MilliSign: mmWave-Based Passive Signs for Guiding UAVs in Poor Visibility Conditions

This paper presents MilliSign, a guidance system based on a batteryless tag to support unmanned aerial vehicles in all-weather conditions. Conventional batteryless guidance systems using visual signs fail to work in inclement weather due to poor visibility. The need for all-weather operation with long-range readability encourages the use of millimeter wave (mmWave) radar, which poses challenges in providing a wide 3-D read range and low-cost operation. To address these challenges, we introduce a corner reflector (CR) array-based chipless RFID tag and a one-shot slant range reading procedure with COTS mmWave radar. We establish a novel design method for the shape and alignment of CR units to decrease the tag's size and expand the 3-D read range. Additionally, we develop a signal-processing pipeline based on Root-MUSIC to achieve accurate power and spatial estimation, which facilitate automatic tag detection. Our evaluation demonstrates that the tag, measuring 292 mm × 600 mm × 19 mm and storing 8 bits, can be read by mmWave radar from a distance of more than 10 m with a viewing angle of more than 30° in elevation and azimuth. Moreover, its performance remains stable in poor visibility conditions and multipath-rich environments.