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We present Platypus, a micro-displacement sensing system that enables sub-millimeter level accuracy by using mmWave backscatter tags and their modulations as carrier waves to shift the phase changes due to tiny displacements to clean frequency bins for precise tracking. It then reconstructs the tag phase changes with sub-millimeter level accuracy even from extended ranges (over 100m) or in NLoS situations where the tag is behind a wall or blocked by other objects. While Platypus enables many different applications, we demonstrate the proof of concept in structural health monitoring, where mmWave tags are attached to building models and continuously track the structural micro-displacements, achieving a median of 0.3 mm accuracy.
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