[amsat-bb] going digital
karn at ka9q.net
Mon Jul 21 03:18:43 UTC 2014
On 07/20/2014 06:13 AM, Simon Brown wrote:
> If both side have good Doppler correction then it can be done, but you have
> to choose the correct mode as there are other issues even if the Doppler
> correction is perfect, especially when the range is changing at its maximum,
> for example as the satellite passes overhead.
Correct, but Doppler actually gets much easier to deal with as the data
rate increases. What matters is the *ratio* of the Doppler to the data
rate (actually the channel symbol rate, which is usually higher).
At the very high rates possible from LEO on the microwave bands, Doppler
on a BPSK signal can be easily tracked by a Costas loop even without
orbit prediction steering (though it's still a good idea).
E.g., a relative velocity of 3 km/s at 10 GHz is a Doppler of 100 kHz,
considerably less than the symbol rates possible from LEO with even
small transmitter powers and antenna gains. Compare this with the
numbers for current amateur satellites on VHF and UHF.
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