The Laser Positioner is a unique system that determines the position of a mobile robot in a wide-open field. The measurement system employs a new technique in which particular laser beacons are used to emit two symmetrical laser beams that travel in opposite directions around the beacon and a sensor then detects the beams to measure the arrival time ratio between the beams. The two beacons are set at reference positions to enable beam-detection by the sensor mounted on the robot to immediately determine the exact two dimensional location of the robot (according to x- and y-coordinates) wherever robot moves. Tests using a prototype system revealed a measurement error of 4 mm or less in a 26m x 16 m square area. Consequently, the measurement occur of this system is adequate for construction surveying applications. This paper describes the principles of detection, test results, and system applications.