so ive been licensed for about 3 years now and mostly just done HF but i finally got a decent dual band yagi setup on an az/el rotor and figured id try working some of the linear transponder birds. specifically been looking at FO-29 and AO-73 passes over my QTH.

my question is about the doppler correction — i get the concept, the satellite is moving toward you so the uplink frequency shifts one way and downlink the other. but when i use gpredict to track the pass and it feeds into my rig via hamlib, it seems like the offsets it applies are way more aggressive than what ive read about. like on a good overhead pass the total swing is something like 10kHz on 435 downlink? that seems like a lot but maybe im wrong

also the other thing thats confusing me is the inverting transponder thing on FO-29. so if i tune up in frequency on my uplink the downlink goes down? ive read about this several times and i sort of understand it mechanically but in practice when im on a pass im losing track of what im doing. does anyone have a tip for internalizing this that actually works or is it just muscle memory after a few passes

the doppler numbers you're seeing are totally normal, dont let that throw you off. at 435 MHz the total doppler swing on a low-inclination LEO pass can be around 10kHz yeah, more like 8-9kHz for a 400km orbit and it varies depending on the elevation angle of the pass. a high overhead pass actually has a faster rate of change near AOS and LOS. gpredict is probably doing it right if you've entered your QTH coordinates correctly and selected the right doppler correction mode for split operation.

on the inverting transponder thing — yeah it trips everyone up at first. the way i think about it is this: your uplink on 145 MHz and the downlink on 435 MHz are linked but inverted. so if you hear yourself drifting high on the downlink, you tune your uplink DOWN to compensate. it genuinely just takes a handful of passes before it clicks. what helped me was not trying to chase myself and instead just finding a clear spot, setting my uplink, and letting gpredict handle the gross doppler while i fine-tune manually. also make sure your split is set up right in gpredict, there's a tracking mode specifically for inverting transponders and if you have it set to the wrong one everything will fight you.

FO-29 has been kind of hit or miss lately in terms of when its actually in linear mode vs FM beacon only, worth checking the AMSAT status pages before a pass or you'll sit there confused why you cant hear anything. AO-73 is more reliable for scheduled linear operation windows i think, its in amateur mode during eclipse periods which you can figure out from the keplerian data roughly.

anyway yeah what the other guy said about the inverting transponder, once it clicks it clicks. i remember my first few passes i was basically fighting myself the whole time and never made a contact. then one evening something just made sense and it got way easier. dont get discouraged if the first few passes feel chaotic