Patent ID: 8786488

Claim:
A method for measuring distance between an RFID tag reader and an RFID tag, the RFID tag having a plurality of backscatter reflection states; the method mitigating detrimental effects of noise, clutter and multi-path signals, comprising the steps of: transmitting an RF signal to the tag; receiving a first backscattered RF return signal from the tag corresponding to a first backscatter reflection state of the tag; receiving a second backscattered RF return signal from the tag corresponding to a second backscatter reflection state of the tag; signaling by the tag reader to a pulse radar timing of said first and second backscatter reflection states; transmitting a first radar signal by said pulse radar during said first backscatter reflection state of the tag; transmitting a second radar signal by said pulse radar during said backscatter reflection state of the tag; receiving reflected return radar signals by said pulse radar from the tag during said first and second backscatter reflection states of the tag; processing each of said received reflected radar signals using a fast Fourier transform (FFT) to produce complex signal spectrums; forming a complex conjugate of the FFT of one of said transmitted radar signals; multiplying said complex conjugate of the transmit signal with the FFT of each received signal individually to create first and second products; producing a correlation for each received signal individually by performing an inverse FFT of each of said first and second products; subtracting the correlation obtained with the tag in said first backscatter reflection state from the correlation with the tag in said second backscatter reflection state to produce a signal having a peak; determining round trip time delay to the RFID tag from the time of said peak; calculating the distance between the RFID tag and the reader based on said round trip time delay.