What is Doppler shift in reference to satellite communications?

The Doppler effect is a phenomenon where the frequency of a wave changes due to the relative motion between the source and the observer. A classic real-world example is the pitch of a train whistle dropping as it speeds past you. In satellite communications, because Low Earth Orbit (LEO) satellites move very fast relative to your Earth station, the radio frequency shifts. The signal appears higher in frequency as the satellite approaches and lower as it moves away. Distractors suggest this is a change in the physical orbit or a digital mode. Remember that Doppler shift only affects the apparent frequency of the radio signal, not the satellite's physical path or mode of operation.