## 5 Satellite Remote Sensing Techniques for Infrastructure Monitoring
### Stand-Alone Applications of Satellite Remote Sensing for Infrastructure Monitoring
#### 5.3.4 Railways
Two different SAR datasets were processed, including (i) Sentinel-1 (C-band) data in both the acquisition geometries (i.e., ascending and descending), and (ii) COSMO-SkyMed (X-band) data. Furthermore, measurements were also compared with those collected on the runway using the ground-based levelling technique. The SAR datasets covered a time frame from April 2017 to December 2019. The aim of the study was to test and investigate the viability of using medium-resolution SAR interferometry data in monitoring ground displacements on runways at the millimetre scale, in a complex scenario affected by a well-known subsidence of different severity affecting the runway. A geostatistical analysis was developed to (i) study the statistical variability of the datasets, (ii) interpolate the Sentinel-1 data through an ordinary kriging (OK) method and (iii) compare the interpolated displacements to the values collected on-site by topographic levelling. Figure 13 shows the main outcomes from the use of ground-based topographic levelling data and satellite remote sensing data on the runway. A comparison between the PSI outcomes from the Sentinel-1A SAR data interpolated through OK and the ground-truth topographic levelling data demonstrated a relatively high accuracy of the medium-resolution data. This was proven by the high values of the correlation coefficient (r = 0.94) and the multiple R-squared coefficient (R\({}^{2}\) = 0.88) comparing levelling and SAR data [125]. Figure 13: Integrated monitoring of runways through Sentinel-1 SAR products processed by MT-InSAR and interpolated by geostatistical methods and levelling, modified from [108]. Displacement velocity trends of the Sentinel-1 PSI data interpolated by a Kriging geostatistical approach (red cross markers) and longitudinal survey profiles (**a–e**) by topographic levelling (blue squared markers); (**f**) scatter plot of the interpolated Sentinel-1 and levelling deformations, with regression line. In [125], an InSAR investigation was conducted into the differential settlements at one of the runways at the Fiumicino International Airport in Rome, Italy. Several COSMO-SkyMed X-band SAR images were processed by means of the PS-InSAR approach, highlighting the presence of vertical displacements. These were compared to the outputs from a dense levelling campaign conducted over four years. The accuracy of the results confirmed that PSI analyses of high-frequency images are very effective in reconstructing vertical deformations in airfield runways. Bridge monitoring is a further area of investigation where the integrated approach has proven effective. Literature shows increasing research for use of ground-based NDTs, especially GPR, as fundamental investigation tools in bridge diagnostics [126; 127]. Although the above methodologies can collect reliable information, a fully comprehensive and routine diagnostics of the bridge conditions cannot be achieved if these techniques are utilised individually. To this end, the integration of multi-source, multi-temporal and multi-resolution datasets is a challenge and an area of further research development. Research in [128] explored how InSAR datasets can be combined with traditional measurement techniques including automated total stations and sensors installed on the Waterloo Bridge, a 434-m long concrete bridge carrying the A301 across the River Thames in London, UK. A novel approach to InSAR bridge monitoring was adopted by the installation of corner reflectors at key points of structural interest on the bridge, to supplement the bridge's own reflection characteristics and ensure that the InSAR measurements could be directly compared and combined with measurements collected on site. A strategy to combine and interpret various data from multiple sources was presented, outlining the practical applications of this data analysis to support wider monitoring strategies. In [129; 130], the viability of using X-band SAR imagery for effective monitoring of historical bridges was investigated.