Vibrational monitoring of structures traditionally relies on contact sensors such as accelerometers, displacement transducers, and strain gauges, which provide reliable physical information for structural health assessment. However, these sensors require manual installation and direct access to the structure, resulting in practical limitations in terms of installation time, safety, and accessibility, while long-term maintenance may compromise the overall Structural Health Monitoring (SHM) system reliability. These constraints often hinder the systematic implementation of SHM, particularly for large-scale infrastructures such as bridges. Alternative approaches present complementary limitations: vision-based techniques, such as digital image correlation, are sensitive to lighting conditions, camera calibration, and line-of-sight occlusions, whereas fibre-optic sensing systems, despite their high accuracy and distributed capabilities, require permanent installation and physical integration within the structure, limiting their suitability for rapid or temporary monitoring campaigns.
To address these challenges, Ground-Based Interferometric RADAR (GB-InRA) technology has emerged as a promising non-contact alternative for vibration monitoring, enabling safe measurements where visual inspection or contact sensor deployment are impractical. Interferometric radar enables the detection of sub-millimetric displacements by measuring the phase difference between transmitted and received signals. In this context, Real Aperture Radar (RAR), which provides one-dimensional line-of-sight measurements, is typically preferred for vibration monitoring over Synthetic Aperture Radar (SAR), which enables two-dimensional imaging. Nevertheless, radar-based measurements remain sensitive to instrument-to-target distance and antenna tilt, which must be carefully calibrated to ensure reliable results.
In the perspective of bridge monitoring, this work presents an experimental study conducted on a simply supported flexible steel beam (Figure 1). Vibrations were measured using an IBIS-FS microwave interferometric radar and compared against conventional displacement transducers adopted as ground truth. Three corner reflectors were installed at distinct beam locations to enable the estimation of the first three vibration mode frequencies and shapes. Impulsive hammer excitations induced sub-millimetric displacements, allowing a quantitative assessment of radar sensitivity and measurement accuracy. Preliminary results indicate displacement amplitude errors ranging between 0.02 mm and 0.2 mm for reference amplitudes between 0.5 mm and 12 mm, when compared to displacement transducers. Different configurations of vertical tilt and radar-to-target distance were also investigated to evaluate their influence on displacement estimation and modal identification. Overall, the findings demonstrate the effectiveness and practical applicability of GB-InRA as a robust, non-invasive tool for vibration monitoring and modal identification of civil structures.
Chiara Suppi, R. Andreotti, Vikram Kumar et al.· e-Journal of Nondestructive...· 0 citations
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.
A. Vattulainen, A. Lotti, Chiara Suppi et al.· e-Journal of Nondestructive...· 0 citations
