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

[tlang@xxxxxxxxxxxxxxxxxxxx (Tony Langdon)]

Hello Bob!

26 Feb 01 12:56, you wrote to Jon Ogden:

 BB> Each TRANSMITTING station can pre-compute the Doppler for his UPLINK
 BB> so that it arrives at the satellite with 0 doppler.  And since the
 BB> worst case doppler is less than 600 Hz, then  the 10m RF transmitter
 BB> never needs to be tuned.  The PSK-31 software simply slides the audio
 BB> carrier within the audio passband.  SImilarly, the DONWLINK then
 BB> contains all downlinks with 0 doppler.  THe DOPPLER at each receive
 BB> site can now be accomodated easily within the PSK-31 decoding software
 BB> since it is still within 600 Hz so no RADIO controls are required...

This conforms to the One True Rule, and would seem to be a good idea.  Of
course, it would depend on the computer's clocks being on the right time, but
that's no different to the issue facing current computer compensated stations.

 BB> OK, it takes a little more software, but still seems very easy to do
 BB> Thanks for pointing this out...

No probs.  Certainly not insurmountable.

 BB> By pre-computing for 0 on the satellite, then, the position of each
 BB> user in the single 3 KHz downlink remains fixed making it easy to join
 BB> or find a QSO.   Also since all doppler comnpensation is only self
 BB> relative, then it is entirely doable within the PSK program...

Agreed.

 BB> BUT WAIT!  Maybe the PSK-31 would NOT EVEN Need a orbit model!  As
 BB> someone else suggested there should just be a single PILOT channel
 BB> with a unique RYRY sequence.  The PSK-31 RECEIVE software just looks
 BB> for that channel and then OBSERVES its offset.  Thus he now konws
 BB> everything he needs to compensate his uplink in the opposite
 BB> direction...
 BB>
 BB> Except for one thing.  He needs an absolute reference to start with...

Yes, to determine the amount of Doppler shift to apply, as there are two
variables involved here - Doppler shift and the accuracy of the receiver's
tuning, so the extra piece of information is needed.

 BB>
 BB> OK..!  How about the BOTTOM 600 HZ of the transponder is reserved for
 BB> "seeking".  Each new USERs PSK-31 software, after it finds the PSK-31
 BB> RYRY PILOT, it transmits its own unique code 600 HZ lower.  Then it
 BB> SERACHES the downlink to FIND it.  Once it finds its OWN signal, then
 BB> it has instantaneous real-time knowledge of the exact Doppler offset.
 BB> At this point the software knows everything it needs and the "SEEK"
 BB> transmission is no longer needed.  Now the user is LOCKED ON and is
 BB> free to join any QSO anywhere in the passband.   And from this point
 BB> on the PSK-31 transmitter and receiver can be fully tracked by simply
 BB> observing the drift of any, OR all OR JUST THE pilot.  It doesnt
 BB> matter, because everyone has 0 DOppler at the bird....

Interesting approach, certainly worth a fiddle. :-)

 BB>
 BB> Wow...
 BB> Just a little software....
 BB>
 BB> de WB4APR, Bob
 BB>
 BB>
 BB>
 BB>
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Tony, VK3JED

.. ve got to do with it
--
|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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