[amsat-bb] Re: [psk31] Re:Re;psk31 om ham satellites

[Bob Bruninga <bruninga@xxxxxxxx>]

On Wed, 20 Nov 2002, Butch Mason wrote:

> Bob flat out said my scheme will NOT work because of the high Doppler.
> First, Bob, I in no way want to detract from your effort.  ...I submit
> to you, the Keplarian elements now available for AO40 are sufficiently
> accurate to track within a few Hertz if each user has set his clock to
> an accuracy of one tenth of a second...

Yes, I agree, it "can" be done, and in fact, I would encourage efforts in
that direction.  PSK-31 is a neat weak signal mode...

I was WRONG.  Here is what I was "going" to say in response:

But I was only saying that it cannot be done with existing hardware and
software.  Yes, you can set your PC clock to within a 10th of a second and
yes, there are tracking programs that can tune your radio for Doppler, but
I think that these do it in quantized steps.  Certainly retuning the radio
is a "step function" that will introduce bit errors in reception.

Thus every time the satellite tracking program retunes the radio, there is
a step function change in radio frequency.  The PSK-31 DSP demodulator
does have an AFC, but any step function change in frequency will cause
bit errors until the DSP re-locks.  And all rigs have PLL's that only tune
in discreet steps...

If you re-tune the radio at 1 HZ steps to keep within the AFC of the
PSK-31 software and AO-40 is moving along at a rate of say  2 Hz per
second...then....... oops...

Ah-HAAAAAAA!  you are right!  This is no worse than my LEO PSK-31
transponder on 10 meters!  And Peter Martinez says that PSK-31 should be
able to be set up to follow that kind of rate in its AFC.

So Yes, the Doppler OFFSET is high, but the RATE OF CHANGE at appogee for
AO-40 is no worse than for the 10m bird at LEO altitudes.  I have not had
time to work this out thoroughly, but back of the envelope shows this
might be the case..

de WB4APR@amsat.org, Bob

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