My error, typo I meant GMSK!!
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The narrowest channels are 600 bps MPSK, although in Inmarsat parlance they call it "A-BPSK". There are 1200, 4800, 10200 bps channels as well, plus faster ones I have not really considered yet.
The spec document describes it all.
I find the signal to be quite strong, perhaps check your polarization or antenna pointing. You'll want RHCP for Inmarsat. Signals are well above the noise floor for me. I'm using an Ettus B200.
Also, check out a program called "JAERO", the web site has lots of relevant information.
My program isn't on github yet, I'm an old timer so I'm developing it locally. I can release it once it's ready!
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So the control channels are the narrow ones? I’m guessing the really wide ones are the traffic. (On a side note, I’m kind of surprised that people [you and killmore231] can get Inmarsat with a patch. I just barely see the signals with 9A4QAV’s helix and an SDRplay. However, Iridium comes in like a blowtorch.) Anyway, can’t wait to see your program. Is it on Github?
I have an Inmarsat demodulator working for the lower speed control channels. I am writing a program to decode the traffic, but this is not a trivial task. Think on the order of 1000 lines of C. It is "almost" working, I am hoping for valid output in the next week or so, depending on my free time.
My program is currently a standalone program, but once it's done it could conceivably be converted to one or more gnuradio blocks.
I suggest the following document to give you background on the channel specifications for Inmarsat:
The demodulation is fairly easy, read the spec document and the channel characteristics are pretty well described. My gnuradio demodulator is based on the GMSK demod block. I'm using a home brew patch antenna, and the LNA4ALL low noise amplifier.