Trajectory-Tracking of UHF RFID Tags, Exploiting Phase Measurements Collected From Fixed Antennas

In this article we propose a novel method for tracking a moving UHF RFID tag in 3D space. The tag moves in an area monitored by at least two antennas fixed at known locations. Phase measurements from each antenna are collected. Assuming a grid of N initial positions, we calculate N possible trajectories, by deriving closed form expressions, exploiting only the phase measurements collected by each antenna separately. Then, using the measured phase-difference from each antenna-pair, a hyperbola's branch on which the tag must be located is calculated. We evaluate the distance of each trajectory from the hyperbolas. Finally, we select the trajectory that minimizes the distance. Experimental results, conducted with a SLAM (Simultaneous Localization and Mapping)-enabled robot show a mean error less than 20cm with estimations derived in the order of ms. Compared to prior art, the proposed method does not assume neither knowledge of the trace, nor of the initial position of the tag. Furthermore, it accomplishes high accuracy at reduced computation time.