Demodulating slow phase-modulated data

[Philip Pemberton]

Hi all,

I'm working on a project to reverse-engineer the data format used by an old navigation system called Datatrak. I've got a project page on my website about it: https://www.philpem.me.uk/datatrak/start

The data is sent by phase-modulating a ~133kHz or ~146kHz (longwave) carrier at a very slow rate -- the modulating wave is sinusoidal, either 37.5Hz or 50Hz depending on the part of the signal being sent. The signal bandwidth is about 200Hz.

I've been given some single-channel WAV format recordings from LW SDR receivers, 44100Hz sample rate, with a LO frequency of 128kHz. Sadly they're not I-Q recordings. I'd like to try and demodulate the signals in these recordings to recover the 64 bits of "Goldcode" synchronisation code (from the Trigger slot) and the 128 bits of Clock signal (from the Clock slot). I already know what the Goldcode value should be.

Can anyone suggest a way I could do this with Gnuradio/GRC or some other tool?

I figure I need to convert the WAV file into a complex (I-Q) signal, mix the signal of interest down to baseband, then filter it with a 200Hz bandpass filter. I'm not sure what I need to do after that to recover the modulating phase signal.

Thanks,

--

Phil.
philpem@philpem.me.uk (preferred)
philpem@gmail.com (alternate)
http://www.philpem.me.uk/