Ah yes you could use time of flight even in the MHz range, where the wavelength of light is 300 meters if you have an accurate enough receiver, but that should

Ah yes you could use time of flight even in the MHz range, where the wavelength of light is 300 meters if you have an accurate enough receiver, but that should be a problem. I think the precision should be on the order of ~ (A/D relative precision) * (wavelength), so for mm accuracy you need an A/D converted with more then 10^(-9) precision (that is, a >30 bit D/A), which I don't think is cheap (but doable perhaps?). The other problem is this conflicts greatly with multipath and other kinds of interference.