defines a bearing to the station, like we already saw with QDR definition. The TO/FROM concept is used if we are flying towards the station (TO) our away (FROM) the station.Regarding this last concept, TO/FROM, I'll advise all pilots to fly using the FROM flag since it's much more understandable and easy to use than to fly with the TO flag.
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The above two aircrafts fly along to the EAST (090). Aircraft A flies on RAD090 outbound the station, Aircraft B flies on RAD270 inbound the station so we could define the positions of the two aircrafts using the following ways:
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The position of the aircraft related to the VOR is given to us by the RADIAL we are at that moment. To know the RADIAL we are from a VOR the pilot only has to do 3 things:
1- Tune and identify the VOR
2- Rotate the OBS until the Deviation Indicator is centred and the TO/FROM indication shows FROM.
3- Read the value on the bearing scale. That's the RADIAL we are from the VOR.

Using the VOR a pilot can do basically two things; fly from to or away of the VOR station directly, fly to or away of the VOR station using a given RADIAL.
The first case is not very difficult. To fly from the actual position to the station, first you must identify your actual RADIAL from the station (we can call it also QDM VOR) as we just did above, them turn your aircraft to the opposite heading (if you are on RAD170 FROM to fly directly to the VOR you should maintain RAD170 FROM and fly heading 340), if you want to fly away from the VOR you should fly an heading equal to the RADIAL value (in the case above RAD170 FROM you should maintain RAD170 FROM and fly on heading 170).
The second case is a little more complicated, let's look at a diagram to better understand what happens:
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(The pictures next to the airplanes are the representation of the lectures we'll get using any of three type of the Navigation Indicator we shown above)
Position 1- We are flying to the VOR and have to intercept and proceed on RAD210 to the VOR. To perform the intercept manoeuvre we must first know where we are, so following the steps to know our position related to the VOR (see above) e know we are flying on RAD182 FROM VOR.
Position 2- Once we know where we are we are going to enter our targeted RAD210 on our Navigation Indicator using the OBS. Our instruments shown that we are flying to the right of the wanted RADIAL so we turn left.
Position 3- The needle of the Deviation Indicator "comes to live" and gently starts moving to the center of the display... We are approaching our targeted RADIAL!
Position 4- The needle has reached the central position so we are over the RADIAL we choose, since we are going to fly to the station we must turn the aircraft to heading 030.
Position 5- Here we are established on RAD210 flying directly towards the VOR, remember that if we choose to fly away from the VOR we would choose heading 210.

Passing over the VOR, the so called "overhead" position, can be easily recognised by the oscillation of the CDI needle, the TO/FROM indicator passes from one to the other value and could appear the OFF flag. Once passed the VOR the indications start to reappear and to estabelizing.

The VOR could help us also to perform some other tasks like:
To define a fix if we are able to do a cross check using two VORs.
To define a fix if the VOR is associated with a DME equipment.
To know our time of flying to station, using the formula:
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For instance, our RADIAL from a VOR changes 10 in 3 minutes, so (3*60)/10=180/10=18. We should fly 18 minutes to arrive over the VOR station.

As any other Navaid the VOR also have some limitations, we already spoke about the absence of signal when we pass right over the VOR or even CDI and TO/FROM indicators oscillations.
There are also some problems when passing "abeam" the VOR (i.e. making an 90 angle with the VOR) usually the TO/FROM flag starts to swap values making from 70 to the 110 with the VOR. When passing exactly abeam (90) the OFF flag can appear also.
The radio waves from the VOR station can be reflected on buildings, hills, etc. causing the receptor onboard the aircraft to compute with mistakes the signal received.
There are also errors due to long distance propagation and error by interference of two VORs signals using the same frequency when the aircraft is at high altitudes.