[amsat-bb] Doppler shift and oscillator offset

[Peter Kazakoff]

Hi folks,

I'm in the process of designing the modem for a university-developed cube
satellite. We'd like amateurs to be able to use the packetized data channel
for interaction with the satellite. The plan is for amateurs to be able to
do stuff like initiate file downlink (there's a small camera on board),
write text files for use as a message board, query the telemetry files, and
possibly route packets from ground -> satellite -> ground.

Right now I'm trying to gauge the capabilities of amateur ground stations.
Obviously, the ground stations will be attempting to correct for Doppler
shift, but there's only so far that can go. Does anyone have a good idea
what the typical velocity error is between a NORAD TLE and a cube satellite
TLE? This will go into my maximum frequency error specification.

The other issue, frequency wise, is oscillator drift and offset error. The
satellite will be using a very stable Stratum-3 TCXO with 280 ppb or better
stability, which gives me good accuracy on the satellite side (within 100
Hz or so on 70cm). The other side of the link would be the amateur
transceivers. What's a reasonable oscillator stability spec for a typical
SSB transceiver used on amateur satellite frequencies?

For modulation scheme I'm leaning towards either pi/4 DQPSK or standard
QPSK with a BPSK barker code preamble on each packet. Both schemes will use
convolutional coding for FEC. Which scheme I go with depends on which ends
up being more robust under severe doppler while still fitting into hardware
constraints - the comms system is all going to be done in SDR. Suggestions
for alternate schemes are welcome though.

Thanks for your help,

-- 
*Peter Kazakoff*
ECOSat <http://csdc.uvic.ca/> Communications Lead
4th Year Electrical Engineering Student
University of Victoria
*(250) 920 - 6870*