Very High Frequency Omni-directional Range (VOR)
A VOR is a radio station that transmits radio navigation signals in the very high frequency (VHF) band. The station transmits radio signals, called radials, in every direction away from the station. Pilots use 360 radials, one for each degree in a circle, to determine position. If the equipment in the airplane says the airplane is on the 360 degree radial, then the airplane is north of the VOR. An airplane on the 90 radial is east, while one on the 180 degree radial is due south of the station.
Since pilots rely on the compass for directional guidance, the radials transmitted by VORs are aligned with magnetic north.
Tuning and Identifying a VOR
Before using a VOR for navigation, it must be tuned and identified. Tuning in the VOR is a simple matter of selecting the proper frequency on the receiver. Next, configure the airplane's audio system so that you can monitor the audible Morse code identifier, which is broadcast by the VOR station. Each VOR has a unique three letter identifier. By hearing the Morse code for this three letter identifier, you have positively identified that the signal on your display is being received from the intended VOR.
Finding Your Location From a VOR
Turn the omni-bearing selector (OBS) knob on the VOR receiver display until the needle centers and the TO-FROM flag displays "FROM". Once the needle is centered, your magnetic bearing, or radial, from the VOR is shown on the top of the display. For example, if the reading was 135, this tells you the airplane is located over the 135 radial. In other words, the airplane is southeast of the VOR.
One thing that can be very confusing when learning VOR navigation: The radial you are on has nothing to do with your heading. You could be on that 135 radial and be heading north, south, or any direction.
Finding the Direction to a VOR
Rotate the OBS until the needle is centered with a "TO" indication. The number shown on the top of the display is the bearing from your airplane to the VOR. This is exactly the opposite of the direction from the station to the airplane. Therefore, your bearing, or radial, from the station is also shown, on the bottom of the display.
Flying Directly to a VOR
With the needle centered with a "TO" indication, turn the aircraft to the bearing shown on the top of the VOR receiver display. Since the VOR is displaying the direction to the station, and your airplane is flying that heading, you will be heading directly to that VOR. However, the airplane will drift with the wind, unless a crosswind correction is applied.
To correct for the wind, maintain your heading and observe the movement of the needle, which is referred to as the course deviation indicator (CDI). If the needle moves right or left, it indicates the need to move the airplane right or left to stay on course to the station. This is another confusing concept of VOR navigation: The needle is not telling you to turn left or right, it's telling you to move the entire airplane left or right. In order to do this, you will, of course, have to turn the airplane. However, your goal is to adjust the heading in order to move the airplane laterally. Remember, the needle is giving you position information, which has nothing to do with your airplane's heading.
For example, if you were directly south of a VOR, on its 180 degree radial, then centering the needle with a "TO" indication would result in 360 being shown, since the VOR is directly north of the airplane. Turning the airplane to a 360 heading will get us heading toward the station. While maintaining a 360 heading, observe the movement of the CDI. Lets say the CDI begins to move to the right. In this case, the instrument is telling us that we are drifting to the left with the wind whenever we maintain a 360 degree heading.
At this point, we must choose a new heading to counteract the airplane drifting to the left. We'll turn right and establish the airplane on a 015 heading. Maintaining this new heading, observe the movement of the CDI. Lets say it continues to move to the right. We can gather that a 015 heading is not enough to counteract the wind. A greater correction is required, so we choose a further right heading of 030 degrees.
While maintaining a 030 heading, the needle begins to move back toward the center of the display. This means a 030 heading more than counteracts the wind, and we are returning to our desired course. Maintain the new heading until the CDI is centered.
Once the needle is centered, we make a guess as to what heading will perfectly counteract the wind. We know from our experiences so far that a 030 heading more than counteracts the wind, but that a 015 heading fails to counteract the wind. So, we should pick a heading between the two. We decide to try a 020 heading. After maintaining that 020 heading for a few minutes, the CDI needle is still centered.
This process of determining the proper heading for wind correction is known as bracketing, since you progressively narrow the range of headings the desired wind correction heading lies within.
Flying Directly Away From a VOR
With the needle centered with a "FROM" indication, turning the airplane to the bearing shown at the top of the VOR receiver display results in the airplane flying directly away from the station. This is because you have turned the airplane to a heading that is the same as the direction your airplane is from the station.
Track the radial away from the station using the same bracketing technique as when flying to the station.
More on the TO-FROM Flag
If you attempt to track a radial from the station, with a "TO" flag, the needle will work in reverse. The pilot would need to fly away from the needle, not towards it, in order to keep it centered. The same is true if flying to a station with a "FROM" flag. To alleviate this reverse sensing, make sure the TO-FROM flag displays the correct indication. If it does not, rotate the OBS until the desired flag appears.
The TO-FROM flag may also display an OFF indication. The OFF indication is shown when neither a TO or FROM indication is appropriate for the OBS selection, such as a selection that would take the airplane neither to or from the station. It is also displayed with directly over or very close to the navaid. If none of these conditions do not exist, then the OFF flag indicates a loss of signal from the navaid.
VOR Sensitivity
Since the radials a VOR transmits all originate at the VOR station itself, the distance between radials increases the further the airplane is from the station. This means that the needle is slower to respond at greater distances from the VOR, since the airplane must move a greater distance to move from one radial to another. As the airplane gets closer and closer to the station, the needle will become more and more sensitive, making it more difficult to keep centered.
When overflying a VOR, the needle will become more and more sensitive until, when very close to the navaid, the needle will likely swing all the way to the left or the right. At these close distances, simply remain on your heading. The TO-FROM flag will change from "TO" to an OFF indication during station passage. Once on the other side of the station, the "FROM" flag will appear.
As you track outbound from the station, the needle will become less and less sensitive with increasing distance from the VOR.
Intercepting a Radial
Often times, it is desirable to track a specific radial. To do so, dial the radial you desire to track using the OBS. Turn the airplane to a heading that is the same as the course you selected with the OBS. The airplane is now roughly paralleling your desired course.
Next, select a new heading, which will intercept the desired course. If the needle is not deflected to full scale, only a small heading change may be required, such as 15 or 30 degrees in the direction of the needle deflection. If the needle is deflected full scale, a greater intercept heading will be required.
You may choose to center the needle with a from indication to see which radial you're currently on, then return the OBS to the desired course. Choose an intercept heading, in the direction of the needle, between 30 and 90 degrees left or right of your current heading. When the needle begins to center, turn to the desired course. Track the desired radial, using the bracketing technique to correct for wind drift.
Distance Measuring Equipment (DME)
DME consists of stations on the ground and equipment in the airplane. DME operates on the ultra high frequency (UHF) band of frequencies. When a VOR is fitted with DME equipment, it is referred to as a VOR/DME. Each VHF VOR frequency is paired with a specific UHF DME frequency. This means the pilot only uses the VOR's frequency, and needs not be concerned with the DME's UHF frequency.
Before using DME, identify the DME to be used by listening for its Morse code identifier. DME equipment does not have its own unique identifier. Instead, it uses the identifier of the VOR to which it is attached. The DME will transmit its parent VOR's Morse code identifier once every 30 seconds.
DME operates by measuring the time it takes for a radio signal to be relayed from the airplane to the ground based DME equipment and back. The result is displayed in nautical miles to the pilot.
VORTACs
A TACAN is a ground based navigation aid used for tactical air navigation by military aircraft. Its operation is similar to that of a VOR/DME, but the azimuth transmitted from a TACAN can only be received by military aircraft. Civilian aircraft can receive DME from a TACAN, however.
It is common for a TACAN transmitter to be fitted with equipment which allows it to be used as a VOR or a TACAN. These joint civil/military navigation aids are called VORTACs.
From the civilian users perspective, a VORTAC is identical in use and function to a VOR/DME.
VOR Test Facilities (VOTs)
A VOT broadcasts a test signal to permit pilots to check the accuracy of their aircraft's VOR receiver. VOT locations and frequencies are listed in the airport/facilities directory.
To use a VOT, simply tune its frequency. The VOT does not broadcast a signal usable for navigation. Instead, it broadcasts a test signal which tells your aircraft it is directly north of the VOT station, regardless of actual position. This means that your onboard VOR will always indicate a 0 degrees FROM or 180 degrees TO when the CDI is centered.