[amsat-bb] marks from space

Lizeth Norman normanlizeth at gmail.com
Fri Aug 22 10:08:41 UTC 2014

Hi all!
Latest craze is trying to decode 1200 baud ax.25 packets from
satellites. Satellite telemetry is not a busy terrestrial packet
frequency. As the number of chances that a ground station get at
receiving are limited and the environment different in the extreme,
I'd like to get some input on how recover more data.

Will concentrate on using a Fun Cube dongle pro + as the receiver.

Have never had much success. Received a few frames from the ISS with a
Byonics Tiny Tracker 4. Last time I chased beepsats was a year and a
half ago. Blew up the rotator control box it spun around so much.
Since then, it's been one failure or another. Nothing has torched
itself in a week or so. It's time to be daring.

Found a (software modem) program called "Dire wolf" amongst others. It
does a good job on the local APRS/packet traffic. I will try my TT4
again at some point. Did ask a similar question on that mailing list.
Also asked about the 9k6 TT4 rx feature. Was told that it was a
software switch, nothing more needed to be done. Enabling it should
allow the tnc to decode the data. Since it won't encode that rate,
setting one up with a pc in another part of the house and using remote
desktop to monitor/control it and test isn't an option.

Doing some reading about software decoders gives me the impression
that if the center of the passband is off 100 Hz or more, the
algorithm that decides what is a "mark" and what is a "space" fails.
How does a hardware modem compare? Are they different?

Obviously, ensuring the receiver is calibrated and centered on the
correct frequency with the correct mode/ IF filter is one factor.
Another is transmitter LO temperature drift. Doppler correction is a
factor. Amplitude of the audio input is a variable.

There is a copy of virtual audio cable on my pc. It is possible to
route the signal from the receiver to an audio spectrum analyzer as
well as the software modem. Would this be any help troubleshooting?

Norm n3ykf

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