so i've been wanting to do LEO satellite work for a while now and last weekend i finally gave AO-73 a proper shot. been doing FM sats for a few months (EO-88, RS-44 occasionally) but linear transponders are a whole different animal honestly

managed to make two contacts on my first real pass but i'm still pretty confused about the doppler correction side of things. i'm running a FT-818 and a IC-9700 split — 818 on uplink, 9700 on downlink — with a pair of handheld yagis, not the fanciest setup but it gets out there. my issue is i cant tell if im supposed to be correcting for doppler on uplink, downlink, or both. i've read different things in different places and some of the older guides seem to contradict each other

also using gpredict for tracking and it does have the doppler tuning function but ive only gotten it to control the 9700, cant figure out if theres a way to run rig control on both radios simultaneously from gpredict or if people usually just do it manually on one end. the pass windows are short enough that fumbling with corrections is kind of killing me

anyway any advice from people who actually do this regularly would be great, not just looking for the theory version

yeah the doppler thing trips everyone up at first. short answer for a linear transponder like AO-73 is you correct on the downlink and let the uplink ride, or at least thats the most common approach people use in practice. the reason is that if everyone tried to perfectly correct uplink you'd still hear yourself drifting around on the downlink anyway because other stations aren't doing the same thing in sync with you. so most guys just tune the downlink to keep their own signal centered and nudge the uplink occasionally if they feel like they're getting too far off

as for gpredict dual rig control, i dont think it natively supports two separate rig instances talking to two different radios cleanly, at least not the version ive used. some people run two instances of gpredict which is kind of hacky but it works. honestly though for handheld yagis doing it manually on the uplink isnt that bad once you get a feel for the doppler curve — AO-73 passes aren't long but the rate of change slows down a lot when its near overhead so you really only need to actively correct near AOS and LOS

rs-44 is great for learning linear because the passband is pretty forgiving and it tends to have lighter traffic than AO-73 during peak hours so you have a bit more time to fumble around without someone walking over you lol. i actually started on that one before moving to AO-73 and it helped a lot

one thing that helped me was marking the expected doppler shift at AOS, mid-pass, and LOS on a sticky note for each satellite before the pass, kind of a rough cheat sheet. gpredict shows you the instantaneous doppler but when youre also trying to point an antenna and work a contact its easy to lose track. eventually you just internalize it but in the beginning having that reference was clutch