What causes spin fading of satellite signals?

Many satellites are designed to spin in space as a way to maintain physical stability or to evenly distribute heat from the sun. As the satellite rotates, the antennas mounted on it rotate as well. This constant spinning causes the polarization of the satellite's radio signal to continuously change from the perspective of an Earth station. If you are receiving the signal with a fixed linearly polarized antenna, the signal strength will rapidly rise and fall as the satellite's antenna aligns and misaligns with yours. This rhythmic fluctuation in signal strength is called 'spin fading'. Other options, such as Doppler shift or solar noise, cause frequency changes or background interference, not the rhythmic fading caused by physical rotation.