Remote sensing techniques, particularly Interferometric Synthetic Aperture Radar (InSAR), offer a promising, cost-effective solution for civil infrastructure monitoring without the need for on-site instrumentation. Among InSAR methods, Persistent Scatterer Interferometry (PSI) exploits temporally stable scatterers to measure ground displacements with millimetric accuracy.
However, the PSI technique faces inherent limitations when applied to complex structural systems such as long-span bridges. In these cases, the deformation may differ from the simple, linear models typically assumed for ground motion, leading to loss of phase coherence and phase ambiguity issues. Consequently, potentially valuable – but low-coherence – scatterers are often discarded from analysis, resulting in an incomplete interpretation of the structure behaviour.
This work introduces a novel framework to integrate physics-based structural models (e.g. finite element models) with PSI to overcome these limitations. Rather than focusing on individual pixels, the model accounts for spatial correlation of the persistent scatterers using the structural model of the bridge. The method enables the inclusion of low-coherence Persistent Scatterers that would otherwise be excluded, enhancing the spatial density and reliability of displacement data. The methodology is applied to the Colle Isarco Viaduct (Vipiteno, Italy), a reinforced concrete bridge monitored with multi-temporal COSMO-SkyMed X-band SAR data. The infrastructure is also monitored with topographic survey measurements, which are used in this work as a validation benchmark to assess the accuracy of the results.
Results demonstrate that the proposed framework successfully reduces uncertainty in LOS displacement for poorly coherent PSs from approximately 8 mm to 3 mm, within the uncertainty bounds of the benchmark. Furthermore, the enhanced interpretation of low-coherence points provides valuable insights into the bridge structural response and thermal deformation patterns.
A. Lotti, S. Zorzi, Enrico Tubaldi et al.· e-Journal of Nondestructive...· 0 citations
Many existing methods for vibration-based monitoring of bridges rely on expensive sensor networks or require direct physical contact with the structure, which limits their widespread adoption, especially across large networks of ageing infrastructures. Recent advancements in non-contact radar technology enable precise and multichannel vibration measurements from a distance, allowing operation safely without interfering with traffic or requiring complex installation. This feature offers a practical and scalable approach to monitoring hard-to-access bridges or structures. However, commercial solutions remain expensive and are often constrained by limited beam coverage, focusing on single-point measurements.
Addressing the existing drawbacks and augmented past research, this study presents an application of low-cost frequency modulated continuous wave (FMCW) radar-based vibration monitoring bridges through experimental validation. This research also investigates multi-point vibration measurements, thanks to the use of multiple transmitting and receiving antennas in radar enabling full-field vibration measurements of the structure. The fundamental working principle of the radar systems is by emitting electromagnetic waves continuously with a linearly changing frequency and comparing its frequency to the reflected waves from the target. To monitor sub-millimetre-level displacements, which are more often observed in the real-world infrastructure systems, the radar systems use the interferometry technique to analyse the phase shift in the reflected signal to accurately estimate the small displacements.
The present study adopts the two-step strategy to validate the predictive capabilities of the FMCW radar system, as shown in Figure. The initial step involves the validation of low-cost sensors against commercial solutions on a corner reflector attached to a shake table, enhancing the signal-to-noise ratio and thereby avoiding interference from static clutter. Subsequently, validation of multiple-point vibration measurements for different targets located within the same range is carried out using multiple transmitting and receiving antennas. This approach increases confidence in the radar system's capability to evaluate the full-field vibration of a structure, rather than relying solely on single-point measurements. Second, the radars are tested for the real-world application on a bridge under the traffic or railway loadings. The field campaign is carried out to evaluate the dynamic behaviour of the bridge, such as its modal properties under ambient traffic conditions. The results reveal that the low-cost radar system effectively captures the dynamic behaviour of both the corner reflectors in the laboratory setup and the bridge during the field campaign. This enables informed decision-making for the stakeholder and asset managers for timely, actionable insight for maintenance and safety interventions by relying on the low-cost FMCW radar systems.
Vikram Kumar, Enrico Tubaldi, Carmine Clemente et al.· e-Journal of Nondestructive...· 0 citations
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