so ive been trying to work AO-73 for a few weeks now and i keep running into this thing where maybe 3 or 4 minutes into a pass the downlink just kind of wanders off and i cant figure out if its my doppler correction thats off or if the bird itself is doing something. im using SDR# for the downlink and a handheld for the uplink which i know isnt ideal but its what i have right now. my tracking software is Gpredict and i have it set to update TLEs every couple days from celestrak.

the thing that confuses me is it doesnt happen every pass. some passes it works pretty clean the whole time, other times it just falls apart around the middle where the satellite is highest. i thought maybe it was AGC or the transponder going into compression but i honestly dont know enough about how the linear transponders on these birds work to say for sure. anyone dealt with something like this on FO-29 or the other linear birds? wondering if its just a known quirk or if im actually doing something wrong

yeah AO-73 has that quirk where the transponder output can shift depending on how much load is on it. if theres a pileup or a few strong stations on the passband at once the AGC on the bird backs things off and everything kind of squishes together. happens more around AOS and LOS when the geometry is better for more stations to hit it. mid-pass at high elevation is actually when the most people can access it simultaneously so thats probably what youre seeing.

the other thing i'd check is your doppler correction rate in gpredict — make sure its actually feeding correctly to SDR# because if that handoff lags even a little bit the signal drifts and sounds like the bird is doing something when its really just your rx chain. i had that problem for weeks before i figured out the virtual audio cable routing was introducing latency in the correction loop. once i sorted that it was way cleaner.

handheld for uplink is gonna cause you problems on the linear birds honestly. you need to be compensating doppler on transmit too not just receive, and with a handheld thats pretty much not happening unless you're manually tuning which is basically impossible to keep up with during a pass. FO-29 is a bit more forgiving but AO-73 youll notice it. even a few hundred hz off on uplink shifts your downlink spot around and if you dont realize its you causing the drift you'll chase your own signal all over the passband. been there