A) An observed change in signal frequency caused by relative motion between the satellite and the earth station
The Doppler shift, named after the Austrian mathematician and physicist Christian Doppler, who proposed it in 1842, is the change in frequency for an observer moving relative to its source. With respect to sound waves, it is commonly heard when a vehicle sounding a siren or horn approaches (rising pitch), passes (neutral), and recedes (falling pitch) from an observer. A fast moving train blasting its horn while passing a stationary observer at a crossing is one of the more dramatic examples.
Doppler shift also occurs in radio communications with satellites. When this happens the receives signal frequency appears to change (though the transmitted frequency remains constant). The change is due to the satellite's high speed of motion relative to the ground station.