## 5 Satellite Remote Sensing Techniques for Infrastructure Monitoring
### Stand-Alone Applications of Satellite Remote Sensing for Infrastructure Monitoring
#### 5.3.4 Railways
When anomalies are detected, a more detailed PSI analysis based on high-resolution SAR products (i.e., CSK) can be achieved over smaller areas and/or complemented with on-site inspections. Lastly, the PSI method can be subject to processing errors, mainly when selected reference points, which are assumed as stable, are subject to local movements. Currently, these issues still limit the stand-alone use of MT-InSAR methods as a routine asset management tool for the detection of deformations in transport structures. Hence, the synergistic use of PSI techniques with complementary ground-based NDT methods stands as the most viable and up-to-date solution to cover these gaps. The use of ground-based NDT methods for the assessment of transport infrastructure has increased significantly in the past few decades. Several applications and case studies carried out by means of indirect non-invasive technologies have been successfully presented in the literature, proving their contribution to enhancing the productivity of asset monitoring activities. Research advances on the use of ground-based NDT methods have been reported in many areas of transportation and pavement engineering, including applications on highways [112], railways [113; 114] and airfields [115; 116]. A wide availability of multi-source, multi-scale and multi-temporal information on infrastructure conditions, as well as the advancement of hardware and software technologies, have created new opportunities for further expansion in the application of NDT methods. The information produced lends itself to incorporation into existing infrastructure management models [117]. Relying on established design and verification guides [118; 119], the rationale is to exploit the higher productivity of the ground-based NDT techniques and use the outcomes for incorporation into existing models for network infrastructure asset management. Within this context, it is observed that compliance to increasingly challenging budget and environmental goals is moving the research focus towards the development of more advanced, rapid and reliable NDT methods for pavement assessment. Efforts are being spent on the provision of continuous and faster measurements to replace existing non-destructive technologies based on discrete methods of data collection [120]. This is undoubtedly related to the higher requirements for the frequency of testing in certain types of infrastructure (e.g., railways and airfields) as well as to the inherent configuration of linear transport structures, where stop-and-go operations and partial or full-service interruption (e.g., lane closures) can dramatically affect their functionality and operability. New paradigms based on the use of non-destructive technologies have been therefore introduced, and other existing NDTs have been integrated in attempts to overcome resolution and data collection time limitations [13; 113] and to exploit their full potential. The following paragraphs report an overview of studies and applications within the context of the use of InSAR and other NDTs techniques in infrastructure engineering, sorted by infrastructure type, i.e., roads, railways, airfields and bridges. Research has confirmed the very high suitability of the InSAR techniques in detecting displacements for highways, motorways and road intersections, reaching a millimetre-level accuracy in the measurements. These applications have proven that, despite the abundant availability of PSs in urban areas, it could be challenging to associate the information with the concerned structural element, especially when medium-resolution SAR data (e.g.,C-band) are utilised. This issue has been overcome by using last-generation SAR data acquired in X-band [121], where a pixel resolution up to 1 m can be reached. In this context, [PERSON] et al. [122] conducted an experimental activity on the Aylesford area in Kent, UK, where a novel \"integrated\" holistic health monitoring approach--including the GPR and InSAR techniques--was proposed.