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Re: FM Satellites (Millennium Project)



At 02:44 PM 07/28/1999 -0400, John, KD2BD wrote:

>Why wait?  How about designing/building handheld equipment to work the
>existing analog LEO birds?  If FM is somewhat practical, just imagine how
>much better SSB would be.  Or how about giving ACSSB a whirl (using the
>pilot carrier and an AFC circuit to compensate for Doppler shift)?

Now that's a promising suggestion, which could wring out a few more dB than
regular SSB.  

Here's another.  I hope that one of our software wizards combindes G3PLX's
PSK31 scheme with the 400 bps PSK used for digisats and Phase III command,
implements it for a sound card and PC platform, and adds two layers of
real-time doppler correction: one adjusting the sound card audio frequency
(AFC), and the other tuning the rig with mic clicks via the COM or parallel
port.  You would only need the mic click tuning for LEOs at 144 MHz and
above--the audio AFC would suffice for most of P3D's orbit.

The result would be a robust, easy-to-use keyboard-chat mode that would not
require linking, would be usable at signal levels well below that required
for an SSB contact, and wouldn't require an expensive additional modem.
Folks without killer antennas could use it for reliable communications over
any CW/SSB satellite.

I realize this is not for the so-called average ham with only an FM HT.
But it does give someone on a budget digital advantages using the PC and
sound card they already own.  

This suggestion is based on my tests of PSK31 on Oscar 10 (including a QSO
with DL6DBN).  The major problem was transponder instability, not doppler
shift.  As long as I was able to stay tuned in, I could copy PSK31 signals
very well, under conditions that would cause all but the most skilled
weak-signal CW operators to give up. 

For what it's worth, I hope there will always be an FM bird or two around.
But for real, real-time communications, give me an analog transponder.  You
can fit a lot more people into them.  And all kinds of signals we haven't
tried yet.

73 from KD7MW,

--- Peter
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