Circular polarized noise interference radiated from the sun
Rotation of the satellite and its antennas✓Correct answer
Doppler shift of the received signal
Interfering signals within the satellite uplink band
Explanation
Most satellites rotate, and as they turn, the orientation and pattern of their antennas relative to your station change, so the received signal rises and falls in a regular rhythm. That periodic variation is spin fading and is distinct from Doppler shift, which changes frequency rather than amplitude.