## 1 Introduction  
Implementing effective maintenance of transport infrastructure assets is a crucial task, as it involves important macro-economic and safety implications. The development of maintenance approaches based on the critical condition of infrastructure has triggered new demand for the use of non-destructive testing (NDT) methods in their assessment and health monitoring. Continuous monitoring of transport structures, including roads, railways and bridges, is a priority for asset owners and administrators. This ensures structural stability and operational safety, and prevents damage and deterioration, leading to expensive rehabilitation, failures and collapses [1]. Conventional technologies and methods for on-site infrastructure monitoring have been successfully applied in the past. Several ground-based NDT technologies and sensors are currently available for subsidence monitoring and displacement mapping. Amongst others, accelerometers [2], strain gauges [3], global positioning systems (GPS) [4], levelling [5], ground penetrating radar (GPR) [6; 7; 8], laser scanners [9; 10], infrared thermography (IRT) [11] and terrestrial SAR interferometry [12] have been used in the sector.  
However, the costs of the equipment are substantial, and the on-site survey operations can be demanding and difficult to implement, with a high temporal frequency at the network level. This is above all due to economic and administrative budget constraints [13]. To tackle these limitations, several advanced satellite-based remote sensing techniques, i.e., the persistent scatterers interferometry (PSI) methods, amongst which are the Permanent (or Persistent) Scatterers Interferometry (PS-InSAR) [14; 15] and the Small BASeline Subset (SBAS) [16], have gained momentum in the last few years for the monitoring of transport assets and the investigation of the surrounding environment. Several successful applications have proven the viability of using satellite-based remote sensing techniques in this sector, paving the way for the development of further research and applications [13]. In addition, the wide land coverage and the ability to collect data with a high temporal frequency have contributed to the promotion of the use of this technology in combination with other monitoring techniques. The concept of data integration of the information collected by non-destructive testing and satellite-based techniques with different spatial and temporal resolutions stands as a future challenge in research [13; 17; 18]. New investigations have recently emerged with the aim of defining novel algorithms, methods and surveying procedures for the integration of multi-source, multi-resolution and multi-temporal information. A major advantage of this approach is in the provision of additional information that is not available when technologies are used individually [17; 18; 19]. This paper is organised as follows: an introduction is given in Section 1, followed by an overview of significant stand-alone applications of satellite remote sensing and ground-based NDT methods for infrastructure monitoring (Section 2). Section 3 reports an overview of the main maintenance strategies and monitoring procedures for transport assets, followed by Section 4, describing the aim and objectives of this paper. Section 5 reports an overview of PSI applications for infrastructure monitoring. Section 6 presents recent studies on the combined application and data integration between ground-based and PSI methods in the monitoring of transport structures. Advances, benefits and main challenges for network-level monitoring with satellite remote sensing are reported in Section 7. Conclusions, challenges and future perspectives are discussed in Section 8.