so ive been licensed for about two years now mostly doing HF and some local 2m stuff but i finally decided to try working through one of the LEO birds with a linear transponder, specifically been looking at FO-29 and AO-73. got myself a copy of gpredict running and i can see the passes fine, i can predict when theyre overhead, thats all good.

my problem is actually making a contact. i understand the concept of doppler shift, like i know the frequency is gonna be moving as the satellite comes at you and then moves away, but i just cant get my head around how to actually tune in real time while also trying to find someone to work and talk and everything else. do you just set a fixed frequency and let gpredict do the auto-doppler correction or do you tune by ear? my radio is an IC-9700 so it can do CAT control, ive got it hooked up to the laptop but i havent fully figured out the correction side yet.

also is there a good starting point frequency for calling on AO-73 or do you just sweep the passband and hope for the best? feels like there's a lot going on at once and im probably missing something obvious

yeah the doppler thing is confusing at first but honestly once you do a few passes it starts to click. with the IC-9700 and gpredict you can set up the CAT control so gpredict handles the VFO corrections automatically, thats the easiest way to start. just make sure you have the right doppler correction set for both uplink and downlink frequencies because on a linear transponder they're inverting, so as the sat approaches your downlink goes up and your uplink goes down, you have to correct both directions or youre gonna be chasing your tail.

for AO-73 the linear transponder passband is fairly narrow, like 30kHz or so, so youll hear everything pretty quickly just by scanning across it. what i do is tune around on the downlink until i hear signals, then figure out where i am in the passband and set my uplink accordingly. honestly forget about the auto-doppler stuff for the first few passes, just do it manually so you actually understand whats happening. once you get the feel for it then hand the correction off to gpredict.

one thing that trips people up is remembering its an inverting transponder so if you tune your downlink up, your uplink needs to go down to stay in the same spot in the passband. took me an embarrassingly long time to internalize that

FO-29 has been a bit hit or miss lately from what ive seen, might wanna check the current status before spending too much time on it. AO-73 is generally more reliable for getting started. also dont underestimate how much antenna pointing matters on these passes, even a handheld yagi manually tracked is gonna outperform a fixed vertical by a lot, especially near the horizon where youve got the most time but the most doppler shift happening fastest