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For boats there is and has been for years, a device known as an E.P.I.R.B. (Emergency Position Indicating Beacon) They are automatically activated when a vessel sinks or may be activated manually in an emergency. They are detected by radio receivers on shore, afloat and in aircraft. I do not know if the modern ones have a GPS unit built into them or not (I have been retired for about ten years and things change) but that would seem a logical step in their development. They are also rather large and bulky and would not be convenient to carry very far. Although Cell technology has advanced and is probably capable of doing this fiunction, most places I hunt are well out of cell phone range, or at best have very spotty coverage, we may see major improvements in this regard in the future, but I am not counting on it being available in the near future.

I don' t recall the name of it , but it does exist . A device was developed to help locate lost skiers that uses sattelites/GPS with a repeating beacon for SAR . Each unit has it' s own unique signature to identify the owner .