A VHF Omni-directional Range (VOR) is used as a navigation beacon for aircraft. Aircraft with a receiving unit will be able to determine their position relative to the VOR ground station. The ground station sends a highly directional signal, by making use of a so called phased array antenna. Together with this signal, the VOR sends a 30 Hz reference signal which is equal in all direction.

The phase difference between the reference signal and the highly directional signal, is the bearing from the VOR station relative to Magnetic North. A VOR receiver works by phase comparison. The VOR compares the phase difference between a reference signal and a variable signal.