Monitoring bridge vibrations via spaceborne SAR micro‐Doppler

The advent of Synthetic Aperture Radar (SAR) imaging has presented the possibility of remote monitoring of civil infrastructure on a large scale. Although well established for observing slow and long-term phenomena, its application to vibration-based structural health monitoring (SHM) remains relatively unexplored in the current literature. This study demonstrates the use of micro-Doppler SAR (MDSAR) using data from spaceborne platforms for measuring structural vibrations of a real bridge, specifically the line of sight velocity time histories of the deck. These measurements are compared to synchronous ground truth data to validate the method and assess its accuracy. Experimental results show that MDSAR measures vibration with an error in velocity on the order of 1 mm/s and successfully identifies the bridge’s dominant frequencies from two separate SAR acquisitions at different times. Spectral correlation with ground truth data reaches values up to 0.88. Frequency estimation errors are essentially controlled by the resolution of the spectrum, which in turn is limited by the acquisition time. In this work, a frequency resolution of 0.06 Hz is achieved for an acquisition duration of 16 s. Given these results, it is expected that MDSAR could be suitable for monitoring natural frequencies and performing modal recognition for bridges. Further improvements in the technology and in the analysis algorithm could potentially enable the accurate measurement of mode shape components.