I want to design a GNU Radio project to extract the information of a VOR signal, then find the angle to the north and the Morse code of the station.

Is there any example that shows how to do it?


I'm not aware of any existing application.

Azimuth Estimation

However, wouldn't VOR (VHF Ominidirectional Ranging) consist of two relevant signals:

  1. the reference signal, a 30 Hz signal frequency-modulated onto a carrier not too far of the nominal system frequency
  2. the rotating beam signal, which is phase-delayed proportionally to the azimuth

with the direction information in the phase difference between reference signal and beam signal?

so your first implementation would look something like

  1. demodulate the FM signal and estimate the phase
  2. estimate the phase of the pulsed beam signal at its maximum
  3. compare these two

A more stable second implementation would phase-lock an internal oscillator to the reference signal. An even more advanced receiver could then e.g. apply a Kalman filter to the observed azimuth.

Morse Decoding

I'm also not aware of any Morse/CW receiver in existence (there probably is – a good idea is just asking for this isolatedly on http://ham.stackexchange.com , or the GNU Radio mailing list.

Decoding human transmissions is probably non-trivial, since humans tend to have suboptimally stable symbol clocks, but an automatic transponder might be relatively easy to decode – just find the shortest reasonable "dit", everything significantly longer is a "dah".

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  • 2
    $\begingroup$ Decoding a human operator is relatively simple too: Start with a reasonable assumption for a threshold, maintain a histogram of tone durations, it will be bimodal, one mode around the short durations, one mode around the long durations. Put a threshold between the modes. Update threshold periodically. $\endgroup$ – A_A Jan 23 '17 at 20:20
  • $\begingroup$ @A_A I must admit that the hardness is kind of a claim I adopted from more active ham operating friends. They might have especially referred to low-SNR situations, where you get a high uncertainty about detection in the first place. Intuitively, I'd agree with you. It doesn't seem hard at all. $\endgroup$ – Marcus Müller Jan 23 '17 at 21:31

Maybe this can help: https://github.com/hpux735/VOR-SDR http://www.f4gkr.org/tag/vor/ I've used some code from the the last link to do some calculations on VOR samples.

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