so ive been messing around with FO-29 and AO-73 for a few weeks now and i think i finally understand the basics but the doppler thing is still tripping me up. like i get the concept, the satellite is moving so frequencies shift, but in practice im not sure if im supposed to be correcting my uplink, my downlink, or both and in which direction.

my current setup is an FT-818 for uplink on 2m and a separate SDR for the downlink on 70cm. im running gpredict on the laptop and it has that doppler correction thing but i cant figure out if it only adjusts the radio or if i need to manually tune as well. during a pass last week i heard what i think was my own signal come back but it kept drifting off and i had to chase it around the whole time, which felt wrong. is that normal or am i doing something backwards

also does anyone actually use full duplex rigs for this or is the two radio approach pretty common? ive seen people mention the IC-9700 a lot but thats a lot of money just to chase satellites

yeah the doppler on linear transponders is a bit of a head scratcher at first. the short version is you want to keep your downlink signal stable — so you tune your receiver to compensate for doppler on the downlink side, and then let your uplink drift a bit or correct that separately. gpredict can talk to both radios via hamlib so in theory it adjusts both, but the tuning resolution and timing matters a lot especially near AOS and LOS when doppler rate is changing fast.

what i do is lock onto my own downlink signal by ear and tune the uplink manually to compensate. sounds primitive but it actually works pretty well once you get a feel for it. the two radio approach is totally fine and a lot of people run it that way. the IC-9700 is nice but its not magic, you still have to chase the signal a bit on fast overhead passes. the real advantage is full duplex so you can hear yourself in real time, which makes tuning way easier

I went through this exact same thing last year. What finally clicked for me was thinking about it from the transponder's perspective — the inverting linear transponder on FO-29 means if you tune your uplink UP, your downlink goes DOWN, so overcorrecting in the wrong direction makes everything worse fast. I bricked a few contacts before i figured that out lol

Also gpredict doppler correction works okay but ive had it lag a little on faster passes, so i usually have my hand on the VFO anyway. dont stress about the IC-9700 yet, i worked a bunch of stations on AO-73 with an FT-817 and a pair of handheld yagis. the two radio setup is honestly pretty standard for satellite ops.