Conference Agenda

In the framework of interferometric applications devoted to ground instabilities, artificial reflectors, both passive corner reflectors and active transponders, are usually designed and exploited to calibrate the interferometric measurements as well as to provide displacement measurements where natural coherent targets are missing within the area of interest. Passive devices have drawbacks related to their large size (specifically for long SAR wavelengths) and weight that make deployment problematic in difficult areas, as well as to possible loss of phase signal coherence due to geometric deformations and material degradation. Compact active transponders have been proposed as alternatives to corner reflectors, showing promising results but also some issues related to their relatively high cost and signal stability. The aim of this study is to assess the reliability and limits of corner reflectors and active transponders for supporting multi-temporal SAR interferometry (MTInSAR) in displacement measurements, with a specific focus on urban areas.

An experimental calibration site consisting of two corner reflectors and three active transponders was established on the roof of the Physics Department of the Polytechnic University of Bari (PhyBA) (Southern Italy). The site is operated by Geophysical Applications Processing (GAP), in collaboration with two research institutions (CNR and PoliBA). The corner reflectors s are trihedral structures with internal edge lengths of 69.5 cm (CR0) and 1.05 m (CR1). The active reflectors include a C-band Electronic Corner Reflector (ECR-C) developed by MetaSensing and two Active Radar Calibrators, a C-band unit (ARC-C) and an X-band unit (ARC-X), both developed by the Remote Sensing Group of PhyBA. The ARC devices, designed in the early 1990s for NASA/JPL airborne campaigns and the SIR-C/X-SAR mission, use military-grade Gallium Arsenide Field-Effect Transistor amplifiers and horn antennas. In this setup, ECR-C and ARC-C are tuned for the ESA Sentinel-1 SAR mission, while ARC-X is used for the ASI COSMO-SkyMED constellation. ECR-C can switch electronically between ascending and descending passes, whereas ARC-C and ARC-X require mechanical reorientation. This calibration site provides a controlled environment to test passive and active reflectors under real urban conditions and by simulating displacements at different rates and directions.

Data acquired up to 2026 from Sentinel-1 and COSMO-SkyMED along both ascending and descending passes were processed through the SPINUA MTInSAR algorithm to derive time series of both SAR amplitude and displacement values. These estimations were then analysed for assessing the quality of backscattering and interferometric phase with respect to reflector size, geometry, polarisation, and distance from other reflectors and structures. Results show that measured backscatter is consistent with theoretical expectations. Reflector size proved to be a key factor: CR0, being smaller, was noisier and more affected by nearby structures, while CR1 provided more stable responses. Active reflectors offered strong amplitude signals, though their phase stability was sometimes lower than expected. Performance was also influenced by interactions among artificial reflectors as well as with other structures present on the building roof.

Specific experiments were designed and performed to investigate the possibility of monitoring subpixel differential displacements by exploiting both the polarisation diversity and the time diversity induced by a delay line. Since January 2024, ARC-C has been operated in VH polarisation, while CR0, which responds in VV polarisation, has been positioned at about 1 m from ARC-C and occasionally moved upward up to 1 cm. By processing both VH and VV Sentinel-1 datasets, it was possible to identify the two reflectors and correctly measure the line-of-sight component of the induced CR0 displacements.

Similar results were obtained by setting ARC-C in VV polarisation and by delaying its response through a 30 m long coaxial cable calibrated for C-band. The delayed ARC-C signals were imaged a few pixels away from the CR0 in the VV Sentinel-1 data, thus allowing us to analyse separately the two targets and thus distinguish their behaviour in time. These configurations allowed us to test the measurement of the differential displacement between two point-like targets located within the same Sentinel-1 resolution cell, opening opportunities for monitoring closely spaced scatterers and for exploiting the VH polarisation, which has so far been underused in MTInSAR applications.

Moreover, since in urban settings the precise geocoding of the structures becomes pivotal, we assessed the performance of geocoding of Sentinel-1 data in relation to the increase of the Sentinel-1A orbital tube from mid-April 2024, which improved the precision of target elevation estimation through MTInSAR.

Further work focused on innovative reflector geometries and low-cost transponders. Specifically, a hexagon-periodic corner reflector with a side length of 46 cm was designed to achieve a more compact and less visually intrusive structure, suitable for architecturally sensitive environments. A prototype was built and tested after 3D electromagnetic simulations. The reflector showed good performance at C-band, but it exhibited a highly directive response. Despite this drawback, the design remains promising for applications where aesthetics is a primary concern. In addition, a prototypal low-cost transponder was designed and built up by PoliBA. Experimental tests have so far been performed using both Sentinel-1 C-band data and COSMO-SkyMED X-band data. There is, however, a growing interest in extending these investigations to the L-band, where transponders could provide a major advantage compared to passive corner reflectors in terms of physical size. Moreover, L-band is expected to ease the preservation of transponder phase stability due to the lower frequency with respect to C/X bands.

In the future, the proposed calibration site may become an open experimental facility, providing data available for testing and validating interferometric techniques.

Acknowledgment

This work was supported in part by ESA under the project “Quantum computing for ground motion measurement.”, ESA/RFQ/3-17708/22/I-DT-lr. The authors also wish to acknowledge all researchers and professionals involved in research activities related to the Experimental SAR Calibration Site of Bari (Italy) under self-financed projects.

Europe’s bridge stock is aging, and many structures are approaching or exceeding their design life, which increases the need for continuous, objective evidence to support safety assurance, maintenance prioritization, and life-extension strategies. Periodic visual inspections alone can miss early-stage deterioration and are difficult to scale at the frequency required for risk-informed management. At the same time, Structural Health Monitoring solutions based on a single technology often provide only a partial view of structural behaviour; they can capture local changes but may fail to contextualize long-term trends or external drivers acting at the scale of the surrounding environment.

ISABHEL (acronym of Integrated SAtellite and ground-based monitoring for Bridge HEalth Lifetime assessment) addresses this gap by demonstrating an integrated monitoring service that combines satellite A-DInSAR, contact sensors, Photomonitoring™, and Finite Element Modelling within a single decision-oriented workflow and a unified user-facing platform. The project is co-funded by ESA and implemented with the Municipality of Turin as pilot user. The service is deployed on two river bridges in Turin, Italy, namely the Amedeo VIII bridge and the Regina Margherita bridge. These case studies were selected because they represent different structural typologies and monitoring needs while sharing exposure to hydraulic hazards such as potential scouring. The Regina Margherita bridge also provides an opportunity to capture wider-area ground processes in the surrounding environment, including possible slope-related phenomena that can influence structural performance.

The ISABHEL concept is built on complementarity. Satellite A-DInSAR techniques provide millimetric-scale deformation information over long time spans and wide areas. This supports historical screening and trend interpretation and can reveal subtle anomalies that are hard to detect with on-site instrumentation alone. In ISABHEL, high-resolution COSMO-SkyMed data are processed with a Persistent Scatterer Interferometry (PSI) approach; the availability of archived imagery since 2009 enables retrospective analysis and the reconstruction of long-term displacement histories. For the Turin demonstration, hundreds of SAR images in both ascending and descending geometries are exploited over the 2011–2024 period. Results are referenced to stable points and quality-controlled using standard deviation and temporal coherence indicators, then calibrated and validated against a GNSS station in the municipality area. Beyond standard line-of-sight deformation products, ISABHEL applies vector decomposition using both geometries to derive vertical and east–west components through Synthetic Measurement Points. The processing and exploration of results are supported by a dedicated toolchain integrated in QGIS, enabling infrastructure-oriented queries, time-series inspection, and the extraction of deformation indicators around specific elements.

Ground-based monitoring complements EO by delivering high-frequency, high-accuracy measurements of structural response and enabling near-real-time tracking of key parameters. The monitoring strategy is differentiated across the two bridges. The Amedeo VIII bridge is primarily monitored statically, targeting rotations and displacements that can indicate progressive settlement or changes at key structural interfaces. The Regina Margherita bridge combines static and dynamic monitoring to detect potential scour-related effects and to assess deck behaviour and load-bearing capacity through vibration-based indicators. The sensor suite includes biaxial inclinometers, triaxial accelerometers, strain gauges, displacement transducers, and temperature probes.

A resilient communications architecture supports operational continuity, using 5G for high-speed transmission and satellite communication as a backup channel. The test results confirmed reliable data acquisition and transmission, with sampling at 200 Hz for accelerometers and 1 Hz for static sensors, and stable data integrity and continuity. Automated processing routines transform raw measurements into standardized outputs and compute basic statistics on an hourly basis. Dynamic data are further processed through Automated Operational Modal Analysis to estimate modal parameters such as natural frequencies, damping ratios, and mode shapes, producing hourly results suitable for long-term trend tracking.

Photomonitoring™ adds a non-contact layer based on image sequences acquired by site-installed cameras; it supports both displacement analysis and change detection, enabling the identification of surface modifications and localized anomalies that may be relevant for inspection planning. The service includes controlled image synchronization and a processing chain implemented through dedicated software. Image transfer can be completed in less than one minute, and processing times were verified at less than one minute per image pair in the tested configuration, with sub-pixel registration accuracy and quantitative acceptance criteria for change-detection overlap against reference polygons.

A central differentiator of ISABHEL is the Digital Twin layer based on Finite Element Models developed for each bridge. The modelling strategy progresses from simplified representations to more detailed formulations as needed to capture deck behaviour. The models are informed by technical documentation, historical A-DInSAR trends, which in some cases enable the observation of the seasonal thermal behaviour of the structures, and on-site inspections and are calibrated using load-test data and progressively refined as monitoring data streams become available. This physics-based layer supports interpretation of measured responses and reduces false alarms by accounting for expected structural behaviour under operational loads and environmental forcing. It also enables the definition of model-driven thresholds tied to structural mechanisms and limit states rather than relying only on statistical deviations.

ISABHEL implements a multi-level thresholding framework: an initial level reflects operational conditions derived from preliminary model evaluations; then, a second level captures statistically significant deviations from normal behaviour using baselines built during the first monitoring period. Finally, a third level is linked to ultimate limit states derived from numerical simulations and represents conditions approaching failure. Temperature is treated as a key explanatory variable across the workflow because thermal effects can dominate both long-term and cyclic responses in many bridge typologies. Separating temperature-driven variability from anomalous behaviour is essential for robust alerting and for reliable interpretation of both EO and in-situ measurements.

All information streams converge in a web-based platform designed for operational use by different stakeholder profiles. The platform aggregates satellite deformation products, Photomonitoring™ outputs, contact-sensor time series, and Digital Twin results. It provides a dashboard and geospatial views and issues alerts when thresholds are exceeded. The performed tests validated end-to-end functionality across modules, including SAR processing and geohazard layer integration, sensor acquisition and transmission with SatCom backup, Photomonitoring™ displacement and change detection, and FEM model generation and calibration activities.

Early EO results already illustrate the value of integration. For the Regina Margherita bridge, COSMO-SkyMed analysis highlights localized deformation at midspan and indicates asymmetry between carriageways that can guide targeted inspections, sensor placement, and modelling priorities; for the Amedeo VIII bridge, deformation appears within stable ranges with time series dominated by seasonal cycles around a near-zero average trend, supporting a differentiated interpretation of behaviour and a tailored threshold definition. Overall, ISABHEL demonstrates a practical pathway from satellite-to-sensor monitoring to decision-ready bridge lifetime assessment. The integration of wide-area EO evidence, local high-frequency instrumentation, image-based diagnostics, and model-driven interpretation supports early detection of change and a more reliable classification of what constitutes normal behaviour. The Turin pilots provide a replicable blueprint for scaling the service to additional bridges and networks, enabling more proactive maintenance, improved intervention prioritization, and enhanced resilience of critical transport assets, with planning for critical transport corridors.

Differential land subsidence affects many world metropolises, impacting their public and private infrastructure, including housing, transport and utility networks, social, healthcare and education facilities and, in turn, causing socio-economic impacts. This work showcases an innovative workflow based on geospatial data for exposure-vulnerability rating, hazard quantification and risk assessment.

The methodology integrates Interferometric Synthetic Aperture Radar (InSAR)-derived information on ground displacement from Copernicus European Ground Motion Service (EGMS), with land cover and settlement characteristics from freely and openly available global datasets including the Copernicus Global Human Settlement Layer (GHSL) and DLR’s World Settlement Footprint (WSF). Such an integrated approach represents a significant step forward from InSAR displacement velocity-based approaches that are nowadays common in the specialist literature, to actionable risk information that are still rare. Land subsidence-induced deformation and structural stress on urban assets are quantified within the 15 metropolitan cities of Italy, along with the distribution and amount of residential/non-residential infrastructure and population exposed. Deformation-induced risk is assessed via the implementation of a tailored risk matrix enabling the geospatial intersection of four hazard (H1 to H4) and four exposure-vulnerability (EV1 to EV4) classes into 16 combinations of likelihood and impact (or also, probability and severity), and the consequent classification of risk in three levels (R1 to R3).

The analysis shows that a total of 1.44 out of 2665 km² urbanised land within the 15 cities is at high risk (R3) due to significant angular distortions (and, sometimes, additive threat from horizontal strain) affecting very high exposure-vulnerability infrastructure. Moreover, it is estimated that, for more than 2700 buildings within the 15 cities, there is high likelihood of already occurred/incipient structural damage. The reference knowledge-base on present-day subsidence-induced risk can inform land and risk management at national scale, and provides a baseline for future assessments to build upon with a look to the next decades and sustainable urban development.

This work is funded by the European Union – Next Generation EU, component M4C2; project SubRISK+ (https://www.subrisk.eu/), 2023–2026 (CUP B53D23033400001).

Value-added risk mapping outputs and statistics are openly available via the SubRISK+ ‘Control Room’ web platform (https://controlroom.subrisk.eu/).

Full details about the workflow and results are available in the full paper: Cigna, F., Paranunzio, R., Bonì, R., Teatini P. 2025. Present-day land subsidence risk in the metropolitan cities of Italy. Scientific Reports, 15, 34999 (https://doi.org/10.1038/s41598-025-18941-8).

Keywords (7): InSAR, TerraSAR-X, Bridges, Viaducts, Infrastructure, Monitoring, Segmentation

Introduction: Monitoring Needs within the Dutch National Infrastructure

Monitoring the stability of critical infrastructure is essential for ensuring safety and maintaining asset performance within the portfolio of the Ministry of Infrastructure and Watermanagement managed by Rijkswaterstaat. Responsible for a safe, sustainable and liveable environment, Rijkswaterstaat maintains thousands of kilometres of road, waterways and dikes, and hundreds of civil engineering structures on a national scale. As many of these assets were built after 1945, they are approaching the end of their designed service life. To facilitate renovation and replacement on a national scale, Rijkswaterstaat must be able to prioritise assets based on objective indicators of structural condition. Reliable deformation information is therefore essential for identifying structures that require attention urgently.

Traditional geodetic measurement techniques such as levelling, GNSS, and total station surveying provide highly accurate deformation information, but they require on-site access, traffic management measures, and extensive safety precautions. These operational constraints make large-scale and frequent monitoring costly and logistically challenging. With a growing number of civil structures that require monitoring, Rijkswaterstaat must adopt methods that can provide broader coverage without increasing operational burden. Therefore, InSAR is being implemented as a scalable technique for stability assessment. Its wide spatial coverage, frequent revisit rates, and remote, non-intrusive data acquisition make it well suited for monitoring large and dispersed assets, alleviating some of the challenges inherent to traditional methods.

While InSAR-based monitoring has been implemented successfully for relatively uniform assets such as roads and dikes, dynamic civil structures such as bridges and viaducts introduce a fundamentally different set of challenges. Features including expansion joints, liftable bridge decks, tall support pillars and steel trusses create strong spatial variability in deformation behaviour and complicate radar imaging through shadow, layover and double-bounce effects. These structures also exhibit operational dynamics, such as bridge openings or variable loading, that alter their appearance in the radar signal. Tailoring InSAR-processing chains accordingly is therefore necessary to accommodate these structural and imaging complexities and ensure that deformation patterns are captured and interpreted in a physically meaningful way.

Study Area: Botlek Bridge and Adjacent Viaducts in Rotterdam, The Netherlands

To investigate these challenges in practice, this study analyses the Botlek Bridge and its adjacent viaducts in Rotterdam, The Netherlands, as a representative example of a complex civil structure. The system comprises two large liftable bridge decks that can be raised vertically to accommodate waterway traffic and that carry both road and rail transport. Its spans are supported by concrete pillars and segmented by expansion joints at multiple locations, leading to structural elements that respond differently to temperature, loading conditions and operational states. The presence of steel trusses, barriers, rail infrastructure and other load-bearing elements introduces a heterogeneous set of radar reflectors, while the liftable nature of the bridge decks periodically changes the observable geometry. Together, these characteristics make the Botlek bridge an ideal case study for evaluating how InSAR products behave on complex, dynamic structures and for identifying the adjustments required to make InSAR operationally useful for this object type.

Methodology: From Radar Imagery to Deformation Timeseries

For this analysis, high-resolution TerraSAR-X StripMap images spanning a period from January 2022 until July 2025 are used. Both an ascending and descending track are processed (level 2), resulting in Line-of-Sight (LOS) point deformation time series with an interval of 11 to 22 days. Combining information from each track allows us to decompose LOS deformation into horizontal and vertical displacements elementwise (level 3) under the assumption that all elements only move vertically and longitudinally in the horizontal plane. Element boundaries are derived from structural properties such as expansion joints to delineate sections that are expected to move homogenously. Finally, model-fitting is done on level 2 and level 3 data to visualise the spatial distribution of the deformation profile.

Additional sources including a high-resolution DEM (0.5 m), sensor-derived bridge opening times, high-resolution aerial imagery and a digital object registration bank are used to enhance the InSAR-processing and form an initial structure-aware segmentation.

For this project, SatSense was responsible for the InSAR-processing under commission of Rijkswaterstaat, keeping the technical challenges in mind. Fruitful discussions and cooperation between SatSense and Rijkswaterstaat resulted in the findings discussed next.

Key Findings

Thermal deformation dominates and varies strongly across structural boundaries

The Botlek Bridge exhibits pronounced longitudinal thermal expansion and contraction, with horizontal amplitudes up to 15 mm. These signals vary sharply across expansion joints and viaduct segments, confirming that bridges show strong internal deformation gradients that require element‑wise analysis rather than treating the structure as a uniform object. Horizontal movement is opposed around expansion joints, highlighting that network selection and element segmentation must follow structural boundaries to ensure that the deformation behaviour of each component is represented accurately.

Internal expansion of viaduct spans is observable when segment length allows subdivision

Viaduct spans show internal thermal expansion when their length permits subdivision into multiple structural elements. Opposing horizontal motion at the boundaries of these elements reveals span-specific thermal behaviour that would be obscured if the viaduct were analysed as a single unit. This demonstrates that detectable internal deformation is contingent on both segment length and appropriate element delineation, illustrating the value of structure-aware segmentation for capturing localised responses.

Structural complexity creates characteristic radar artefacts that influence observable deformation

Tall pillars, steel trusses, barriers and overhanging components generate radar shadow, layover, and double‑bounce reflections, leading to uneven spatial coverage, occasional mislocated scatterers and mixing of deformation signals. These artefacts are not noise but predictable consequences of structure‑specific geometry in relation to the satellite viewing angle, reinforcing the need for object‑aware interpretation and scatterer filtering. Radar shadow and layover maps can be used to partially assess these effects a priori, helping set expectations for the achievable spatial coverage.

Operational behaviour influences the temporal coherence of InSAR observations

When bridge openings coincide with SAR acquisitions, the lifted decks change their radar appearance, breaking temporal consistency in the deformation signal. For the Botlek Bridge, 8 out of 112 descending‑track acquisitions and 10 out of 59 ascending‑track acquisitions were removed after visual inspection confirmed that deck geometry was altered during acquisition. Accounting for such operational events is critical in workflows for dynamic structures to avoid introducing artefacts into the displacement time series, especially since the bridge sensor data did not always reliably coincide with the opening events visible in the radar imagery.

Discussion and Conclusions

The results highlight that InSAR can capture the complex deformation behaviour that aligns well with the expected behaviour. Both internal expansion and opposed deformation around expansion joints are captured as a result of structure-informed segmentation. Additionally, the detection of subtle long‑term trends, such as minor subsidence and horizontal divergence near expansion joints, confirms that multi‑year InSAR time series can reveal gradual, persistent changes that may be difficult to observe with traditional measurements alone.

Overall, this study shows that while InSAR provides valuable insights into the deformation behaviour of bridges and viaducts, its effective use depends on incorporating structural knowledge, accounting for radar‑specific artefacts and applying object‑appropriate interpretation guidelines. The insights gained from the Botlek bridge contribute to the development of object‑type‑specific InSAR methodologies for Rijkswaterstaat. The lessons-learned serve as a basis for the upcoming European tender to develop object-type-specific InSAR methodologies, supporting more efficient monitoring of Rijkswaterstaat’s nationwide infrastructure portfolio.

Satellite-derived surface motion measurements have become routinely available through Interferometric Synthetic Aperture Radar (InSAR) techniques applied to data from multiple SAR missions. Even the complexity traditionally associated with InSAR processing has been significantly reduced through the emergence of platform-based solutions, which promise to minimise data-handling requirements while enabling straightforward execution of advanced processing chains for the generation of displacement measurements. However, the transition from surface motion to meaningful estimates for engineers and, ultimately, to actionable information that allows the characterization of structural stress, remains a critical challenge.

The concept of differential settlement causing deformation affecting urban infrastructures, expressed through the calculation of angular distortion, provides an important step in this direction. In existing approaches, higher-level products of angular distortion are typically derived solely from surface motion measurements through gridding and the calculation of local gradients. While effective at lower spatial scales, these methods do not explicitly incorporate the geometry and orientation of the infrastructure undergoing deformation. The explicit inclusion of the infrastructure itself allows for more customized calculations and improved local estimates, considering the actual direction along which tilt or differential motion occurs.

In this study, Copernicus Sentinel-1 data over the metropolitan area of Rome, Italy, for the period 01/2022 to 12/2024 (approx. 3 years) are processed using both ascending and descending tracks through the SNAPPING PSI service on the Geohazards Exploitation Platform (GEP). Line-of-Sight (LoS) measurements are then combined to retrieve vertical motion time series, subsequently used as input for differential settlement calculations. A parallel assessment is performed to investigate the effect of gridding during the 3D decomposition stage on the resulting settlement estimates.

The proposed methodology introduces an infrastructure-aware framework for angular distortion estimation. Enhanced triangulation of Persistent Scatterer (PS) is considered to link neighboring observations, followed by linear interpolation of displacements over structural elements. OpenStreetMap (OSM) building footprints and road network data are thus integrated in the processing chain to guide spatial analysis. Angular distortion is subsequently calculated in a manner tailored to the geometry and orientation of these structures, including not only the average motion rates, but the entire displacement time series. This infrastructure-aware geospatial analysis allows for more robust and physically meaningful estimates compared to conventional grid-based approaches. An intercomparison between the proposed methodology and existing approaches for deriving angular distortion from spaceborne point-like InSAR measurements demonstrates its capability to highlight localized differential motion patterns while also capturing the directionality of the deformation effect.

The results demonstrate that angular distortion derived from spaceborne surface motion measurements can provide meaningful indicators of structural stress when calculated in an infrastructure-constrained framework. Such metrics can support the identification of zones where differential settlement may pose a risk to buildings and transport networks, thereby contributing towards large-scale, remotely derived assessments of structural stability in urban environments.