# Interpretation of Bridge Health Monitoring Data from Satellite InSAR Technology
###### Abstract
Recently, the research community has started to investigate the possibility of applying such innovative technology for monitoring the long-term response of civil infrastructure [12; 13; 14]. Satellite InSAR exploits the Synthetic Aperture Radar (SAR) sensors carried by many satellites orbiting around our planet, which provide high-resolution weather-independent imagery of the Earth's surface. Some natural and artificial elements (e.g., rocks, light poles, house roofs, road guardrails) strongly reflect the electromagnetic signal emitted by SAR sensors, which receive it back and generate radar images of Earth's surface: the SAR images. Through appropriate datasets and data-analysis techniques, it is possible to obtain displacement time series of some of those reflective elements--called Persistent Scatterers (PSs)--along the Line of Sight (LoS) connecting the SAR sensor to the reflective elements up to millimetric level [15]. In principle, SAR technology allows the monitoring of large territory and civil infrastructures without installing traditional sensors on-site. Moreover, it potentially gives the possibility of going back in time and studying the past behaviour of structures based on archive SAR imagery acquired by satellites in the past years. However, the use of such technology for these purposes is still in its early stages, and only a limited number of case studies have been reported in the literature. As far as buildings are concerned, visual inspection campaigns were conducted in [16] to confirm the reliability of satellite technology in determining differential movements that would decrease servicability, and the negative effects of the subsidence phenomenon have been observed in [17]. The reliability of structural monitoring of dams through InSAR technology was studied by [18], even though it was carried out by first-generation sensors (ENVISAT) with low spatial/temporal resolution. However, this study encouraged monitoring dams with higher spatial resolution sensors, as in the case of [19]. InSAR has also been used for monitoring ground surface movements following the construction of underground tunnels. Two applications are reported by [20]: the first emphasises the integration between InSAR monitoring and traditional monitoring; the second studies the temporal evolutions of settlements along a landslide slope hosting two tunnels using Multi-Temporal InSAR. Regarding roads and railways, the possibility of large-scale monitoring of these structures through InSAR is demonstrated in [21]. Similarly, [22] presents a completely automated monitoring solution for road networks, highlighting possible damages or unexpected displacements. Delving into the published studies on bridge monitoring using InSAR technology, [12] extracted important information regarding the effects of bridge scour. Through the analysis of historical series, the authors observed unexpected behaviour of the pier affected by scour already one month before its collapse. In addition, [23] studied the progressive displacement of some bridges in California (USA), observing how these movements were not attributable to structural defects but were caused by subsidence phenomena resulting from the continuous water pumping from the surrounding aquifers. Also, [24] demonstrated that it was possible to measure past anomalous deformations of the Morandi Bridge (Italy)--which collapsed in 2018--using archived InSAR images, highlighting a continuous increase in the relative deformation between the pier and the deck of this bridge since 2015. An attempt to reconstruct the two-dimensional displacement field was conducted by [25], acquiring displacement information from ascending and descending geometry for the Albiano-Magra viaduct, which collapsed in 2020. Finally, an interesting study published by [13] utilises satellite monitoring as an early warning tool for unexpected displacements.