Playing a bit with the RTL-SDR stick I hoped to decode APRS traffic.
APRS is Automatic Packet Reporting System
a standard for radio amateurs to broadcast digital messages reporting
location, status, weather and lots of other things. Nowadays a lot of
traffic is location information, but there is a lot more. Information is
transmitted at 1200 bps AFSK Frequency shift keying - Wikipedia
so it can easily be
handled by equipment for FM phone transmissions.
Looking around for options for decoding APRS traffic with gnuradio
I ran into SDR with Raspberry Pi and DVB-T receivers - JPMeijers.com
where it was described to skip a lot of steps and use
utility from the rtl-sdr source and multimon
to do the decoding.
Reception of the signal via the NOXON DAB+ USB dongle
is very noisy but playing with gain settings helped in eventually getting
something decoded. Signal via the Wouxun transciever sounds a lot
better. So maybe an audio cable from the Wouxun to a sound-card is in order
to help with decoding this and other digital signals.
$ rtl_fm -f 144800000 -s 22050 -o 4 -g 100 -C - | tee >(nc -l 8080) | multimon -a AFSK1200 -t raw /dev/stdin
multimod (C) 1996/1997 by Tom Sailer HB9JNX/AE4WA
available demodulators: POCSAG512 POCSAG1200 POCSAG2400 EAS AFSK1200 AFSK2400 AFSK2400_2 HAPN4800 FSK9600 DTMF ZVEI SCOPE
Enabled demodulators: AFSK1200
Found 1 device(s):
0: NOXON, DAB Stick, SN: 0
Using device 0: Terratec NOXON DAB/DAB+ USB dongle (rev 1)
Found Fitipower FC0013 tuner
Oversampling input by: 12x.
Oversampling output by: 4x.
Buffer size: 7.74ms
Tuned to 145064600 Hz.
Sampling at 1058400 Hz.
Output at 22050 Hz.
Exact sample rate is: 1058400.010094 Hz
Tuner gain set to 100.00 dB.
AFSK1200: fm PE1REO-9 to APERXQ-0 via PI1APU-0 UI pid=F0
And indeed: PE1REO
shows up on aprs.fi
command is to check audio with
$ nc thompson 8080 | play -t raw -r 22050 -e signed-integer -b 16 -c 1 -V1 -
parameter in rtl_fm
controls the gain, I had to search
for a value which gave me a reasonable signal, at -g 100
of the databursts were decoded. It helps to hear the audio that rtl_fm
is decoding to get an idea of reception.
It is truly amazing that this all can be done in software, with really
My general idea is to be able to decode other digital amateur radio signals
such as the telemetry from amateur radio balloon flights.