Michael R. McCarthy Paul Duff Henk L. Muller Cliff Randell Accessible Ultrasonic Positioning. 86-93 2006 5 IEEE Pervasive Computing 4 http://doi.ieeecomputersociety.org/10.1109/MPRV.2006.65 db/journals/pervasive/pervasive5.html#McCarthyDMR06