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[psk31] PSK31 satellite ideas

Good ideas from the Author of PSK-31!

---------- Forwarded message ----------
Date: Tue, 27 Feb 2001 12:26:07 +0000
From: Peter Martinez <Peter.Martinez@btinternet.com>

My own idea for a PSK31 satellite (based on a proposal made 30 years
ago by G3VEL) would have a 3 kHz wide linear uplink on a low band
like 29MHz and an FM downlink, probably on a VHF or UHF band. In
effect the satellite consists of an SSB receiver feeding it's audio
to a voiceband FM transmitter. This is virtually the opposite to
Bob's earlier proposal. On the ground the users transmit on 29MHz
PSK31 using an SSB transmitter and receive on (say) 145.9MHz FM,
connecting the audio from the FM receiver to the soundcard input.

There are several advantages to this idea:-

1) the satellite transmitter doesn't have to be linear so it's more
efficient. It's much easier to build a linear FM modulator than a
linear transmitter, so there should be no problems with downlink

2) There is no downlink Doppler on the demodulated baseband audio, so
a VHF or UHF downlink doesn't give an impossible problem for PSK31.
But more importantly, all stations receiving the downlink will see
exactly the same spectrum. This removes the confusion between uplink
and downlink Doppler shift and guarantees that if I can set my
transmitter frequency so that it isn't causing QRM in my own
downlink, then it isn't causing QRM in anyone else's downlink either.
In an SSB-to-SSB transponder you get lots of Doppler-confusion QRM
which cannot be avoided even by the most careful operating practices.

3) Because the modulation level of the received FM downlink is
independent of downlink signal strength, the level of audio that I
receive from my own return signal is precisely related to my uplink
signal level received at the satellite. Downlink fading doesn't
affect it assuming my FM receiver limits properly. Thus, by
monitoring the audio level of my own return signal, I can accurately
keep my uplink signal level at the satellite within acceptable
limits. It's almost impossible to do this well on an SSB-to-SSB
transponder where the uplink and downlink paths fade differently. We
could define, for example, that all users should aim for a downlink
signal that was no stronger than the downlink beacon. This beacon
would be a PSK31 telemtery broadcast at one end of the audio band. An
automatic uplink power-control feedback loop is therefore possible
and can be done entirely in PC software on the ground.

4) Although this scheme doesn't, in itself, give a way to cancel
uplink Doppler shift, it will be possible for individual users to
monitor their own signal coming back and  tweek the transmitter
frequency to keep their own downlink signal on-channel. This, like
the power control, can be done in software. This technique is
difficult on an SSB-to-SSB transponder because you cannot easily
separate the uplink and downlink Doppler shifts. With the SSB-to-FM
transponder you see only uplink Doppler in the downlink baseband
which means that end-to-end AFC actually works and cancels out the
Doppler completely for all receivers.

5) Not only does this give the possibility for perfect end-to-end
Doppler cancellation but it cancels out ALL the frequency errors.
This means, for example, that someone could plan to broadcast a
bulletin on a specific AUDIO frequency, and listeners will just need
to tune to that audio frequency in the downlink baseband and be spot
on tune throughout the pass, regardless of RF frequency errors
anywhere in the path. The only complexity is at the uplink broadcast
station, who has to AFC his transmitter onto the nominal downlink
audio channel and do the power-control. He doesn't even need an
accurate RF frequency standard to do this. The receiver (in a
school?) is a simple 2m FM receiver and a PC with a soundcard.

It's possible to extend this idea to work with an SSB-to-SSB
transponder with a pilot tone in the downlink passband and suitable
software in the ground receiver to track the amplitude and frequency
of the pilot tone and one's own return signal and make all the
neccesary adjustments.  This then becomes an extension of Bob's
latest proposal, but from my own past experience with amateur LEO
satellite working, the workload is high enough already, what with
antenna tracking as well, and it would only be usable by very skilled

The SSB-to-FM scheme would be much easier for amateurs at all skill
levels to use effectively, especially for downlink-only listeners.
There are lots of other interesting possibilities for narrow-band
satellite working this way. The only worry I have is that some idiot
high-power SSB stations might hog the whole passband!

Bob: copy to AMSAT please?

Psk31 WWW Site at http://aintel.bi.ehu.es/psk31.html
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