There are a lot of posts on this thread, so apologies if I've missed some reason why this wont work, but how about triangulation by radio beams? If the base location can accomodate antennas a few meters apart you could get some reasonable estimate of distance.

I thinking of trying it on our errant cat who doesn't know when it's curfew time (actually he's a smart cat and probably knows exactly when it's curfew time, but chooses to avoid it)