## 8 Conclusions, Challenges and Future Perspectives  
This paper reviews new developments and applications of satellite remote sensing and ground-based non-destructive testing (NDT) methods in transport infrastructure monitoring. Special focus is given to the integration between these two areas of technology and their most recent applications in the field. Overall, it is observed that excellent progress has been made in terms of the provision of high standards of data quality and accuracy. Figure 14: Flowchart of the proposed integrated health monitoring model for transport infrastructure management. However, a stand-alone use of these technologies is naturally constrained by physical (e.g., limitations in terms of nominal data resolutions of the equipment) and productivity (e.g., limitations in terms of land coverage or data acquisition pace) factors, which poses an issue on how to enhance their applicability in the sector. The concept of \"technology and data integration\" is one of the options in answering this fundamental research question. In this regard, it has emerged that new research has been carried out on combined applications and data integration studies involving satellite remote sensing and ground-based NDT methods across all the transport infrastructure modes (highways, railways and airfields). Furthermore, historical bridges have been a focus of recent research involving these two areas of technology. At present, ground-penetrating radar (GPR) stands as a popular technology for integration with Interferometric Synthetic Aperture Radar (InSAR) techniques. This is due to the possibility of collecting data on the subsurface features and providing a more comprehensive diagnostics of the causes of ground displacements. On the other hand, the concept of integration triggers some challenges that can be summarised as follows. * Existing permanent scatterer (PS) approaches have proven to work effectively. However, there are limitations in terms of target accuracy that can limit their applicability in certain areas of transport engineering. * The variety of techniques available as well as the differences in the physics and working principles of the inspection equipment make it complex to identify the actual gaps in the quality and type of information provided. A clear matching between these gaps and the actual needs in transport infrastructure monitoring is still a point of debate. * Although several NDT methods have gained official recognition as fundamental tools for integration in infrastructure management systems (IMSs), it is observed that satellite remote sensing techniques have not yet entered that stage. At present, this condition could stand as a limitation for any potential development based on the integration between these two areas of technology. The above challenges could be also looked at as prospective points of research development, as follows:  
* To investigate more deeply into the development of new potential SAR analysis methods using innovative permanent scatterer (PS) approaches (e.g., non-linear displacement models, integrated persistent scatterers interferometry (PSI) -small baseline subset (SBAS) approaches and distributed scatterers (DS) methods). * To orient research towards filling the gaps left by the stand-alone use of individual technologies, promoting their integration. A comprehensive theoretical and practical knowledge of these techniques as well as of the actual needs of several transport infrastructure sectors is essential to identify the right direction. It is the authors' opinion that the implementation of advanced machine learning and deep neural networks (DNN) algorithms can support this process. * To invest in the development of more advanced IMSs with the capacity and resources to integrate satellite remote sensing and ground-based technologies at the network level. Conceptualization, V.G., F.T., M.L.B. and A.B. ; methodology, V.G. and F.T. ; formal analysis, V.G., F.T., L.B.C., L.D. and A.B. ; investigation, V.G. ; data curation, V.G.