A little History
The first radio direction finder was invented by John Stone Stone an American electrical engineer and physicist.
I'm reading through several papers on the equivalence principal, which I used to think said that you can't tell the difference between a gravitational field and say a centrifugal force. It looks like, at least at relativistic speeds, I may have been wrong. There's been quite a bit written on this, and it's taking me a while, but I'll keep you up to date.
1. Awesome listing of direction finding radios
2. Equivalence principle
3. Ham radio study exam
Muñoz G. & Jones P. (2010). The equivalence principle, uniformly accelerated reference frames, and the uniform gravitational field, American Journal of Physics, 78 (4) 377. DOI: 10.1119/1.3272719
open access version: http://arxiv.org/abs/1003.3022
Rohrlich F. (1963). The principle of equivalence, Annals of Physics, 22 (2) 169-191. DOI: 10.1016/0003-4916(63)90051-4