What is Doppler shift in reference to satellite communications?

Doppler shift is the apparent change in the frequency of a wave caused by relative motion between the source and the observer. Just as a train horn sounds higher in pitch as it approaches and lower as it departs, a satellite's radio signal does the exact same thing. When a satellite is moving rapidly toward your Earth station, the radio waves are compressed, causing you to receive them at a slightly higher frequency than what was transmitted. As it moves away, the waves stretch out, causing the received frequency to drop. This is strictly a frequency phenomenon caused by velocity relative to your station. It has nothing to do with the satellite changing its physical orbit, and it is entirely unrelated to the operating bands or digital protocols the satellite might be using.