In the present invention, a first and a second projector means projecting rotating laser beams rotating at a predetermined period are disposed at two predetermined fixed points, respectively. A first, a second and a third photodetection means are disposed on a moving body to detect the photodetection...http://www.google.com.au/patents/US4818107?utm_source=gb-gplus-sharePatent US4818107 - System for measuring the position of a moving body
