Patent US 6,317,049

A micropower transponder operates in conjunction with a constellation of low-to-medium Earth-orbiting communication satellites. The transponder is attached to a person, animal, or object. The location of a missing person, animal, or lost object is ascertained by locating the transponder associated with that person, animal, or object. The transponder may be hidden in the individual's hair, timepiece, jewelry, or article of clothing; may be swallowed by the individual; may be implanted under the individual's skin; or incorporated into an inconspicuous hand-held device such as a cellular telephone, pager, or calculator. The transponder includes a receiver for receiving an interrogation radio signal and a transmitter for transmitting a response radio signal. The transponder transmits the response radio signal in response to the interrogation radio signal if the interrogation radio signal includes a code matching the access code stored in the transponder. The Doppler shift in frequency of the response radio signal is measured and the Doppler shift is used to determine the location of the transponder.