Communication between a remote locator and a transponder is used to determine the relative position of the transponder. The transponder and locator each include a transmitter and a receiver. The locator transmits an inquiry in the form of a relatively powerful cyclically encoded signal with repetitive...http://www.google.de/patents/US7646330?utm_source=gb-gplus-sharePatent US7646330 - System and method for locating objects and communicating with the same
