## 3 Discussion
### Space-Borne Radar Interferometry System  
There are several satellites acquiring SAR data suitable for DInSAR, including the European Space Agency's ERS-1/2, ENVISAT and Sentinel-1, the Canadian Radarsat-1/2, the Japanese J-ERS-1 and ALOS-1/2, the German TerraSAR/TanDEM-X and the Italian COSMO-SkyMed (four satellites). Other SAR missions are also being planned, e.g., the Radarsat Constellation Mission (three satellites), COSMO-SkyMed-2 (two satellites), TerraSAR-X-2 and the Argentinean CONAE SAOCOM constellation (two satellites) [61]. In space-borne DInSAR, the backscattering characteristics exhibited by the surface features in heritage sites are closely related to the radar observation parameters, such as spatial resolution, wavelength (or radar frequency), angle of incidence, revisit cycle, _etc._ Therefore, the selection of space-borne radar data with appropriate observation parameters is crucial for determining the effectiveness of culture heritage monitoring. We briefly summarize the principle aspects that need to be taken into account in the selection process below:  
1. Spatial resolution: High spatial-resolution InSAR system allows detailed analysis of deformation threats. For example, the range-resolution cell size of the medium spatial-resolution sensors (e.g., ERS and ENVISAT ASAR) at times approximates the size of some historical monuments, leading to the problem that it is unable to separate signals from the ground-wall interface. In this case, it is hard to determine whether an interferometric phase change is caused by ground motion or by structural deformation of the monument itself. Differential radar interferometry system with higher spatial resolution, such as TerraSAR/TanDEM-X or COSMO-SkyMed, provides a potential solution to this problem. Although layover ambiguities and multipath scattering effects cannot be resolved by higher spatial-resolution, especially for heritage sites with a large density of complex archaeological structures, a higher resolution in the range and azimuth direction provides more details for tracing these effects [62]. 2. Wavelength (or radar frequency): According to the principle of DInSAR (\(\varphi_{\text{def}}=-(4\pi/\lambda)\cdot\Delta\text{R}\)), the precision of displacement measurement is higher than the order of the wavelength. The wavelength is in the order of centimeters; hence DInSAR can measure displacements down to millimeter accuracy [63]. Besides, the wavelength and the scattering mechanism are interrelated in a complex way. Since microwaves with different wavelengths are characterized by different penetration capabilities [64], the differences in radar signatures become particularly significant when it comes to scattering distributions from surfaces, such as forests: the l-band (about 23 cm of wavelength) penetrates sparse canopy and gets scattered dominantly from the soil, which may stay coherent for years, while the X-band (about 3 cm of wavelength) is reflected even by leaves and may decorrelate after a few seconds. Temporal decorrelation increases rapidly with radar frequency. Consequently, for heritage monitoring in vegetated areas, the l-band is a preferred choice in most cases. However, since the available l-band SAR data are provided by the Japanese Space Agency JAXA missions, J-ERS and ALOS PALSAR, they are not always available for a complete time series covering the study area, depending on the geographic location (_i.e.,_ outside Asia/Pacific Ocean). Besides, the study conducted by [PERSON] _et al._[31] demonstrated that even C-band (about 5 cm of wavelength) data can provide satisfactory results over vegetated areas in rural sites, depending on the processing algorithms, the spatial resolution of the satellite and the typology of the monitored structures. Furthermore, the theoretical model presented by [PERSON] _et al._[65] assessed the impact of radiometric and geometric parameters on the precision of the MT-InSAR estimates. The phase noise decreases as the wavelength decreases and the spatial resolution increases.