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Meteor-M2 LRPT demodulator
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README.md

README.md

Meteor-M2 Demodulator

This is a free, open-source LRPT demodulator for the Meteor-M2 Russian weather satellite. It supports reading from a I/Q recording in .wav format, and it outputs an 8-bit soft-QPSK file, from which you can generate an image with the help of LRPTofflineDecoder or meteor_decoder.

Please note that, to get the best decoding performance, you should downsample the I/Q recording to about 140KHz.

Compling and installing

As usual, type make to compile the project, make install to install the binary to /usr/bin/. A debug target is available if you want to keep the debug symbols in the executable.

Usage info

Usage: meteor_demod [options] file_in
   -o, --output <file>     Output decoded symbols to <file>
   -r, --symrate <rate>    Set the symbol rate to <rate> (default: 72000)
   -s, --samplerate <samp> Force the input samplerate to <samp> (default: auto)
       --bps <bps>         Force the input bits per sample to <bps> (default: 16)
   -R, --refresh-rate <ms> Refresh the status screen every <ms> ms (default: 50ms in TUI mode, 5000ms in batch mode)
   -B, --batch             Do not use ncurses, write the message log to stdout instead
   -q, --quiet             Do not print status information

Advanced options:
   -b, --pll-bw <bw>       Set the PLL bandwidth to <bw> (default: 100)
   -a, --alpha <alpha>     Set the RRC filter alpha to <alpha> (default: 0.6)
   -f, --fir-order <ord>   Set the RRC filter order to <ord> (default: 64)
   -O, --oversamp <mult>   Set the interpolation factor to <mult> (default: 4)

   -h, --help              Print this help screen
   -v, --version           Print version info

Advanced options explanation

Increasing the PLL bandwidth will allow the loop to lock faster to the carrier, while decreasing it will make the lock more stable.

Increasing the root-raised cosine filter order will slow the decoding down, but it'll make the filtering more accurate.

Increasing the interpolation factor enables better timing recovery, at the expense of filtering accuracy. Typically you'll want to increase the RRC order and the interpolation factor by the same proportion (i.e. multiply them by the same amount).

Live decoding

Meteor_demod now supports live decoding with the help of rtl_fm! Here's how to get it working:

First off, you need to create a named pipe. This is simply what the name implies: a pipe for data. You send data in from one side, and it comes out from the other. It's going to be used to let rtl_fm and meteor_demod exchange samples:

mkfifo /tmp/meteor_iq

Then, you can start up meteor_demod, telling it to read the raw samples from this pipe. Note the -s flag; it must be passed because the stream coming from rtl_fm doesn't specify a sampling rate, so you need to set it manually:

meteor_demod -s 140000 /tmp/meteor_iq

This command will seem to hang, but that's expected. The pipe doesn't open until both the sender and the receiver are connected to it.

So finally, you can start up rtl_fm to capture the live data and send the raw IQ stream into the named pipe. Open another terminal and type in:

rtl_fm -M raw -s 140000 -f 137.9M -E dc -g <gain> -p <ppm> /tmp/meteor_iq

At this point, meteor_demod's ui should pop up inside the first terminal, and the constellation diagram should stabilize within 10~15 seconds.

Once the pass is over, stop the decoding by pressing q: rtl_fm will terminate as well. The named pipe can now be deleted as if it were any other regular file:

rm /tmp/meteor_iq

You can experiment with the sampling rate, as long as you make sure both rtl_fm and meteor_demod are using the same rate.

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