## 4 Perspectives and Summary  
The above analysis demonstrates that the space-borne and ground-based differential radar interferometry approaches have great potential for various scales of deformation monitoring and preventive diagnosis over cultural heritage sites (_i.e._, from the whole site to a single monument). In practice, different approaches can be used in an integrated manner to set up a multi-scale monitoring workflow combined with geophysical and other auxiliary data. Conducting systematic investigations for heritage sites following a particular workflow model enables the setting up of a radar interferometry application system dedicated to cultural heritage monitoring. Such a system will also provide scientific support for planning the protection and sustainable development of cultural heritage. The workflow we propose in Figure 1 can be implemented as follows:  
The first step is to perform a preliminary investigation of the surface deformation over the whole heritage site and its surroundings. MT-InSAR or DInSAR can be applied in large-scale deformation mapping according to the surface features of the heritage site under consideration for highlighting hot spots with deformation anomalies. Besides, multi-LOS DInSAR measurements can be used to reconstruct the large-scale 3D deformation field if the interferometric pairs from both ascending and descending orbits are available. The 3D displacement vectors better reflect the real surface deformation, allowing us to better understand the mechanisms triggering deformation in heritage sites. Secondly, local-scale deformation monitoring can be carried out over specific areas, such as the unstable units detected by the first step and other groups of monuments or a single archaeological structure of interest. GB-InSAR can be used to detect structural displacements of heritage monuments or could serve as a complementary method of space-borne DInSAR in deformation monitoring at chosen directions; while GB-InRAR can be applied in evaluating the influences of surrounding vibration sources on heritage structures. In particular, GB-InSAR enables us to monitor rapid deformation caused by natural disasters or human activities, providing a useful approach for emergency management of cultural heritage sites. Besides, with respect to heritage sites with a large density of vertical structures, the usage of d-TomoSAR should be taken into consideration in order to separate interfering targets in the vertical direction. Thirdly, it is important to analyze the mechanisms triggering structural and ground deformation in heritage sites, based on available auxiliary data from geology, hydrography and historical records. Differential radar interferometry only provides information about the superficial effects of deterioration and instability phenomena, which might be caused by different triggering factors. A correct interpretation of the radar data requires substantial background knowledge of the heritage site and field inspections for verification [33]. Based on these comprehensive analyses, the displacement results obtained by different methods (including different radar interferometry approaches and traditional monitoring methods) can be compared for cross-validation. Then, the spatial-temporal deformation processes and their trends can be parameterized using certain indices, e.g., combining deformation velocity rates and acceleration and the scale, which could be useful for the recognition of factors triggering deformation. Fourthly, the existing and potential influences of natural and human-induced factors on cultural heritage can be quantitatively evaluated, including certain events, e.g., earthquakes, landslides, floods and wars, that lead to rapid deformation and long-term environmental factors or human activities, such as coastal erosion, soil erosion and groundwater pumping, that cause slow deformation. These scientific evaluations are crucial for heritage managers to make appropriate countermeasures for the conservation of cultural heritage.