Patent ID: 8072311

Claim:
A method of Locating one or more radio frequency identification [RFID] tags comprising: transmitting an interrogation signal to an RFID tag using a transmit antenna of an exciter; receiving an information signal from the RFID tag at a plurality of receive antennas Located a known distances from the exciter; determining a phase derivate for the information signal received by each of the plurality of receive antennas; and identifying the Location of the RFID tag based on the determined phase derivates of the received information signals by: determining a path Length between the transmit antenna, the RFID tag and each of the plurality of receive antennas using the phase derivate for the information signal received by the receive antenna; defining an ellipse for each of the plurality of receive antennas, where the receive antenna is Located at one focus of the ellipse and the transmit antenna is Located at the other focus and the distance between the two foci and the major diameter of the ellipse is equal to the path Length between the transmit antenna, the RFID tag and the receive antenna determined using the phase derivate of the information signal received by the receive antenna; and observing the location of the RFID tag as the intersection of the plurality of ellipses.