You can also use acoustic ranging, since sound travels at about 1 foot per millisecond. We did this with ultrasound at MIT -- http://nms.csail.mit.edu/papers/i

You can also use acoustic ranging, since sound travels at about 1 foot per millisecond. We did this with ultrasound at MIT -- http://nms.csail.mit.edu/papers/index.php?detail=9 http://nms.csail.mit.edu/papers/index.php?detail=9 You can get about 1cm of ranging error (median), and say 10cm of positioning error.