What is the VOR reference signal?

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

• en.wikipedia.org/wiki/VHF_omnidirectional_range has a good description Commented Oct 25, 2017 at 15:53
• @mins That's very helpful and complete documentation. The best answer! Thank you for finding time to help me. Commented Oct 25, 2017 at 20:17
• 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)
– mins
Commented Sep 28, 2020 at 10:51

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:

• Blue is 360° sweeping signal
• Green is omni-directional reference signal
• 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! Commented Oct 25, 2017 at 15:42
• 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? Commented Oct 25, 2017 at 18:20
• @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).
– mins
Commented Sep 28, 2020 at 11:05