Re: [Discuss-gnuradio] INMARSAT wide signal question

[Henry Barton]

The wide signal in question is about 168 kHz. I’ve been able to see several side-by-side like in other people’s pictures by using 9A4QAV’s helix.

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From: Henry Barton
Sent: ‎Wednesday‎, ‎May‎ ‎4‎, ‎2016 ‎1‎:‎08‎ ‎PM
To: address@hidden

Thanks, Ian. So the I prefix is Inmarsat?

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From: Ian Buckley
Sent: ‎Wednesday‎, ‎May‎ ‎4‎, ‎2016 ‎12‎:‎47‎ ‎PM
To: Kevin McQuiggin
Cc: Henry Barton, address@hidden

Henry, 

Its important to realize that there are 3 generations of Inmarsat systems currently in use. The I3 birds carry a lot of narrow band services for air to ground and ship to shore as Kevin just described. As you move to I5 via I4 you see higher, more modern broadband service start to appear.

-Ian

On Tue, May 3, 2016 at 9:34 PM, Kevin McQuiggin <address@hidden> wrote:

My error, typo I meant GMSK!!

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On May 3, 2016, at 9:31 PM, Kevin McQuiggin <address@hidden> wrote:

Hi Henry:

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!

Kevin

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On May 3, 2016, at 8:31 PM, Henry Barton <address@hidden> wrote:

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?

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From: Kevin McQuiggin
Sent: ‎Tuesday‎, ‎May‎ ‎3‎, ‎2016 ‎10‎:‎02‎ ‎PM
To: Henry Barton
Cc: address@hidden

Hi Henry:

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:

http://www.icao.int/safety/acp/inactive%20working%20groups%20library/acp-wg-m-iridium-8/ird-swg08-wp07%20-%20old_amss_material_ch.4_plus_attachment.doc

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.

Kevin

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On May 3, 2016, at 12:45 PM, Henry Barton <address@hidden> wrote:

http://rtlsdrblog.rtlsdrblog.netdna-cdn.com/wp-content/uploads/2015/10/sdrplay_lband-1024x573.png

I didn’t see the widest signal (bright yellow) from this picture on sigidwiki.com. Does anyone know what it is? If so, can it be decoded by GNUradio? Thanks.

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