On 02/02/20 18:50, John Brier via AMSAT-BB wrote:
One thing I didn't mention in the video, at the end of the pass around 10 degrees elevation and lower I decoded a lot of frames because the signal was still strong and by that point the frequency wasn't changing much anymore, as the satellite's relative speed stopped changing.
The video is instructive as one can see the plotted constellation "spinning" before symbol lock. Even after locking, the constellation rotates, indicating a frequency error between the transmitter and receiver presenting itself as a continuous phase rotation.
With an antipodal signal, the constellation diagram will tell you a lot about received signal quality, including SNR (fuzzy points), interfering signals (donuts), out-of-lock (one big circle), frequency/phase errors (smeared clusters of points in a radial pattern).
A great technical read on vector modulation analysis:
http://literature.cdn.keysight.com/litweb/pdf/5091-8687E.pdf
--- Zach N0ZGO