Basicaly, the idea would be that you have an emitter and a receiver.
Both components has a really precise clock that would be synchronized.
From this point the emitter would send the time that he emits in an encrypted ultrasound.
This ultrasound would be decrypted by the receiver and would compare the lag between the actual time and the emitted sound.
Because sound travels at a speed of around 320 m/s, I could get the distance between the two objects.
tell me what you guys think
I know there is many factor to consider.
And I post this idea as I think that this idea as to be brainstormed by smart people (which I am not)
although I think its an awesome idea to work on