How does a VOR tell a pilot which radial the aircraft is on?
A ground station sends two 30 Hz signals, the aircraft receiver compares their phase to find its bearing from the station, and the open ground around the station is shared with the airfield lighting.
A VOR (VHF omnidirectional range) is a ground station that tells an aircraft its bearing from the station. The station sends a reference signal that is the same in every direction and a variable signal whose phase changes with direction, both at 30 Hz. The aircraft receiver measures the phase difference, which gives the radial, the line out from the station that the aircraft is on, shown against magnetic north. The pilot can fly to or from the station along a chosen radial.
A VOR gives bearing only, not distance. For distance it is usually paired with a DME, which is why a VOR/DME is common. A conventional VOR uses a central antenna on a raised metal ground plane, and a Doppler VOR adds a ring of antennas to make the variable signal and is less affected by reflections from nearby ground. Both give the same bearing to the aircraft.
The signal depends on open ground around the station, so trees, buildings, fences, overhead lines and large metal structures close to it can reflect the signal and make the bearing wrong. A VOR is not part of the AGL system, but when a station stands on or near an airfield its clear site is shared with lighting masts, cable routes, signs and works. Anything new close to the antenna is reviewed with the navaid owner, work near the station is agreed in advance, and the owner decides whether the VOR needs a check before it goes back into service.
Frequently Asked Questions
Q1What does a VOR do?
A VOR, or VHF omnidirectional range, is a ground station that lets an aircraft work out its bearing from the station. The station sends two signals, and the aircraft receiver compares them to find which direction, or radial, the aircraft is on from the station.
Q2What is a radial, and does VOR give distance?
A radial is a line leading out from the station in one direction, named by its bearing in degrees. A VOR gives bearing only. For distance the VOR is usually paired with a DME, which is why the two are often installed together as a VOR/DME.
Q3What is the difference between a conventional VOR and a Doppler VOR?
Both give the same bearing information to the aircraft. A Doppler VOR makes the variable signal with a ring of antennas and is less affected by reflections from nearby terrain and buildings, so it is often used where the site is not ideal. The type is chosen in the design.
Q4Why must the area around a VOR be kept clear?
Trees, buildings, fences, overhead lines and large metal structures near the antenna can reflect the signal and make the bearing wrong. The site is chosen for open ground, and new structures close to the station are reviewed with the navaid owner before they are built.
Q5Is a VOR part of airfield ground lighting?
No. It is a radio navigation aid owned and maintained by the air navigation service provider or the airport's navaid team. It meets the AGL system on the ground when a station stands on or near the airfield and its clear site is shared with lighting masts, cables and works.
Q6What should be done before working near a VOR?
Tell the navaid owner before the work and agree the dates and the method. Keep tall equipment, large metal objects and new cable routes away from the antenna while the VOR is in use, and have the navaid owner decide whether the VOR needs checking before it goes back into service.
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