I'm a new ham. The past few days, I've had a few nearby passes of the International Space Station. As it was going overhead, I managed to record what I believe to be 1200 baud AFSK transmissions from it (AX.25) - from it's 145.825MHz Packet Radio downlink. I have an audio file of one. I am trying to figure out how to decode it and haven't had any luck. I've tried QTMM from SourceForge (playing the recording into my computer microphone) - but got no decode.

I have an audio file which has three distinct burst of what appear to be data (with very little noise during these bursts). Both nights, these burst only occurred while the station was nearest to me. Thus, I'm positive I have good audio - just can't figure out how to demodulate!

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    $\begingroup$ How did you make the recording? Not with a microphone I hope...and not saved in a lossy format (mp3), I hope. $\endgroup$ – Phil Frost - W8II Aug 6 '14 at 16:06
  • $\begingroup$ Microphone :( don't have the right cable. Nonetheless, I have many samples and they all seem very clear. But even the waveform looks good. However, looking at the signal and taking measurements, it looks more like a QPSK signal than an AFSK - so I'm a bit suspicious... $\endgroup$ – Brad Aug 6 '14 at 17:03
  • $\begingroup$ Well, from the brief research I've done it looks like that frequency from the ISS should be AFSK. I'd suspect that your recording is not of sufficient fidelity to be decoded, and I'd also question whatever methods you are using to analyze the signal and conclude that it is QPSK. $\endgroup$ – Phil Frost - W8II Aug 6 '14 at 17:08
  • $\begingroup$ I'd suggest you make your audio file available somewhere on the internet, so we can have a 'look' at it. I like a challenge! $\endgroup$ – jcoppens Sep 20 '14 at 18:59
  • $\begingroup$ s3.bradgoodman.com/Iss2_pkt2.wav s3.bradgoodman.com/Iss2_pkt3.wav s3.bradgoodman.com/Iss2_pkt4.wav Thanks! ;-) $\endgroup$ – Brad Sep 24 '14 at 13:33

Audio which sounds perfectly fine can be noisy or distorted (notably, by FM deemphasis) enough to prevent digital decoding.

At a minimum, you should get rid of the microphone step and feed your audio directly into the demodulator program.

You don't say what OS you're using, but I have had success demodulating APRS messages using multimon-ng. It can be set to accept raw input (16-bit at 22050 Hz); you can use an audio editor (such as the free Audacity) to convert your recording.

(Note that multimon-ng will take the AFSK audio signal and give you text-format AX.25 messages, but it will not actually parse the APRS content into human-readable text except as APRS has some readable fields already; you would need another program to handle that. At least you'll be able to see if the messages are recoverable at all.)


Just a suggestion, but have you considered the possibility of it being an SSTV transmission? I would download an SSTV decoder and run it through that, using various encoding methods (e.g. Robot32, PD180, etc.) and see what you get.


I had a problem with APRS reception from ISS too. It seems I got frequency shift in received data. Slowing down by 2% (deduced from looking at the spectrum - ymmv) in Audacity helped some, but then I found software with adjustable frequency on site by UZ7HO here: http://uz7.ho.ua/packetradio.htm and there is soundmodem application (file sm2ch57.zip). I set up baud rate to 1200 and then frequency as to have the most packets received. Around 1750 Hz in my case. I got packets decoded with recording both through cable onto my digital voice recorder and with it next to my scanner. But I plugged it through line-in for recording on computer in both cases. My computer gives off a hellish amount of interference at frequencies in 2m band so plugging scanner directly was not an option.


If you are on windows... go with QTMM


Play it in Audacity and make the output the input of qtmm

  • $\begingroup$ Excellent tip, I used QTMM's afsk1200dec program with great success to decode APRS packets. $\endgroup$ – captcha Sep 21 '14 at 22:32

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