Satgen 209   Doppler Part 5, Transmitting and Receiving by GM4IHJ 27Mar 93
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Using a satellite for two way communication, transmitting to it , and ,
receiving from it, produces two separate doppler shifts on the signal.
Fisrtly your uplink transmission is doppler shifted as it enters the
satellite receiver . Then when the satellite transponds and retransmits
the signal back to you , it gets another dose of doppler shift before
entering your receiver. In a typical situation :-
You tx up to the sat at 435 MHz It is received at 435.0093 MHz
The sat mixer downconverts it additively to 145 MHz using 290 MHz local
osc. So the sat transmits 435.0093 - 290 = 145.0093 MHz to you
You receive this with added downlink doppler as 145.0093 + 0.0031 MHz
so you end up with +12.4 kHz approaching doppler and then have -12.4 kHz
of receding doppler by the end of the pass. A total doppler shift of 24.8
kHz, with a tuning rate of about 8 kHz/min in the middle of the pass . A
totally impossible situation to handle with manual tuning.

Amsat builders were aware of this difficulty from the start. So they
adopted a simple correction. Instead of using additive mixing in the sat
receiver they used subtractive mixing, substituting a 580 MHz local
oscillator for the 290 MHz local oscillator . Effectively subtracting
downlink doppler from uplink doppler.
435 up received in sat as 435.0093 . Mixed with 580 Mhz becomes 144.9907
which downlinked to you becomes 144.9907 + 0.0031 = 144.9938 . An apparent
shift of only 6.2 kHz instead of 12.4 kHz . Resulting in an overall shift
going from horizon to horizon of only 12.4 kHz instead of 24.8 kHz. With a
consequent very useful halving of the tuning rate necessary to stay on
tune in the overhead portion of the pass.

In practice the above example refers only to your reception of your own
uplink. Any distant station you communicate with has a different relative
velocity to the satellite and therefore receives a different doppler
shift. But if you start calling directly on his frequency and TUNE YOUR
TRANSMITTER TO REMAIN ON THE SAME PLACE ON YOUR RECEIVER TUNING, you
should get useful communication.

Be warned however that this type of operation has two important features.
Firstly if you use SSB, you must uplink LSB in order to downlink USB,
because subtractive mixing inverts the sidebands of your signal. Secondly
some transceivers cannot "Tune whilst transmitting" - they only tune when
your Tx is off , so tuning to follow doppler is difficult if you cannot
tune whilst actually sending and listening to your returning downlink
signal.

Please ensure you get my callsign right if you have any queries to send to
me. The GBR and EI BBS ops are very good at guessing who you really mean ,
but 3 wrong callsigns in each of the last 2 weeks , tests their kindness
to the limit ,Tnx . 73 de GM4IHJ @ GB7SAN