## 3 Discussion
### Combination of Space-Borne and Ground-Based InSAR Systems  
Space-borne and ground-based InSAR systems work at different spatial and temporal scales. By virtue of the above-mentioned differences and characteristics, the combination of these two techniques could enable us to carry out displacement measurements of the \"whole area\", as well as the \"single monument\" in heritage sites. Satellite SAR data can be used to carry out a preliminary study on surface deformation at a basin scale, providing references and guidance for GB-InSAR monitoring in terms of hotspot selection and device installation planning. [PERSON] _et al._[69] demonstrated the feasibility of such an integrated application using ground-based and satellite SAR data to perform deformation monitoring for cultural heritage at different scales. [PERSON] _et al._[70] proved the efficiency of the integration of PS-InSAR and GB-InSAR for landslide mapping, offering a valid scientific support for post-emergency management of heritage monitoring. On the other hand, in order to fully reflect local-scale deformation threats to heritage sites, the LOS displacement vectors generated from the space-borne and ground-based DInSAR can be integrated to obtain multi-dimensional deformation fields. For near-polar orbiting satellites, the north component of the displacement vector is always the most difficult to determine by LOS measurements. This is because the LOS measurements from both ascending and descending orbits are more sensitive to the vertical and east-west displacements than to north-south displacements [71]. Although solutions have been put forward by using multiple interferograms obtained from ascending and descending orbits, there are still limitations to resolve multi-dimensional deformation fields with high precision [71, 72, 73]. In addition, it is not always possible to acquire interferometric pairs over the same heritage scene from both ascending and descending passes. GB-InSAR can serve as a complementary tool of space-borne DInSAR in measuring multi-direction displacements by virtue of its optimum observing geometry. Therefore, the integrated method combining space-borne and ground-based DInSAR has great potential for multi-dimensional deformation monitoring, especially for monitoring those events whose vertical and horizontal displacements are both important. However, non-simultaneous acquisitions and the different spatial resolutions offered by these two systems also raise challenges to the integration of their deformation results. Although differential radar interferometry has the above opportunities in heritage monitoring, there are still some challenges that need to be addressed and resolved [74]. The most critical problem is the lack of effective workflow, standardized processing protocols and evaluation systems dedicated to cultural heritage. Thus, in order to promote its application in heritage monitoring, the main difficulty to be overcome is to conduct systematic investigations in heritage sites with different geographical features, environmental conditions and land cover. Based on the substantial analysis of these surveys, the application systems of differential radar interferometry can be set up for benefitting the conservation and management of cultural heritage sites throughout the world. On the other hand, in cultural heritage science, there is a communication gap between archaeologists, radar scientists and end-users responsible for heritage management, which strongly affects the effectiveness of monitoring activities. Hence, significant efforts are needed to promote the close interaction and exchange of ideas among these communities of scientists and managers through a range of cooperative activities, including frequent seminars and training workshops. In addition, the availability of satellite SAR data, as well as their cost can also limit the regular use of differential radar interferometry in heritage monitoring.