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Re: AMSAT has a SECRET!



>Yes there is computer tuning. But I don't think the update speed of
>the current doppler tracking software is sufficiently fast or
>accurate enough for the prediction type AFC's (as opposed to feedback
>AFC's such as on AO40RCV) on the LEO's at TCA.

Then you merely need to switch to a modulation scheme that allows
closed-loop doppler tracking. SSB is clearly not the right choice here.

>I'm sure there's better samples, but a couple of months ago I put up
>some scratchy UO-11 and AO-16 S-Band wav files on my site where you
>can hear the rate of doppler. The second one has me attempting to
>manually keep the tuning centered on a PSK-1.

Doppler tracking becomes considerably easier as the receiver
bandwidths and data rates go up.  If you can make your data rate
substantially larger than the peak-to-peak dopper shift, an ordinary
Costas loop will easily track it out.

And where are the link budgets good enough and the available
bandwidths large enough to support high rates? On the microwave bands!

If you still lack the link budget for a high data rate, other
approaches to closed-loop doppler tracking become practical if your
receivers are wide enough and your demodulators are clever enough.

The false lock problems that have long plagued low-speed amateur
satellite PSK demodulators are merely the fault of their
implementation and the perception that narrow band receivers will
always be used because that's all we've ever had available.

Phil



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