GPS can provide the coherent reference, if you mean transmitting signal (say sound) while the panel is illuminated by the sun theres youtube videos of people doing that, with a laser pointer, but in sunlight and without information theoretic justified modulation scheme.

Nothing prevents the satellite to transmit the commands at night, if that feels more convincing to you.

Ask yourself what is the active area of a photodiode in your TV/... ? What is the active area of your light-bucket on a roof?