Cinerath
I dont realy know much about Signals, but ultrasonic sounds (hehe) like it uses soundwaves to transmits information, so how can it work in Space?
I believe the sound waves can indeed travel through these particles. But temperature and differences in air density distort sound.
Cinerath:Well, although the air is not breathable in the thermosphere since the composition is a bit messed up, sound can still find a way to propagate throught these molecules. I don't know if that's the exact case, but I assumed it worked like that.