US Patent 7352652 Ultrasonic tracking and 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. The system comprises a plurality of electronic units, called identification tags, which are attached to the objects that have to be monitored. Each identification tag has its own identification code (ID code) and is equipped with an ultrasonic transmitter, radio transmitter and radio receiver. The ultrasonic signals are received by one or more master and slave units which calculate transit time differences of ultrasonic pulses. This information together with the identification tags' ID code, identification of the room in which it is located, and any additional information are transmitted to a central processing unit which calculates the identification tag's position and presents it to a user of the system.