## 4 Results and Discussions
### Feasibility of SBAS-InSAR Technique in Highway Slope Inspection  
SBAS-InSAR technology was used to investigate potential artificial slope hazards along the Longqing Highway. A total of six potential hazards were identified and surveyed manually in the field. The results of InSAR analysis are basically consistent with the on-site verification, which proves that SBAS-InSAR technique can well catch the deformation of slope and reflect the safe status of the slopes.  
For slope maintenance inspection, the purpose is to find out potential slope hazards and to limit the risk of slope failure to minimum. According to the Technical Specification for Expressway Slope Maintenance (DB 33/T 2099-2018, a provincial standard of Zhejiang, China) [39] (p. 4) and Guide to Slope Maintenance (Geoguide 5, a recommended standard of Hongkong, China) [40] (p. 53), the routine inspection frequency is normally larger than once per two weeks, as shown in Table 1. Moreover, the entire length of highway should be tested. Thus, the InSAR method can meet the frequency requirement of slope inspection. Taking account of slope safety, the frequency of InSAR inspection can be set as once every 2 months, referring to the abovementioned standards. During the rainy season, the frequency can be adjusted to once or twice every month based on the evolution of deformation and duration/intensity of rainfall. To ensure the quality of results, the time span is recommended to be 6-8 months before each inspection date.  
However, there remain several limitations of SBAS-InSAR. As discussed in Figure 10, the orbit track and slope dip influences the deformation detection significantly [38]. In specific cases of slope dip and dip angle, the InSAR method can be insensitive to the deformation. Additionally, most artificial slopes should be regreened after excavation; therefore, high-density coverage of vegetations also reduce the accuracy of InSAR interpretation. In the situation, corner reflector (CR) can be an aid to natural ground objects to supplement PS candidates and to remove the residual topographic phase error [41; 42]. From an economic point of view, the cost of SBAS-InSAR method is relatively low, by saving part of the labor input. Nevertheless, the SBAS-InSAR method cannot replace manual inspections in the maintenance of highway, as signs of drainage leakage and cracking on slope surface can be hardly detected by SBAS-InSAR. In addition, the InSAR method has a capability of deformation velocity detection at the level of mm/yr to m/yr; thus, it can be useful for slow to extremely slow slope events rather than fast slope movement. For fast slope movement, approaches with higher monitor frequency should be adopted, such as terrestrial laser scanning (TSL), unmanned aerial vehicles (UAV), ground-based InSAR, and GNSS [21; 43].  
To summarize, the SBAS-InSAR technique is suitable for assistance in slope routine inspection.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{2}{c}{**Hongkong Standard**} & \multicolumn{2}{c}{**Zhejiang Standard**} \\
**Consequence-to-Life** & **Frequency** & **Maintenance Grade 1** & **Frequency** \\
**Category** & & & \\ \hline Category 1 and 2 & Once every year & Grade 1 & Once every week \\ Category 3 & Once every 2 years & Grade 2 & Once every month \\ Category 3 & & & \\ located in a remote area 2 & React to known hazard & Grade 3 & Once every 2 month \\ \hline \hline \end{tabular}  
* Determined based on safety risk classification and traffic volume. 2 A disturbed terrain feature is in a remote area if it is surrounded by natural terrain such that a failure would bring no significant direct or indirect consequences.  
\end{table}
Table 1: Frequency of routine maintenance inspections.