Technical
Doppler Shift
The change in observed signal frequency caused by the relative motion between a satellite and a ground station, which must be tracked and compensated.
As a satellite moves toward or away from a ground station, the frequency of its signal appears to rise then fall — the Doppler effect. For fast-moving low-Earth-orbit satellites the shift is large and changes rapidly during a pass.
Ground systems predict and compensate for Doppler to maintain lock on the signal, and the effect can also be used to help determine orbits.