A system and method for determining the position of a radio frequency
identification (RFID) transponder with respect to a sensor. In one
embodiment, the system comprises a plurality of stationary sensors located
in an array within certain physical areas. Each sensor comprises a
plurality of antenna coils arranged in unique physical orientations and
capable of transmitting radio frequency signals of differing phase. The
RFID transponder includes an antenna which receives the plurality of
signals generated by the antenna coils, and compares the phase of at least
two of the signals to determine the relative position of the transponder.
In a second aspect of the invention, the aforementioned antenna coils emit
two direction finding mode (DFM) signals in succession; the first signal
with all antenna coils turned on, the second with one of the coils turned
off. The spatial relationship of the transponder antenna and individual
antenna coils precludes all of the signals in each sensor from being
rejected by the transponder during emission of both the first and second
DFM signal. Hence, the transponder is kept in constant communication with
the sensor in all orientations. In another embodiment, the location of the
transponder with respect to two or more sensor(s) is determined through
measurement of the intensity of the signals received by the antenna coil
of the transponder. The invention also includes a system and method for
transmitting data between a sensor and a dormant (motionless) RFID
transponder using a hand-held high intensity RF probe.