[amsat-bb] Need help decoding EO-79 packets.

[Edson W. R. Pereira]

The AX.25 implementations used in some cubesats do not implement the specs
for AX.25 destination address field encoding correctly. The last bit of the
last byte of the destination callsing (or digipeater) field should always
be 1 in order to indicate the end of the destination field, but some
implementations keep the bit as 0. This is the reason UZ7HO's soundmodem
says NON-AX25 frame. Soundmodem does pass the frame to the KISS interface
but any decoding software must be careful during the processing of the
destination callsign (by assuming there are no digipeaters).

Comparing the performance of the 1200 bps BPSK FUNcube-1 FEC encoding with
cubesats using AX.25 1200 bps BPSK makes me wander why in this day and age
we still use AX.25 for satellite links.

73, Edson PY2SDR


---
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On Thu, Apr 21, 2016 at 7:56 PM, J. Boyd (JR2TTS) <
the2belo at msd.biglobe.ne.jp> wrote:

> On Thu, 21 Apr 2016 17:43:36 -0500, "JoAnne Maenpaa" <k9jkm at comcast.net>
> wrote:
>
> > > There were some other things in my Soundmodem settings that did not
> > > agree with the instructions for DeorbitSail, that may have messed it
> up,
> > > so I'll try again this morning during the next pass.
> >
> > I'm continuing to experiment by myself on this subject of receiving
> > telemetry data using an SDR dongle and software. I've appreciated all the
> > bits of information offered by the stations who are already up and
> running.
> >
> > One thing I'm finding with software packet reception is that the software
> > "modem" or software "TNC" (i.e.: soundmodem) seems to expect packets
> with a
> > good checksum. Just a single bit-flip and I don't have data.
>
> In soundmodem, I had "Bits Recovery" set to "SINGLE" initially, while
> the DeorbitSail page seems to suggest it's more properly set to "NONE".
> Not sure if that's significant, but your comment about flipped bits made
> me suspect this. Anything's worth trying once.
>
>
> --
> J. Boyd, JR2TTS/NI3B
> the2belo at msd.biglobe.ne.jp
> http://www.flickr.com/photos/the2belo/
> http://www.qrz.com/db/JR2TTS
> Twitter: @Minus2_C
>
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