Experimental validation of spaceborne SAR for measurement of structural vibrations
Conventionally, contact sensors such as accelerometers are used to conduct vibration-based structural health monitoring (VBSHM), providing accurate measurements but with deployment significantly limited by installation and maintenance challenges. Remote sensing alternatives for VBSHM have consequently garnered interest for alleviating monitoring costs. Several techniques have been developed previously: uncrewed aerial vehicles (UAVs) equipped with cameras for digital image correlation, and both ground-based lidar and millimetre-wave Doppler radar in either real or synthetic aperture modes – however these all require onsite proximity, limiting their coverage. Spaceborne sensing offers a less occluded vantage point from an orbiting platform, both features vastly improving sensor coverage. Synthetic aperture radar (SAR) is a widely used technology, where in an SHM context interferometric SAR (InSAR) has been noted for utility in measuring the long-term displacement of structural elements. As InSAR functions by comparing multiple SAR acquisitions of an area, its sampling rate – dictated by the revisit time of EO missions – is too low for VBSHM purposes. The alternative presented here is micro-Doppler SAR (MDSAR), which measures motion using a single SAR image and can achieve the sampling rates required, providing a complement to conventional SHM approaches. MDSAR functions by estimating the Doppler shift caused by a vibrating target, which is observed in a SAR signal. These shifts can be related to a target velocity along the line of sight, and sampled at rates matching the oscillation frequencies of large structures. This paper presents an MDSAR technique, showing time history and spectral results both of calibration tests and a validation experiment conducted on a bridge. The input data are high-resolution, single-pass SAR images of real-world targets obtained through commercial SAR companies, with synchronous ground truth measurements gathered by conventional sensing. Calibration measurements of radar targets showed good agreement between time histories for velocities down to RMS 0.66 mm/s for 2 Hz oscillation with a 0.4 Hz amplitude modulation similar to that observed for bridges, with RMSE values of 64%, and a spectral residual value less than the frequency resolution of 0.07 Hz. Validation experiments were carried out on the South Portland Street Suspension Bridge in Glasgow, UK, with ground-truth from an installed accelerometer monitoring system. They are consistent with calibration tests and demonstrate the feasibility of measuring vibrational velocities as low as 1 mm/s with MDSAR. In the time domain, the average measurement error is approximately 1 mm/s, comparable to the true velocities of the bridge. In the frequency domain, the technique performs well in identifying the dominant vibrational frequency, with a residual less than the frequency resolution of 0.06 Hz determined by the SAR acquisition duration of 16 s. Full modal identification (of mode shape components) is currently limited by the characteristics of current SAR missions, including radar wavelength, signal-to-noise ratio, and acquisition time. In the absence of modal coupling, partial information e.g. modeshape phase components, is retrievable. Although MDSAR cannot yet supplant onsite SHM methods, it can still provide valuable insights which could integrate into hybrid systems and scope remains for refinement of measurements.