unless the "unused" frequency is assigned by license to either
you or the ground station.

Jim, got a link for how to apply for the permit required?

I learned that (perhaps it's a state law) law-enforcement types don't want you
to use a portable scanner-type radio tunable to cop frequencies when you're
mobile. It's understandable, given the tendency of Bad Guys to use such a
situation to evade enforcement. But a licensed ham radio operator is exempt
from that. I carry a printout of that regulation in my glove compartment since
I carry a portable scanner...even though it's programmed only with aviation
frequencies.

If it's not ferociously expensive I could see getting the required license in
case I can ever afford a mobile radio that transmits on the av freqs, too.