so i finally got around to trying some LEO satellite work after years of saying i would. been licensed about 8 years but mostly HF so this is pretty new territory for me. picked up a pair of handheld yagis and have been using SDR# on the downlink side while transmitting with my FT-818 on the uplink. managed to hear myself in the passband on AO-73 last week which honestly felt pretty great.

my question is about the doppler correction. i've been using Gpredict to track the pass and it'll show me the doppler shift on both uplink and downlink separately but im confused about which one i should be manually correcting on my radio vs just letting the software handle. like do most people just tune the downlink and leave the uplink fixed, or are you constantly adjusting both? i feel like every time i try to actually make a contact im spending so much time chasing the signal that i cant coordinate an actual qso. probably a technique thing more than a gear thing but figured id ask here.

also what's a realistic expectation for how long a usable pass window is on a typical AO-73 overhead pass from mid-latitudes, like 45 degrees north roughly? some passes feel like i barely have time to get set up before its gone.

yeah the doppler thing trips up pretty much everyone starting out with linear birds. the general rule of thumb most people use is to correct the downlink and keep the uplink as fixed as you reasonably can, mainly because if you're constantly retuning the uplink you're going to be jumping around in the passband and other stations wont be able to find you. what a lot of guys do is pick a spot in the passband before the pass, note that uplink freq, and then just ride the downlink dial throughout. Gpredict can actually drive a rig directly if you set up CAT control, which makes the downlink correction basically automatic — i'd look into that because doing it by hand in real time is rough.

as for pass duration, at 45N a reasonable overhead pass — like one with a high elevation, 60 degrees or better — might give you 8 to maybe 10 minutes of workable time but a lot of passes are lower elevation and you might only get 4-5 minutes of decent signal before the link budget just falls apart. AO-73 specifically tends to be a bit inconsistent depending on its power mode too, sometimes the transponder is in low power and the downlink is weaker than you'd expect.

im in almost the exact same situation lol, started messing with FO-29 a few months back. the doppler on the uplink vs downlink confused me for way longer than i want to admit. one thing that actually helped me was just doing a few passes where i wasnt even trying to make a contact, just listening and tracking my own signal to understand how everything drifts. feels like a waste but i learned more from those passes than any of the ones where i was frantically trying to call CQ. also gpredict with CAT is totally the way to go like the other guy said, night and day difference once you get that set up.