## 3 Experiment
### SAR Acquisitions and Data Processing  
A total of 13 repeat-pass TerraSAR X-band Stripmap descending images, provided by the German Aerospace Center, have been collected in this study. Their acquisition period covered from 27 October 2014 to 27 November 2015. The parameters of these TerraSAR-X images are listed in Table 1. The pixel spacing of selected images is 2.198 m along range direction, and 1.965 m along azimuth direction. Shuttle Radar Topography Mission Digital Elevation Model (SRTM DEM) with 30 m spatial resolution  
Figure 2: Study area featured at different scales: (**a**) Optical image. The red rectangle defines the spatial coverage of the TerraSAR-X images in our experiment; (**b,c**) Highway region of interest outlined in red solid lines; (**d**) Location in China.  
provided by National Aeronautics and Space Administration (NASA) is utilized as external topographic data [35].  
The interferometric processing work was accomplished by SARscape 5.2 software. The spatial and temporal baseline thresholds were set to be 130 m and 365 days, respectively. A total of 60 candidate interferometric pairs were generated for both tested highways. Figure 3 shows temporal baseline (\(x\) axis) and perpendicular baseline (\(y\) axis) of each selected time-adjacent interferometric pair. To remove the topographic phases, SRTM DEM was used to simulate interferometric phases subtracted from these interferograms. In order to maintain the resolution of highways, no multi-looking was conducted. Goldstein filter was utilized to further suppress the atmospheric phase. In addition, the interferograms were unwrapped with the minimum cost flow (MCF) method [4; 36]. To improve the quality of coherent points, we used the triple threshold algorithm (the amplitude deviation index, the average coherence value, and intensity value) to detect the high coherence points automatically [37]. The coherence threshold of 0.6 and the amplitude deviation index of 0.4 was adopted to ensure a reliable distribution of high coherence candidates. The average coherence maps for the two test highways are shown in Figure 4. The lighter the color is, the higher the coherence is. From Figure 4, we can see that higher coherence is found in urban areas, mainly higher than 0.9. After the coherent points selection, a total of 27,465 and 83,528 candidates were extracted for Lungui Highway and G1501 Highway respectively. After deleting low-quality interferometric pairs (i.e., with worse unwrapping phase, large orbit error and atmospheric delay), 42 high quality interferometric pairs for Lungui Highway and 49 for G1501 Highway were generated.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Image No.**} & **Acquisition Dates** & **Normal** & **Temporal** \\  & **(yyyy/mm/dd)** & **Baseline (m)** & **Baseline (days)** \\ \hline
0 & 2014/10/27 & \(-\)126.22 & 242 \\
1 & 2014/11/18 & 48.62 & 220 \\
2 & 2015/01/01 & 122.99 & 176 \\
3 & 2015/02/14 & \(-\)10.34 & 132 \\
4 & 2015/03/08 & 15.94 & 110 \\
5 & 2015/05/13 & \(-\)148.94 & 44 \\
6 & 2015/06/26 & 0 & 0 \\
7 & 2015/08/09 & \(-\)27.10 & 44 \\
8 & 2015/08/31 & 57.39 & 66 \\
9 & 2015/09/22 & \(-\)130.33 & 88 \\
10 & 2015/10/14 & \(-\)36.83 & 110 \\
11 & 2015/11/05 & \(-\)111.08 & 132 \\
12 & 2015/11/27 & 111.41 & 154 \\ \hline \hline \end{tabular}
\end{table}
Table 1: List of the interferometric pairs and their parameters with image number 6 as the master (orbit no.119).  
Figure 3: Spatial and temporal baselines of the interferometric pairs. (\(x\) axis represents the temporal baseline, \(y\) axis represents the perpendicular baseline, and the yellow solid point represents the selected master image in our data stack) (**a**) Lungui Highway; (**b**) G1501 Highway.