## 5 Satellite Remote Sensing Techniques for Infrastructure Monitoring
### Overview of the SAR Satellite Missions  
A general overview of the main SAR satellite missions from 1992 to 2022 is reported in Figure 9. Since 2007, the second generation of space-borne SAR missions, including (i) the mission COSMO- SkyMed (CSK), developed by the Italian Space Agency (ASI) in collaboration with the Italian Minister of Defence, (ii) the TerraSAR-X (TSX), developed by the German Aerospace Centre (DLR), and (iii) the European satellites Sentinel-1A/B (SNT) of the European Union's Earth Observation Programme Copernicus, implemented in partnership with the European Space Agency (ESA), provided a spatial resolution of metres for the pixels and a temporal resolution of a few days (i.e., 12 days for SNT and up  
Figure 8: Schematic overview of the processing steps of the PS technique. Figure 7: Processing steps of the persistent scatterer interferometry analysis. to 1 day for CSK). These new advancements increased the density of PSs extracted from high-resolution SAR data up to five times compared to the first-generation SAR missions with medium resolution, i.e., the RADARSAT-1 and the Envisat missions. The provision of a very high spatial density, a wider coverage and the ability to detect deformations with a millimetre-level accuracy, has driven the use of satellite remote sensing towards the analysis of deformations in large infrastructure facilities, including bridges, roadways and airport runways. Specifically, the MT-InSAR techniques have proven very effective in infrastructure monitoring. COSMO-SkyMed represents the largest Italian investment in space Earth Observation (EO) systems and one of the most advanced constellations at the international level. A general description of the architecture of the COSMO-SkyMed system is given in [80; 81; 82]. It is a satellite constellation owned by ASI (75%) and the Italian Ministry of Defense (25%). It is designed for a dual purpose (i.e., civil and defense sectors) and is currently in its second generation. The system was entirely developed and produced in Italy by Thales Alenia Space Italy (TAS-I) as the prime contractor, with the support of Telespazio S.p.A. and Selex Galileo [81; 82]. The first generation of COSMO-SkyMed includes four satellites, launched in the period between June 2007 and November 2010, from the Vandenberg base (USA). Each satellite is equipped with a high-resolution multimode SAR sensor operating in X-band (9.6 GHz) that can acquire images with very high-spatial resolution up to 1 m, and revisiting times of days up to 12 h (i.e., in emergency cases). The system allows for high-quality and millimetre-level accuracy required both for interferometric and monitoring activities, which makes it ideal for the monitoring of transport assets. One of the key features of the CSK SAR sensors is in the possibility of operating with different acquisition modes. The first mode is the \"Spotlight\", which allows for very high-spatial resolution (up to 1 m) by concentrating the sensor on a relatively small area (10 \(\times\) 10 km). The second mode is the \"Stripmap\", which is the most popular amongst researchers, for instance, regarding its use in civil infrastructure investigations. This allows the acquisition of large areas (i.e., a scale of 30 or 40 km) with a spatial resolution of up to 3 m. Lastly, the \"Scansar\" mode is used for applications requiring a wide geographical coverage (swaths of 100 or 200 km) with a larger spatial resolution (up to 30 m). The system is equipped with a Payload Data Handling and Transmission (PDHT) for on-board storage and downlinking of SAR data. The first-generation satellites, despite being designed to guarantee a useful lifespan of  
Figure 9: Overview of the SAR satellite missions taking place from 1992 to 2022. 5 years, are currently still operational. CSK is one of the most used by both institutional and commercial users, providing information for about 15 years in the monitoring of landslides and critical infrastructure on a global scale.