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Re: PSK-31 Satellite Donwlink Opportunity?

On Sun, 25 Feb 2001, MCGWIER ROBERT wrote:

> You have forgotten the contribution to Doppler caused by the uplink.  
> Even with an inverting transponder, and practically no frequency
> tracking in most of the PSK-31 implementations <<that I know about>>,
> this will be a difficulty.


Sticking my neck out, as I have *NO* experience designing receivers or
other flight hardware... 

WB4APR said:

> > On this short fuse experiment, we would simply put 20 PSK-31 audio
> > streams into a single UHF FM uplink having an onboard wideband (no
> > doppler) receiver.  THus, the only doppler is the small 600 Hz
> > experienced on the 10m SSB downlink.  The PSK-31 software on the ground
> > can EASILY track this, since ALL QSO's move together!

I think WB4APR Bob's statement about doppler shift is correct *if* the UHF
uplink receiver is designed as follows:

The receiver is FM.  Use the discriminator output to drive an AFC circuit.  
Assume the UHF receiver and AFC circuit has sufficient bandwidth to
accomodate the audio stream and +/- 9 KHz of doppler shift.  Assume the
uplink has sufficient ERP to overcome the greater noise in an uplink wide
enough to meet this requirement (say 20-30 KHz total).  Convert the signal
to baseband audio before being fed back into the transmitter.

I think this gives us a no-tune uplink.  The uplink receiver is handling
doppler tuning via AFC. 

I recall that the AO-27 uplink uses this approach.  Doppler shift on 145
MHz is +/-3 KHz, so the total receiver bandwidth can be smaller and
therefore provide a better SNR.  I **think** (8+ years is a long time) the
Microsat uplinks use the same approach, albeit again on 145 MHz instead of
435 MHz.

Bob (WB4APR), how do you intend to "process" the uplink - audio bent pipe?
It seems to me that other than a wider receiver bandwidth due to doppler
and therefore greater noise in the uplink, the approach is sound.  What do
you expect the bandwidth to be of the combined uplink signal? (no
experience with PSK31 here.)

73 Steve KA1LM

Stephan A. Greene                            sgreene@patriot.net
Amateur  KA1LM@amsat.org  Grid FM18hx        38 59'83.33"N 77 23'6.15"W

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