A personnel monitoring system includes a transmitting or transponding unit that is worn or carried by the individual being monitored. This transmitting or transponder unit periodically, or upon request, transmits a uniquely encoded signal that identifies the person being monitored, as well as information...http://www.google.com.mx/patents/US4952928?utm_source=gb-gplus-sharePatent US4952928 - Adaptable electronic monitoring and identification system
