## 5 Satellite Remote Sensing Techniques for Infrastructure Monitoring
### Stand-Alone Applications of Satellite Remote Sensing for Infrastructure Monitoring
#### 5.3.4 Railways
The processing of COSMO-SkyMed Stripmap products demonstrated the high suitability of the PSI technique for road applications. More specifically, the analysis allowed the identification of several coherent PS points across various roads, bridges and buildings in the investigated area. The analysis was completed by GPR investigations providing structural details of the subsurface (i.e., the pavement structure of a river bridge). The approach enabled a clear mapping of the main subsurface structural features (GPR) in addition to producing long-term information on the behaviour of structures and infrastructure involved in this investigation (InSAR). Similarly to roads, railways are generally recognised as stable scatterers and good infrastructure targets for InSAR monitoring. Detected ground displacements are caused by a variety of factors that may be linked to the subsurface. In this regard, GPR technology lends itself to incorporation into InSAR investigations of railway infrastructure. In [17], the authors investigated a railway located in Puglia, Italy, using the satellite remote sensing PSI technique and the GPR technology, which was mounted on inspection trains. The PSI analysis allowed identification of critical sections through the detection of PSs affected by subsidence in the examined railway area (Figure 12). Both medium-resolution (C-band) and high-resolution (X-band) satellite data were used for the purpose. An in-depth analysis of the GPR radargrams enabled the association of areas affected by high attenuation of the signal with the critical sections identified by the InSAR analyses (Figure 12). To elaborate, the authors observed weak reflections at the interface between the sub-ballast layer and the subgrade, showing discontinuous patterns in the radargrams. This was likely related to fouling and clay intrusion at the ballast-foundation level. With similar scopes and approaches, [PERSON] et al. [123] monitored a railway section affected by subsidence. A clear match was found between areas with displacements and areas affected by potential fouling through InSAR and GPR techniques, respectively. Sections with high attenuation of the GPR signal at the foundation level were found to match with sections where the  
Figure 12: Integrated monitoring of railways through InSAR and GPR technologies, modified from [17]. GPR B-scan data collected at 0–100 m, 100–200 m and 200–300 m railway sections, showing signal attenuation (dashed orange boxes) at the subsidence sections detected by InSAR (PSs in red). highest subsidence was observed through the InSAR analyses. In general, these studies have proven the effectiveness of integrating satellite and ground-based information for structural health monitoring of railways. In the same study area, [PERSON] et al. [124] used InSAR and GPR technologies for monitoring the rail-abutment transition area in a railway truss bridge overpassing a motorway. GPR was utilised to collect subsurface structural details of the railway superstructure/substructure, including ordinary rail sections before and after the bridge section. Outcomes from the GPR surveys excluded any potential construction-related issue. A section of the railway at one of the approaches was found to be affected by the high reflectivity of the signal at the ballast-foundation level. This section matched with an area of subsidence clearly identified by the PS-InSAR analysis carried out over the entire railway section in this study. By combining the information from GPR and InSAR, the authors concluded that inadequate compaction of the ballasted layers could have been the main cause of differential displacements at the rail abutment transition area. Amongst the most recent applications conducted on airports, [PERSON] et al. [108] presented a study carried out on Runway n. 3 at the Leonardo Da Vinci International Airport in Fiumicino, Rome, Italy.