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Re: PSK-31 Satellite Donwlink Opportunity?





Bob:

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.

If it is a mode A (2m up and 10m down), and
it is an inverting transponder to remove some
of the Doppler,  you will still have over 2 KHz
maximum Doppler offset.  Sorry!

Bob
N4HY



> Satellite and PSK-31 builders...
> There may be an opportunity to fly a PSK31 Satellite 10m downlink
> on a satellite in the next several months.  On 10m the total doppler is
> less than 600 Hz...  So users with a FIXED tuned 10m SSB receiver
> could easily copy 20 or more QSO's without tuning for Doppler.
>
> To calculate the minimum downlink power needed, I need to know practical,
> (not theoretical) values for current implementations...
>
> 1) What is the "typical" noise level on 10 meters with a 1/4w vertical?
> 2) What is the minimum actual SNR required by most PSK31 decoders
> 3) What is the effective noise bandwidth of most PSK-31 implementations?
> 4) What is the gain (- dB) of a 38" dipole matched to 10m.  Thats
>    the only antenna we have on the satellite that could be used...
>
> With the new PSK-31 software that can actually MONITOR 20 simultaneous
> QSO's in real time (W1SQLPSK), whole new concepts in Amateur Radio
> COmmunications are opening up...  But in such wideband code, I guess the
> narrowband  (55 Hz?) processing gain still applies to each channel?
>
> My first cut assumptions for these unknowns might be 15 dB sky noise, 6 dB
> SNR required in a 60 Hz bandwidth using a -12 dB gain antenna...?
>
> 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!
>
> This is ONLY an experiment to demonstrate the practicality of the
> low power, low doppler 10m downlink for possible future designs
> of an INBAND-INVERTING PSK-31 transponder that would have hardly
> any doppler at all (45 Hz?).  Again, this experiment is designed to
> VALIDATE the downlink, not to be a practical TRANSPONDER for general
> QSO's, since its single wideband FM uplink requires pre-combined signals
> for uplink from single participating ground stations.
>
> But it IS  a free opportunity to maybe fly this experiment and show the
> power of PSK-31 and sound card software to bring Amateur Satellites to
> more people...  It could easily be used (as is) for distributing multiple
> simultaneous amateur radio bulletins...
>
> Even as is, dedicated ground stations in each of the amateur radio
> countries could collect streams of PSK-31 data from schools, clubs
> and many sources to uplink for everyone in the footprint to receive..
> This collection could be via the Internet or a variety of other
> means.
>
> I already have the wideband FM UHF receiver so All I need is someone
> to build the small 30 milliwatt or so SSB exciter for 29.4 MHz.
> THey should be able to build it in a few inches of space...  My
> UHF receiver is about the size of a large postage stamp...  We would
> use an audio baseband of about 2 KHz.
>
> I cannot promise anything, but at this point, it looks like we may
> have some spare payload space on a satellite that is only 6 months away
> from launch, and I would sure like to use it for a meaningful experiment.
> And I think it is time to try this new narrowband, user-friendly
> mode... and demonstrate the value of 10m downlinks for their low doppler
> and low satellite power potential.
>
> de WB4APR, Bob
>
>
>
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