so ive been doing terrestrial vhf/uhf stuff for years and finally decided to give LEO satellite operation a shot. picked up a copy of gpredict and been tracking passes for about a week now just watching before actually trying to transmit. the geometry stuff makes sense to me, AOS, LOS, elevation angles, all that. its the doppler correction piece that has me second guessing myself.

been messing around with AO-73 which i know still has the linear transponder active at least part of the time. my setup is a FT-818 running into a small 3 element yagi for 2m and a 7 element for 70cm, both handheld which i know is far from ideal. my question is basically do i need to be correcting doppler on BOTH the uplink and downlink simultaneously or can i get away with just tracking the downlink and leaving the uplink more or less fixed? ive seen conflicting stuff online and honestly some of it reads like it was written 15 years ago so im not sure whats still accurate for current ops.

also is gpredict good enough for tracking or do i need something else? running it on linux if that matters

yeah so the short answer is you really do need to correct both if you want to be serious about it, but the practical reality is most people running handheld yagis and a single radio just track the downlink and manually nudge the uplink occasionally. the linear transponder inverts the sideband so as your uplink goes up in freq your downlink comes down, which messes with people's heads at first. if you transmit USB on the uplink you receive LSB on the downlink side, just how it works with an inverting transponder.

for gpredict on linux its honestly fine, ive been using it for years. the doppler tuning interface can talk directly to some rigs via hamlib which helps a lot. the tricky bit with a single radio like the 818 is you cant easily do full duplex monitoring of your own downlink while transmitting, so youre kind of flying blind on whether you're in the passband. if you can borrow or beg a second receiver even a cheap SDR dongle it changes everything. plug that into gpredict for the downlink and use the 818 for uplink and suddenly it all starts clicking into place.

AO-73 scheduling is worth checking before you go out, the transponder isnt always on.

gpredict is what pretty much everyone uses on linux, dont overthink that part. the doppler thing confused me too when i started. what actually made it click for me was just listening to a busy pass first and hearing how signals glide around, then it made physical sense why you have to chase it.