Short distance, millimeter-level ranging based on backscatter RF tags remains a challenge today. We propose a composite Dual-Frequency Continuous-Wave and Continuous- Wave (DFCW/CW) radar system for localization of backscatter RF tags. The coarse DFCW range result is used to correct the cycle ambiguity of the more accurate, CW range result. A Minimum Mean Square Error (MMSE) based method is employed to combine the DFCW and CW ranging data, given spectrum constraints. The composite radar concept is demonstrated with a custom, 5.8 GHz, backscatter RF tag system, in a typical indoor laboratory environment with a strong Line-of- Sight (LOS). Results show that the MMSE technique can resolve the CW cycle ambiguity for a moving tag and results in a range error of approximately 1 mm after an acquisition period.
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