What is the VOR (VHF Omnidirectional Range) reference signal? I have to calculate the spectrum of this signal.

  • $\begingroup$ en.wikipedia.org/wiki/VHF_omnidirectional_range has a good description $\endgroup$
    – Steve Kuo
    Oct 25, 2017 at 15:53
  • $\begingroup$ @mins That's very helpful and complete documentation. The best answer! Thank you for finding time to help me. $\endgroup$ Oct 25, 2017 at 20:17
  • $\begingroup$ See this answer, the reference is a 30 Hz sine, but depending on the type (CVOR/DVOR), the reference is transmitted either as a AM sideband (DVOR) or as a FM subcarrier at 9.96 kHz (480 Hz swing) sent as DSB-SC (CVOR). The other signal is the variable, also a 30 Hz sine. The corresponding spectrum is this one (missing the id tone and voice channel) $\endgroup$
    – mins
    Sep 28, 2020 at 10:51

1 Answer 1


VOR Ground Station is aligned with magnetic North. It emits two signals:

  • a 360° sweeping variable signal
  • an omni-directional reference signal

When an aircraft receives those signals, its receiver compares those and a measures the phase difference. This gives a precise radial position of the aircraft which is displayed on its Omni-Bearing Indicator (OBI), Horizontal Situation Indicator (HSI) or a Radio Magnetic Indicator (RMI), or a combination of two different kinds.

This picture shows the two signals:

enter image description here

  • Blue is 360° sweeping signal
  • Green is omni-directional reference signal
  • $\begingroup$ This is very enlightening! And also I would like to know more about this omni-directional reference signal (blue one). For example if it is sinusoidal, and all other informations for calculating its spectrum. Thank you! $\endgroup$ Oct 25, 2017 at 15:42
  • 2
    $\begingroup$ Trying to fig out why they made the reference signal flash in that gif. I guess that's indicating when it's at 0° phase? $\endgroup$
    – TomMcW
    Oct 25, 2017 at 18:20
  • $\begingroup$ @TomMcW: Yes it's misleading, it looks like the bearing is determined by timing the interval between a north pulse and the maximum strength of a supposed thin beam (and I saw another post on the site where it's interpreted this wrong way). $\endgroup$
    – mins
    Sep 28, 2020 at 11:05

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