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[Discuss-gnuradio] EPIRB signal decoding, I/Q data

From: Ludovico Cavedon
Subject: [Discuss-gnuradio] EPIRB signal decoding, I/Q data
Date: Wed, 03 Jan 2018 02:03:51 +0000


I am trying to wrap my head around a project, and I have a bunch of question I am not able to find an answer for. Hopefully someone can point me in the right direction.

I am trying to decode an EPIRB signal [1] with gnuradio. The signal (400 bps) is phase modulated +/- 1.1 rad with the data encoded using Manchester encoding.
It starts with 160 ms of unmodulated carrier, followed by 15 bits set to 1, followed by a frame synchronization sequence, followed by the actual data.

I got an I/Q recording of such signal from [2] (if you have more of those, would you mind sharing them with me, please?).
The first question I have is looking at that signal: it looks like the there is a 1.4kHz signal being phase modulated. Why is that? Is that just because the SDR receiver was centered at a frequency 1.4 kHz lower than the actual signal frequency? Was that maybe to avoid DC bias?

I am studying the flowgraph at [3] for the decoding, hopefully I can make some progress with that.
However, the I/Q recording that I have has some initial time with no signal, then the unmodulated carrier starts. Is the "PLL Carrier Tracking" block supposed to work despite the initial silence, or do I need to trim it out of the recording? If I need to remove it, how would I achieve the same in "real-life" where I am acquiring data from a SDR receiver tuned to 406.25MHz? I would need a way to "start" that part of the gnuradio workflow only when the carrier appears...

Thank you in advance for your help,

[1] http://www.cospas-sarsat.int/images/stories/SystemDocs/Current/cs_t.001_oct_2013.pdf
[2] https://www.sigidwiki.com/wiki/File:Epirbsignal.zip
[3] https://lists.gnu.org/archive/html/discuss-gnuradio/2016-03/msg00208.html

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