VOR (full name: VHF Omni-directional Radio Range) is an air navigation radio aid that performs phase comparison of ground-broadcast signals to indicate direction in aviation. VOR is in the category of avionics systems. This term is formed from the initials of the words “Very High Frequency Omnidirectional Radio Range”.
VOR Working Principle
The VOR operates in the 108 – 117.95 MHz band with 50 kHz channel steps. The ground station emits an electromagnetic pattern of 30 cycles/second. This signal generates a 30 Hz sinusoidal wave at the VOR receiver on the aircraft.
The ground station also broadcasts an FM modulated signal into space on a 30 Hz reference mark in all directions. The phase difference between the two aforementioned 30 Hz signals gives the angle of the aircraft relative to the ground station.
The VOR system works with the Line of Sight (LoS) principle due to the characteristic of the VHF band, in other words, there should be no obstacle between the VOR station and the air platform, and they should be in direct sight of each other.
VOR devices are mostly used together with the DME (VOR/DME), which indicates the distance to the station, and the frequency selected for the VOR is also connected to the DME station. Thus, both radial tracking and distance information are provided.
VOR General Information
All VOR stations take 000 degrees as a reference for magnetic north. With the devices (HSI, CDI…) on the aircraft, magnetic bearing information according to the station, in other words, radial information can be obtained or the head that needs to be held to go to the station can be found. For example, if a plane is at 090 radial from the station, the plane is just east of the station.
TO and FROM information
An aircraft can be located in one of two different sectors relative to a selected VOR radial (relative to the ground station):
FROM: If the aircraft is within the selected radial +/- 90 degree area, it is in the FROM (from station) sector.
TO: The aircraft is in the TO (to station) sector if it is “not” within the selected radial +/- 90 degree area.
For example, if 110 radial is selected in the device on the aircraft -regardless of the head of the aircraft- when the aircraft is within the sector from 020 degrees to 200 degrees according to the station, FROM is read on the display, otherwise the phrase TO is read. VOR information is independent of the direction and the position of the CDI and TO/FROM information do not change regardless of the head of the aircraft.
While passing over the VOR station, information cannot be received or the information received cannot be trusted. This area on the VOR stations is called the cone of silence, and when the device is in this region, where the phrase TO FROM is, the phrase FLAG (flag), indicating that the information cannot be received or is not reliable, is seen. After the station is passed -as the sector changes- TO information returns to FROM and FROM to TO.
1. Aircraft Communications and Navigation Systems, M.Tooley, and D.WYATT, First Edition 2007, Routledge, NY, USA.
2. Operational Notes on VHF Omni Range (VOR) Contents, Civil Aviation Safety Authority, Australia.
3. Navigation Instrumentation – VOR, IVAO HQ training department, 2016.