As long as the fob's own delay is very consistent, I don't see why you couldn't time the signal. Edit: there's a discussion down the page somewhere. The issue

As long as the fob's own delay is very consistent, I don't see why you couldn't time the signal. Edit: there's a discussion down the page somewhere. The issue seems to be that (for power reasons) they use low-freq radio, on which it's hard to get timing accurate enough for 10m distance changes.