US Patent 7362656 Ultrasonic locating system

The invention relates to a method and a system for monitoring and position determination of objects and/or living beings within an area, such as, e.g. a room in a building or a road tunnel. The system comprises a plurality of identification tags equipped with an ultrasonic receiver and radio transmitter which is attached to the objects that have to be monitored. The identification tags receive ultrasonic signals whose arrival time they measure. This information together with the identification tags'ID code are transmitted by radio waves to a central unit which calculates the position of each of the identification tags.