Esko O. Dijk Kees van Berkel Ronald M. Aarts Evert van Loenen Ultrasonic 3D Position Estimation Using a Single Base Station. 133-148 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2875&spage=133 2003 conf/eusai/2003 EUSAI db/conf/eusai/eusai2003.html#DijkBAL03