## 5 Satellite Remote Sensing Techniques for Infrastructure Monitoring
### Stand-Alone Applications of Satellite Remote Sensing for Infrastructure Monitoring
#### 5.3.2 Bridges  
In the last few years, research has been produced regarding the monitoring of bridges with satellite remote sensing techniques and the investigation of the surrounding environment. Amongst the most recent applications, great interest has been paid to the monitoring of historical bridges and cultural heritage sites. It is known that vertical and horizontal displacements at the piers of bridges may seriously compromise their structural stability. Thisoccurrence is generally related to geodynamic (e.g., the sliding of the slope) or geotechnical factors (e.g., oedometric subsidence at the piles). The application of the PSI technique, and in general, the SAR-based processing methods, has been successful in reference to the evaluation of the structural components of bridges. To elaborate, the linear deformation trend, the height of the structure over the terrain, and the thermal expansion have been proven to create variations in the SAR phase [92; 93]. Several successful applications can demonstrate the viability of using MT-InSAR techniques on bridge infrastructure and the possibility of detecting displacements and accelerations. More specifically, the MT-InSAR has been used to reconstruct the thermal expansion of reinforced concrete or truss bridges, as reported in Table 2[92; 93; 94; 95; 96]. A further area of investigation is related to the integration of MT-InSAR deformation outputs into 3D finite element models of the inspected bridge [97; 98]. Research works have used historical SAR images to monitor bridge displacements over long periods of observation by assessing their structural stability or predicting collapse conditions in complex scenarios [99; 100]. To this purpose, relevant case studies have been analysed, including the collapse of the \"Morandi Bridge\" in Genova, Italy, which involved 43 victims. [PERSON] et al. [101] present a study conducted after the bridge collapse, using a dataset of SAR images acquired by the Sentinel-1 (C-Band), the COSMO-SkyMed (X-Band) and the ENVISAT missions. A non-conventional PSI approach was here implemented that located several reference points, in one of the bridge decks and detected several down-lifting PS points. For these points, an increased velocity of deformation was observed in the investigated time frame before the structural collapse. The COSMO-SkyMed dataset in the time frame March 2017-August 2018 revealed an increased deformation magnitude of several PSs located near the strands of the deck next to the collapsed pier. [PERSON] et al. [102] published a subsequent comment article in response to the study by [PERSON] et al. [101] where the same SAR dataset was processed with the SBAS and the TOMOSAR techniques. No critical PSs affected by down-lifting displacements were detected over the bridge. The authors excluded the possibility of predicting rapid movements, and hence, of predicting the collapse of the Morandi bridge by MT-InSAR.  
In general, these studies demonstrate an overall good interest of the scientific community for these areas of application and the need to implement additional research in complex scenarios. This includes the possibility of using data from other SAR missions to improve the temporal frequency of the datasets. MT-InSAR techniques have gained momentum and are becoming more consolidated, as their real-life applicability has also been demonstrated against on-site levelling and GPS measurements. Hence, satellite remote sensing monitoring has gained consideration as one of the most promising techniques for the systematic monitoring of transportation assets. An integrated use of remote sensing and on-site sensors (e.g., accelerometers, GPR, UAVs, laser scanners and optical fibres) can also support the use of this technology in monitoring activities as well as in the interpretation of the results.