[amsat-bb] AO-91: *No* uplink frequency changes needed for Doppler shift

John Klingelhoeffer wb4lnm at aol.com
Fri Nov 24 01:56:47 UTC 2017

I am puzzled why I am seeing announcements going out from apparently several sources indicating user UHF uplinks need to be changed to adjust for Doppler shift near to their own AOS and LOS.  I'm seeing this posted from the AMSAT-UK website as well as other places.  

The FOX1-B UHF receiver has automatic frequency control (AFC) that will slew the uplink users signal to the middle of the IF filter and the discriminator if s/he is anywhere within about +/- 10 kHz of the nominal uplink frequency.  From what I remember, the worst case UHF Doppler shift was about +/- 8.5 kHz.  

There should be *NO* need at all to 'predistort' the uplink transmission frequency to correct for uplink Doppler shift - that was the whole purpose of going through the complexity of adding AFC to the receiver I designed.    The AFC was certainly working fine during testing.  Just use the posted nominal uplink center frequency - that's all.  

This, by the way, is true for 1-A as well.  Only one of 1-Cliff or 1-D has the AFC knowingly disabled.  I guess we will see how that 'experiment' works when they get out of the barn.  


AMSAT FOX RF Design Team

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