so ive been messing with LEO sat ops for a few months now and finally got a decent enough setup to try AO-73. running a yaesu ft-818 for the uplink on 435 and a separate sdr++ on a v-dipole for the downlink around 145 mhz. tracking with gpredict which honestly works pretty well once you get the TLEs updated.

the problem is i can hear the transponder passband fine, and i can even hear other stations working each other which is cool. but when i try to transmit i either cant find myself in the passband or i pop up way off from where i think i should be. ive got the doppler compensation running on gpredict and its supposed to be talking to my radio via hamlib but im not totally sure the rig control is actually doing anything. like the dial doesnt seem to be moving the way i expect during a pass.

anybody run into this before with gpredict and an 818? wondering if its a hamlib config thing or if im just not understanding how the doppler correction should work on a linear bird. uplink and downlink doppler are obviously different frequencies and rates so im not clear on whether gpredict handles that automatically or if i have to set it up a certain way.

yeah the hamlib thing is a common headache with gpredict. first thing id check is whether youre using the right rig number for the 818 — its not always obvious and if you picked the wrong one it might connect without throwing an error but not actually change freq. also make sure you have the right baud rate set on the radio itself in the menu, ft-818 defaults are a little weird iirc.

for the doppler on linear birds, gpredict does handle uplink and downlink separately if you configure both in the transponder settings. theres a transponder list you can edit and you want to make sure the uplink and downlink frequencies and the inversion flag are set correctly for AO-73. the passband is inverting so LSB up and USB down or the other way around depending on how you look at it. if you have that backwards youll chase your tail the whole pass.

honestly the easiest way to find yourself is to transmit a short carrier and listen for it on the downlink before a real pass so you can see where youre actually showing up. just dont hold the carrier too long cause youll walk all over other stations.

im in kind of the same boat tbh, been trying to get sat ops going and the doppler correction is confusing me too. one thing i read somewhere is that for low earth orbit birds the doppler shift can be like plus or minus 10 khz on 435 mhz and even more shift rate near AOS and LOS so if your rig isnt tracking fast enough you'll drift out of the passband pretty quick at high elevation parts of the pass. dunno if thats your issue or not but worth thinking about. good luck with it, i havent managed to make a sat qso yet myself so curious how it goes for you.