Re: [amsat-bb] PSK-31 Satellite Donwlink Opportunity?

[tlang@xxxxxxxxxxxxxxxxxxxx (Tony Langdon)]

Hello Bob!

25 Feb 01 12:35, you wrote to All:

 BB> There may be an opportunity to fly a PSK31 Satellite 10m downlink
 BB> on a satellite in the next several months.  On 10m the total doppler
 BB> is less than 600 Hz...  So users with a FIXED tuned 10m SSB
 BB> receiver could easily copy 20 or more QSO's without tuning for
 BB> Doppler.

PSK-31 has a lot of potential here.

 BB> With the new PSK-31 software that can actually MONITOR 20 simultaneous
 BB> QSO's in real time (W1SQLPSK), whole new concepts in Amateur Radio
 BB> COmmunications are opening up...  But in such wideband code, I guess
 BB> the narrowband  (55 Hz?) processing gain still applies to each
 BB> channel?

That would probably depend on the implementation, but I feel it is likely.

 BB> My first cut assumptions for these unknowns might be 15 dB sky noise,
 BB> 6 dB SNR required in a 60 Hz bandwidth using a -12 dB gain antenna...?

One problem I have encountered with PSK-31 in real world conditions is that it
responds poorly to deep fades, due to the time it takes for the decoder to lock
onto the signal.  While the conditions we tested under are probably more severe
than a 10m satellite link (we were doing weak signal tests between a base and a
pedestrian mobile station on 2m), it is something to take into consideration.
Allow some fade margin into the link budget (any of the RS-12/13 ops have any
idea of a sensible fade margin on a 10m satellite link?).

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

Would these be live QSOs or basically a fancy form of a 'beacon'?  If the
latter, why not use a digital audio storage device to store a loop of PSK-31?
This is the sort of device we used for the pedestrian mobile tests.  As
compression wouldn't be a good idea in this case, you'd been a bit of RAM,
enough for 30 secs to a minute's worth of audio.

 BB> This is ONLY an experiment to demonstrate the practicality of the
 BB> low power, low doppler 10m downlink for possible future designs
 BB> of an INBAND-INVERTING PSK-31 transponder that would have hardly
 BB> any doppler at all (45 Hz?).  Again, this experiment is designed to
 BB> VALIDATE the downlink, not to be a practical TRANSPONDER for general
 BB> QSO's, since its single wideband FM uplink requires pre-combined
 BB> signals for uplink from single participating ground stations.

The near zero Doppler is only true for (1) the uplinking station and (2)
stations close to the uplinking stations.  Others will see more Doppler shift.

E.g.  Station just after AOS transmits signal.  It appears at the satellite
approx 600 Hz above its original frequency.  Inverting transponder retransmits
signal at 600 Hz _below_ the expected frequency, which is received another 600
Hz (total 1200 Hz) lower by a station near LOS...

 BB> But it IS  a free opportunity to maybe fly this experiment and show
 BB> the power of PSK-31 and sound card software to bring Amateur
 BB> Satellites to more people...  It could easily be used (as is) for
 BB> distributing multiple simultaneous amateur radio bulletins...

Agreed, and this would be a good experiment to try.  PSK-31 is a mode which is
easy to generate and has good weak signal properties.


Tony, VK3JED
--
|Fidonet:  Tony Langdon 3:633/284.18
|Internet: tlang@freeway.apana.org.au
|
| Standard disclaimer: The views of this user are strictly his own.


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