## 3 Status and Application
### Single-Pass Interference
#### 3.2.1 CT-InSAR  
Compared with optical satellites, InSAR systems basically do not have limitations on imaging ground targets due to conditions like day-night cycles and weather. Especially in areas with perennial rain, uninhabited areas, and foreign regions, InSAR systems have \begin{table}
\begin{tabular}{c c c c} \hline \hline
**InSAR Technology** & **Advantages** & **Disadvantages** & **Accuracy** \\ \hline \multirow{6}{*}{D-InSAR} & \multicolumn{3}{c}{Two or more high-quality SAR images are} \\  & Easy processing. & required. & \\  & Suitable for deformation monitoring in a small area. & Low sensitivity. & \\  & & a small area. & Not suitable for large-scale deformation & \\  & & monitoring and non-linear changes in & \\  & & surface deformation. & \\ \hline \multirow{3}{*}{SBAS-InSAR} & High monitoring accuracy. & A large amount of SAR data is required. & \\  & Suitable for medium-range & Complex processing. & \\  & deformation monitoring. & Sensitive to the choice of baseline and & \\  & & requires caution. & \\ \hline \multirow{6}{*}{PS-InSAR} & High precision and high sensitivity. & \multirow{2}{*}{Complex processing. & \\  & Suitable for the monitoring of complex & & A large amount of SAR data is required. & \\ \cline{1-1}  & terrain and non-linear deformation. & Stable surface targets such as buildings, & High requirements for the stability of ground & \\ \cline{1-1}  & telecommunication towers, etc., can be & objects. & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Advantages and disadvantages of three technologies. Figure 9: Geometry of repeat-pass. (**a**) CT (Cross-Track)-InSAR; (**b**) AT (Along-Track)-InSAR. unparalleled advantages. Compared with SAR stereo mapping technology, the InSAR technology with phase measurement has higher measurement accuracy. Table 4 lists the comparison between InSAR and other DEM acquisition technologies. Terrain measurement using InSAR technology can already achieve operational applications. Examples are the SRTM system, TanDEM-X system and the mapping project for the blank areas of 1:50,000 topographic maps in western China, etc. Since interferometric measurement requires two SAR images with high coherence in the same region, and the interferometric phase cannot be directly obtained from the complex images, the generation of digital surface models (DSM) requires several steps such as complex image registration, interferogram generation, interferometric phase filtering, phase unwrapping, baseline estimation, and DSM reconstruction. The specific processing flow is shown in Figure 10.  
In 2000, the SRTM system of the United States [61; 62] acquired SAR data ranging from 60\({}^{\circ}\) N to 56\({}^{\circ}\) S. This system extended a 60 m telescopic boom to project an X-band and a C-band antenna outside the cabin, forming a dual-antenna InSAR system with the main antenna inside the cabin. Using InSAR technology, it produced DEM products covering 80% of the global land surface. The product had a planar resolution of 30 m \(\times\) 30 m, a relative elevation accuracy of 6 m, and an absolute elevation accuracy of 16 m, meeting the DTED-2 standard in Table 5. \begin{table}
\begin{tabular}{c c c} \hline \hline
**DEM Acquisition Technique** & **Coverage** & **DEM Accuracy** \\ \hline Ground & Local, large-scale mapping range & 0.01–0.1 m \\ Airborne photogrammetry & Region & 0.1–1 m \\ Airborne lidar & Region & 0.5–2 m \\ InSAR & Regional to global & 1–20 m \\ Shadow mapping & Regional to global & Slope \(\leq\) 2\({}^{\circ}\), 22 m \\ Stereo mapping & Region & 10–100 m \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of InSAR and other DEM acquisition techniques. Figure 10: Flow of CT-InSAR DEM generation [4]. In 2010, the German satellite TanDEM-X was successfully launched and operated in a HELIX configuration with the previous TerraSAR-X satellite [63; 64; 65], pioneering true spaceborne dual-station SAR interferometric measurement.