I have found that FT4 provides an interesting, fine-grained measurement of how accurate my Doppler calibrations are on linear satellites. While running FT4 on a satellite pass, I compare my FT4 transmit frequency, say 1000 Hz, to the FT4 frequency at which I receive my own transmissions in the downlink window. If my Doppler calibration and everything else were perfect, FT4 would show my own signal decoding at 1000 Hz.
Of course nothing is perfect, beginning with the op’s starting alignment. And even though my IC9700 is reference locked to a GPSDO, there is still a little variation in the 9700 itself. Then there is some degree of error in keplerian data, and the possibility of some drift in the bird’s transponder during and/or between passes.
But FT4 does provide a way to measure calibration down to the level of a few Hz. My RS-44 calibration is currently within 10 Hz and generally doesn’t wander beyond that during a pass or between passes. The Zhuhai Twins, CAS-4A/B, are currently good to within 40-60 Hz but do show a bit more wander during and between passes.
So the next time we have a satellite phone QSO and I’m off frequency, it’s either because I don’t know what you sound like in person and have given you a little too much Donald Duck or Darth Vader, or my ambient S2+ noise level is making it hard for me to hear you.🙂
73,
Ray