# Interpretation of Bridge Health Monitoring Data from Satellite InSAR Technology
###### Abstract  
This paper presents a study on applying satellite Interferometric Synthetic Aperture Radar (InSAR) technology for the remote monitoring of road bridges and interpreting the results from a structural standpoint. The motivation behind this study arises from the widespread deterioration observed in many road bridges worldwide, leading to the need for large-scale, economic, and effective structural health monitoring (SHM) techniques. While traditional contact-type sensors have cost sustainability limitations, remote sensing techniques, including satellite-based InSAR, offer interesting alternative solutions. The objective of this study is three-fold: (i) to process InSAR data specifically for road bridges in operational conditions through the Multi-Temporal InSAR technique and extract displacement time series of reflective targets on their decks; (ii) to interpret the observed millimetric bridge displacements to verify the consistency with expected response to environmental loads and the possibility to detect unexpected behaviours; and (iii) to investigate the correlation between bridge displacements and environmental loads as temperature and river water flow variations. The study focuses on the multi-span prestressed concrete A22 Po River Bridge in Italy, utilising a dataset of X-Band HIMAGE mode Stripmap images acquired over eight years by the satellite constellation COSMO-SkyMed. The study demonstrates the effectiveness of InSAR-based SHM in detecting temperature-induced displacements and identifying different bridge spans simply by studying the sign of the correlation between displacements and temperature variation. It also reveals an unexpected behaviour in a portion of the bridge retrofitted to prevent scour issues a few years before the dataset start date. Furthermore, the correlation between pier displacements and river level variations underscores the importance of considering environmental factors and the geotechnical characteristics of the foundation soils in bridge monitoring. The results obtained from this study are significant with a view to using this satellite InSAR-based monitoring for early detection of anomalous bridge behaviour on a large scale. structural health monitoring; interpretation; InSAR; road bridges; remote sensing; satellite; persistent scatterers; displacement time series; correlation analysis; temperature deformation 2023  
13 October 2023  
27 October 2023  
2023  
2023 by the authors. Liconse MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ([[https://creativecommons.org/licenses/by/](https://creativecommons.org/licenses/by/)]([https://creativecommons.org/licenses/by/](https://creativecommons.org/licenses/by/)) 4.0/). traditional contact-type sensors: (i) scalability: cost of intensive instrumentation of a bridge ranges from 50k \(\epsilon\) to 500k \(\epsilon\); therefore, SHM systems are typically installed on few strategic bridges [5]; (ii) adaptability: when a contact-type SHM system is installed on a bridge, its components can be hardly re-used for other structures; (iii) reliability: extensive faults of contact SHM system components occur during extreme events, precisely when they are expected to provide critical information on the structural health condition [6]. Furthermore, like any other monitoring system, contact-type SHM is affected by data quality issues: large amounts of data are generated and must be appropriately analysed and managed to support decision-making [7]. Several remote sensing techniques have been explored in the last few years to cope with the limits of contact-type sensors. Among them, satellite Interferometric Synthetic Aperture Radar (InSAR) technology has emerged as a solution to remotely monitor large-scale phenomena such as subsidence, uplifting, or landslides [8; 9; 10; 11].