so ive been messing around with AMSAT stuff for a few weeks now, got my antennas sorted out (pair of yagis, az/el rotor) and im finally getting solid passes on FO-29 and the AO-7 when its in mode B. the issue im running into is the doppler correction. i understand the concept, like the uplink and downlink shift in opposite directions because of the linear transponder being inverting, but when im actually in a pass i feel like im always chasing my own tail trying to stay on frequency.

my radio is an IC-9700 and i have gpredict running on a laptop doing the doppler tuning automatically but something feels off. like my downlink drifts one way and i push my uplink the other way and then i lose myself in the passband completely. been reading about how you lock onto a beacon first but AO-7 doesnt really have a useable beacon most of the time depending on which mode its in.

is there a better workflow for this? i feel like im missing something fundamental about how to set the rig up with gpredict. does anyone actually have the IC-9700 CAT control working smoothly for sat work or is it kind of a pain

yeah the inverting transponder thing trips up pretty much everyone at first. the key thing to burn into your brain is that when you tune UP on your uplink, your downlink signal moves DOWN in the passband. so if you hear yourself drifting high, you need to transmit lower, not higher. once that becomes instinct it gets way easier.

for gpredict and the 9700, make sure you have the doppler tuning set to full duplex mode and that youre using the correct TLE file thats actually recent — old TLEs will throw off the prediction enough to make the whole thing feel broken. also the 9700 CAT can be a little finicky with gpredict depending on which version youre running, some people have better luck with hamlib vs the native rigctld setup. i'd check the amsat-bb archives because there was a whole thread on 9700 CAT weirdness like 8 months ago that might be exactly your issue.

once you get a couple passes where it clicks, the whole thing kind of snaps into place and you stop thinking about it so much. AO-7 is tricky anyway because of the whole eclipse/sunlight mode switching thing, dont get discouraged by that one specifically

honestly the doppler on LEOs still makes my head hurt sometimes and ive been doing this for years lol. one thing that helped me was just doing a few passes in receive only — dont even transmit, just track yourself listening to other guys working the sat and watch how the signals move across the passband as the pass progresses. gives you a real intuitive feel for the rate of change before you add the complexity of trying to work someone at the same time.

also are you accounting for the transponder inversion offset correctly in gpredict? theres a setting where you put in the transponder passband limits and whether its inverting, and if that data is wrong the auto doppler correction is going to fight you the whole pass. the AMSAT website has a satellite status page with all those parameters updated pretty regularly