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[Discuss-gnuradio] Re: Mode S and ADS-B

From: Eric A. Cottrell
Subject: [Discuss-gnuradio] Re: Mode S and ADS-B
Date: Fri, 13 Apr 2007 16:08:32 -0400
User-agent: Thunderbird (X11/20060911)


I am making good progress.

The first couple of weeks or so (months?) I was collecting information.
 I managed to get some design articles about Mode S transponders,
various protocol specifications, and comments.  I was able to decode
some packets by sight using the oscilloscope program.  The bit timing is
very tight.  Thanks go to Matt Ettus for designing a board that can
handle a 4 MSPS sampling rate.  It was now coding time!  I reviewed some
GNURadio design guidelines and how to write a module.

Of course the name is most important.  I decided on a subdirectory off
gnuradio named gr-mode-s.  gr-transponder would make it look like
sending as well as receiving.  My project is focused on Mode S data and
derivatives (TCAS, ACAS, ADS-B, TIS-B, etc).  I am open to a better name
as the mode-s name forced me to use the prefix of mode_s for things.

I decided to use gr-pager as the model as it is the closest to what I
want to do.  I eliminated some blocks but may put them back in.  I
basically have a slicer, decoder/framer, and parser C++ blocks.  I use
python code to put the blocks together into a mode S decoder superblock.

I appreciate the autoconf/automake and the whole gnuradio build scheme.
   For a programmer who has only done small gcc projects it looks like
it is real work to do things from scratch.  I was able to figure out
what most of the autoconf/automake files are for.  I replaced the pager
prefixes with mode_s, add the required .m4 files, etc.  I was amazed
that I got my new project integrated into the build tree quickly.  The
major part of the time was spent getting my code to work.

So I am jumping up and down with joy as I can now decode Mode S.  I can
now work on improving the slicer, add decoding of the messages, etc.  It
seems like I am not getting hundreds of miles of range due to the
antenna setup but I get enough to work on the decoders.

A future to-do is to submit my code.  I guess I need to join the patch list.

It would be neat if I could get hardware that is smaller sized.  Maybe a
USRP Jr board that allows one receiver or one receiver and one
transmitter board,  I could also use one with a LFRX board and dedicate
it to my AR5000.

73 Eric

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