The apparatus contains two fixed ultrasound transmitters, an ultrasound receiver forming part of the movable object, and a calculator. A trigger signal from the calculator releases ultrasonic pulses on the ultrasound transmitters and the calculator determines the coordinates of the ultrasound receiver...http://www.google.com/patents/US4758691?utm_source=gb-gplus-sharePatent US4758691 - Apparatus for determining the position of a movable object
