What is Doppler shift in reference to satellite communications?
A change in the satellite orbit
A mode where the satellite receives signals on one band and transmits on another
An observed change in signal frequency caused by relative motion between the satellite and Earth station✓الإجابة الصحيحة
A special digital communications mode for some satellites
الشرح
Doppler shift is a change in the observed frequency caused by relative motion between transmitter and receiver: the frequency reads high as the satellite approaches and low as it recedes, with the shift passing through zero at closest approach. It is a property of the observation, not a change in the satellite's orbit or a mode of operation.