VOR Principles and Operation

VOR Principles Explained

VOR navigation relies on transmitting two signals: a reference phase (omnidirectional) and a variable phase (rotating or simulated rotation). The airborne receiver measures the phase difference between these signals, which corresponds directly to the aircraft's bearing from the station. This allows pilots to determine their radial—essential for position fixing and course tracking.

Types of VOR Stations

• Conventional VOR (CVOR): Uses a rotating antenna to create the variable phase signal.
• Doppler VOR (DVOR): Employs fixed antennas and the Doppler effect for improved accuracy and reduced site errors.
• En-route VOR: High-power stations for airway navigation, typically spaced about 80 NM apart.
• Terminal VOR (TVOR): Lower-power, short-range stations (about 25 NM) used near airports for approach and departure procedures.
• Test VOR (VOT): Special ground facilities for checking VOR receiver accuracy, transmitting a constant phase difference.

How Does VOR Work?

• The reference phase is transmitted omnidirectionally.
• The variable phase is transmitted in a way that simulates rotation (physically or electronically).
• The phase difference between the two signals, as received by the aircraft, indicates the bearing (radial) from the station.

Airborne Equipment

A typical VOR installation in an aircraft consists of:

• Antenna: Mounted to optimize reception of horizontally polarized VHF signals.
• Receiver: Tunes and decodes the VOR signal.
• Indicator: Displays the selected radial and deviation, such as an OBS, RMI, or HSI.

Identification and Monitoring

• Each VOR transmits a Morse-code identifier and sometimes voice or ATIS information.
• Stations are equipped with automatic ground monitoring. If the signal goes out of tolerance, the station removes its identification and navigation signal, alerting pilots not to use it.

VOR Frequencies

• Operates in the VHF band: 108.00–117.95 MHz.
• Lower part of the band (108–112 MHz) is shared with ILS; only even 100 kHz channels are used here.
• Above 112 MHz, 50 kHz spacing is possible.

Using VOR

Pilots tune the frequency, identify the station via Morse code, and use the indicator to select and fly desired radials. VOR navigation is immune to many errors affecting older systems, but accuracy depends on line-of-sight and remaining within the published service volume.