so ive been licensed for about 3 years mostly doing HF and some local 2m stuff but last week i finally decided to give the linear transponder satellites a shot, specifically AO-73 and SO-50. got gpredict set up on my laptop and it seems to be tracking fine, the pass predictions look reasonable at least compared to what heavens-above shows.

my problem is the doppler correction. i understand the concept, the satellite is moving so the frequency shifts, but when i actually try to work it in real time im fumbling with the radio while also trying to point the yagi and its just chaos. im using an FT-817 and a handheld yagi, nothing fancy. i tried the AO-73 linear transponder on a pass yesterday and i could hear some signals that sounded like SSB voices but they were drifting all over the place and i couldnt tell if that was me not correcting or other stations not correcting or both.

is there a way to set up gpredict to do the rig control automatically for doppler on the 817? i know it has CAT but i never really used it. or is most people just doing it manually and getting used to it over time? feels like im missing something obvious here

yeah gpredict absolutely can do rig control via CAT, thats actually how most people handle it once they get tired of manually chasing doppler. for the 817 you need a CAT cable, either the official yaesu one or a cheap FTDI-based one works fine, and then in gpredict you go into the interfaces section and set up the radio with the right COM port and baud rate. 9600 i think for the 817 but double check that.

once its connected gpredict will push frequency updates to the radio in real time and it makes a huge difference honestly. the linear transponders are inverted so when you're correcting for doppler on the uplink you have to remember the relationship is flipped relative to the downlink, gpredict handles all that automatically which is nice. one thing though — even with CAT control you still need to find your own signal on the downlink first and kind of tune around it, the correction gets you close but you still have to do a little fine tuning by ear especially near AOS and LOS when the doppler rate is changing fastest.

SO-50 is FM so forget all this for that one, doppler barely matters on FM for a beginner just set it and forget it roughly. AO-73 is where the linear transponder fun and frustration lives.

the drifting you heard was probably a mix of both honestly. a lot of guys on the linear birds especially newer ones dont correct properly and it makes the whole passband sound like a mess. you kind of learn to identify your own signal by its audio characteristics and then just follow it. takes a few passes before it clicks.

i went through the same thing last year. what helped me was doing a few passes just listening, not even trying to transmit, just to get a feel for how the passband sounds and how fast things drift. then started transmitting and could at least recognize when i was off. the 817 with CAT is a pretty common setup for this stuff so theres a lot of info out there for it.