## 5 Discussion
### Analysis of Unwrapped Phase Closed-Loop Residuals  
Based on research and experience [69], a closed loop is considered a problem loop when the RMSE (as explained in Section 3.5) is greater than 1.5 rad. If all the closed loops related to a certain interferogram have problems, it is considered that the interferogram may contain more phase unwrapping errors [62]. As shown in Table 5, in this experiment, 55 interference pairs generated by Sentinel\(-\)1A data constituted a total of 64 closed loops, of which one scene datum (20220607) could not form a closed loop with the other data due to the large space-time baseline. Among the closed loops, there are 20 closed loops whose RMSEs are less than 1.5 rad and 44 closed loops whose RMSEs are greater than 1.5 rad, and the problematic loops account for 68.7% of the total. Thus, 21 problematic interference pairs were identified and eliminated in subsequent processing. ALOS/PALSAR\(-\)2 formed 52 closed loops, 17 of which had an RMSE greater than 1.5 rad, accounting for 32.7% of the total, and eight interference pairs were included in the problematic closed loop. Sentinel\(-\)1A has a 36% higher rate of problematic loops compared with ALOS/PALSAR\(-\)2. For further analysis, samples of unwrapped images and closed-loop residuals of the two data sources were selected, as shown in Figure 15. Even if the temporal baseline is short, the image quality of Sentinel\(-\)1A in mountainous areas will still be extremely poor and the RMSE of all closed loops will exceed the limit (Figure 15b). It is clear that large closed-loop residuals are mostly concentrated in areas with high vegetation. However, when the vegetation coverage is low and the surface is more bare in winter, its overall RMSE is small and the interference quality is better than that of ALOS/PALSAR\(-\)2 images  
Figure 14: Evaluation of coherence coefficients. (**a,b**) represent the average coherence coefficients of Sentinel\(-\)1A and PALSAR\(-\)2, respectively. The values of coherence coefficients from small to large (0 to 1) are represented by shading from blue to yellow, respectively. (**c**) is the statistics of the coherence coefficient of two data sources. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Indicators** & **Sentinel\(-\)1A** & **ALOS/PALSAR\(-\)2** \\ \hline Total number of closed loops & 64 & 52 \\ Number of closed loops in the problem data (\textgreater{}1.5 rad) & 44 & 17 \\ Proportion & 68.7\% & 32.7\% \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Proportion of problem loops among all unwrapped phase closed loops. taken at the same time (Figure 15a,c). In addition, the Sentinel\(-\)1A data also showed better interference quality for areas containing more buildings. ALOS/PALSAR\(-\)2 data are less affected by vegetation or seasonal changes and can still maintain relatively good interference quality for a long time (Figure 15a). From the RMSE of the phase closed loop of the interference pair, even if the same time coverage is selected, Sentinel\(-\)1A is prone to obtaining continuous low-quality interference pairs in summer and thus should be completely eliminated in the subsequent processing, resulting in the direct loss of data in some continuous time series. This problem can lead to the phenomenon in which the deformed regions detected by ALOS/PALSAR\(-\)2 are stable in the Sentinel\(-\)1A results and the overall agreement between the two in terms of the deformation rate is also poor. In addition, with the increase in vegetation coverage, the periodic and seasonal characteristics of landslides in the Qinling-Daba Mountain Area are becoming increasingly prominent [2; 3]. The lack of summer data will directly lead to failure to detect many sudden landslides. Un this experiment, the low-vegetation area with excellent Sentinel\(-\)1A performance is in the city, where it is uncommon for landslides to occur.