Field Guide · technology

Also known as: VOR

VOR (VHF Omnidirectional Range) is a short-range radio navigation aid that lets an aircraft determine its magnetic bearing to (or from) a ground station — its “radial” — without any inertial reference. The station radiates two 30 Hz signals whose relative phase encodes direction: one is a fixed reference, the other varies with the compass angle at which the receiver sits. Measuring the phase difference between them yields the bearing directly.1 Operating in the 108–117.95 MHz band, VOR has been a backbone of airway navigation since the 1950s.

aircraft on 060 radial N (000) reference 30 Hz (FM subcarrier) variable 30 Hz (rotating AM) phase difference = radial
A VOR beacon radiates a reference and a rotating signal; their phase difference tells the aircraft which radial it is on.

How it works

A conventional VOR combines two 30 Hz components. The reference phase is carried as frequency modulation on a 9960 Hz subcarrier, so it is identical in every direction. The variable phase is produced by a rotating antenna pattern (or its electronic equivalent) that amplitude-modulates the carrier; because the lobe sweeps around the compass, the phase a receiver sees depends on its bearing. At magnetic north the two are in phase; at due east they differ by 90°. The receiver demodulates both, measures the phase offset, and displays the radial. The signal uses conventional amplitude modulation for the carrier and identity tone, with the reference riding as an FM subcarrier.

The widely deployed Doppler VOR (DVOR) inverts which signal is AM and which is FM to reduce siting errors from reflections, but the phase-comparison principle is identical. Each station also transmits a Morse identifier so pilots can confirm they have tuned the right beacon.

Relevance to SDR

VOR is a classic teaching signal for software-defined radio: an SDR tuned to a local VOR can capture the AM envelope, recover the 30 Hz variable tone, demodulate the 9960 Hz FM subcarrier for the reference tone, and compute a bearing — a compact exercise in AM/FM demodulation and phase estimation. VOR often shares a site and frequency pairing with DME for distance, and it complements the ILS localizer on approach. GopherTrunk is a land-mobile trunking scanner and does not implement VOR bearing recovery; VOR is included here as RF context, not as a GT decoder feature.

Sources

  1. VHF omnidirectional range — Wikipedia, for the VOR phase-comparison principle, the reference/variable 30 Hz signals, the 108–118 MHz band, and the Doppler VOR variant. ↩

See also