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The technology used is RFID. There is an antenna inside each of the doors that supports the fob and in the cabin. The system can tell where you are, so that having the fob by the driver door doesn't allow someone to unlock the passenger door with a touch. One feature of using RFID is that it doesn't absolutely need battery power to work. That way the fob can be used even if the battery is dead, by using the physical key to unlock and holding the fob against the start button when you start the car.