I know a lot of older communications satellites (incl. commercial ones) were just dumb repeaters - receive a signal on one frequency, retransmit it in real time on a different one, don't need to decode it or authenticate anything (although you still need the specialized radio transmitter hardware). This is how Captain Midnight [0] was possible.

I can't imagine modern satellites work the same way.

[0] https://en.wikipedia.org/wiki/Captain_Midnight_broadcast_sig...