so ive been licensed for about 3 years now mostly doing HF and some 2m FM but i finally decided to try working through the linear transponders on AO-73 and FO-29. got a pair of yagis mounted on a cheap az-el rotor setup and SDR# running on one machine while i try to transmit on another. the tracking software i went with is Gpredict which seems pretty solid but im genuinely confused about how to handle the doppler correctly when youre using a linear transponder.

like i understand the concept — satellite is moving so the frequency shifts — but when youre working SSB through an inverting transponder do i adjust my uplink or my downlink or both? ive seen people say you tune your receiver to keep your downlink constant and let the uplink drift but that seems backwards to me because then the person youre working hears you moving around on the passband. or do i have doppler correction running on both radios simultaneously? gpredict can apparently do CAT control to two rigs at once but i havent gotten that working yet. had one pass yesterday where i could hear beacon fine and even heard some SSB QSOs but couldnt figure out what i was doing wrong, my signal just wasnt showing up anywhere on the passband that i could tell

ok so the inverting transponder thing trips everybody up the first time. basically what happens is the passband is flipped — higher on the uplink becomes lower on the downlink. so when doppler makes your downlink appear to slide up during a pass, you actually hear other stations going down on the passband relative to each other if that makes sense.

the typical approach most guys use is to tune your downlink rig to wherever you want to hear yourself, and then adjust your uplink to find your own signal. the doppler correction in gpredict when you have CAT set up will try to keep a fixed offset between the two but you still kind of have to babysit it especially on a bird like FO-29 which has a pretty wide doppler swing. AO-73 is a bit more forgiving for starting out. honestly just get on a pass and find the beacon first, confirm youre hearing it, then try calling in the passband and just hunt for your own echo. once you hear yourself once it all clicks into place real fast. the latency through the bird is basically nothing so you hear yourself almost instantly.

yeah gpredict CAT to two rigs simultaneously works but getting the COM port assignments right is kind of a pain depending on your OS. what radios are you using for up and downlink? that matters a lot for how smooth the CAT tracking actually works in practice. some rigs respond to CAT commands way faster than others and if theres lag you end up with the correction always being slightly behind.