so ive been doing FM birds for a while now, SO-50 and the like, and figured it was time to try the linear transponder sats like AO-73 or FO-29. got my az/el setup running with a yaesu 5500 and an ic-9700 and i thought i understood how doppler worked but apparently i was wrong because everything sounds terrible and nobody can hear me.

the issue is i know you have to correct for doppler but on a linear transponder its inverting which means if im chasing doppler on the uplink i have to go the other way on the downlink or something? ive been using gpredict to control the radio and it shifts both vfos but i feel like its not doing it right or maybe im not understanding how to configure it. like i can hear signals on the downlink that sound like theyre varying in pitch and when i transmit i can sometimes faintly hear myself come back but way off where i expect and then it drifts.

also does anyone actually use sat-pc32 anymore or is gpredict basically the go-to now for linux guys

yeah the inverting transponder thing trips up a lot of people coming from FM birds. basically on a linear transponder the passband is inverted so LSB up means LSB down but the doppler correction goes in opposite directions for uplink vs downlink. gpredict handles this but you have to make sure you have the transponder configured correctly in the satellite database, specifically that the inversion flag is set. if its not set it'll shift both vfos the same direction which is wrong.

the way i think about it is — the transponder passband is fixed relative to the satellite. so as the sat comes toward you, your downlink frequency appears higher (positive doppler) but because the passband is inverted, your uplink signal needs to go lower to stay in the same spot in the passband. gpredict should handle all of this automatically if the transponder data is right. check that you have the correct center frequencies loaded and that inversion is ticked. also make sure your CAT control is actually talking to both vfos and not just main.

FO-29 has been a bit iffy lately with its schedule but AO-73 is pretty solid for practice passes

sat-pc32 still works fine honestly, i use it on a win10 machine with no issues and some people swear by it especially for the older sats. but yeah most of the newer guys seem to be on gpredict. i tried switching a while back and it does most things well but the CAT implementation felt a little clunky to me for the 9700 specifically, there was some latency that bugged me. might have been my setup though.

one thing that helped me early on was just listening for a full pass without transmitting at all, find a signal in the passband and just track it manually and see how the pitch changes. once you actually hear what correct doppler compensation sounds like vs what it sounds like when its wrong it kind of clicks. also the 9700 has that satellite mode built in which handles some of this but i never fully got it working the way i wanted with external software at the same time