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DSP software on Sunsat SO-35

Hi all,

Sunsat is equipped with a high resolution imager capable of filling 
the 64 MB ramtray in a matter of seconds.  However, because of the 
failed 40 Mbit/s S-band downlink, we are forced to download data at 
only 9.6 kbit/s.  With about 40 minutes of useful download time per 
day this is getting extremely boring (not to mention the frustration 
for not being able to capture such wonderful images more often).

We are investigating ways to better utilise the NBFM data channels 
available on VHF and UHF.  The satellite's 40 MHz Motorola DSP56L002 
platform has been commissioned at the end of last year.  The SNR on 
the 8W downlink is extremely favourable (it still has to be measured 
for capacity calculations).

The effective data rate could be increased by pre-processing of the 
image data and/or by implementing more complex modulator-demodulator 
pairs.  A simple compression routine running on the DSP already 
results in an effective rate of 15k36 bit/s.  Because of the high SNR 
the 9k6 modems can also be overclocked with limited success (up to a 
30% increase in the data rate before ISI becomes a limiting factor).  
This option could be pushed a little further by increasing the IF 
bandwidth of the receivers.

Any pointers to existing DSP56000 code for compression algorithms 
(like JPEG, wavelet and the like!) and/or modems implementing complex 
constellations would be most welcome.  Also ideas on the suitability 
of ITU recommendations for telephony modems (such as V.34).  Can 
these protocols be used for simplex transmissions?  Are there maybe 
other ways to increase the data rate?

Johann Lochner, ZR1CBC

   JG Lochner  ESL, Universiteit van Stellenbosch
   e-pos:      lochner@ing.sun.ac.za
   webtuiste:  http://esl.ee.sun.ac.za/~lochner

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