## 2 Materials and Methods
### Extraction and Analysis of Rice Coherence in the Sentinel-1A Image
#### 3.1.2 The Change of Coherence Coefficient of VH Polarization  
Different from the change in VV polarization coherence coefficient, the change in VH polarization coherence coefficient of rice shows a wave shape (Figure 14). The minimum coherence coefficient is observed in June, August, and October, whereas the maximum coherence coefficient is observed in July and September. Similar to the VV polarization coherence coefficient, the VH polarization coherence coefficient of artificial buildings is  
Figure 13: Sentinel-1 VV polarization coherence coefficient map.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **12-June** & **18-July** & **11-August** & **16-September** & **10-October** \\ \hline Artificial buildings & 0.52 & 0.50 & 0.52 & 0.50 & 0.46 \\ Rice & 0.06 & 0.07 & 0.04 & 0.04 & 0.07 \\ Water area & 0.08 & 0.09 & 0.09 & 0.09 & 0.08 \\ Vegetation & 0.10 & 0.11 & 0.10 & 0.09 & 0.08 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Coherence coefficient of ground objects of VV polarization.  
greater than 0.37, which is far higher than that of rice, water area, and vegetation. The change in the VH polarization coherence coefficient of water is similar to that of the VV polarization coherence coefficient. Due to time variation and low single-to-noise ratio, the surface shows low interference coherence. The study area is located in the subtropical monsoon climate zone, and thus the growth period of rice overlaps with the rainy season. The seasonal sudden rainfall will cause the rise of water level and the expansion of water area. However, the coherence coefficient of the water changes by not more than 0.01. The change in the VH polarization coherence coefficient of vegetation is also similar to that of VV polarization (Table 4).  
On the basis of the VV and VH polarization coherence coefficients of ground objects, the VV polarization coherence coefficient of artificial buildings is greater than the VH polarization coherence coefficient. No remarkable difference can be observed between the VV and VH polarization coherence coefficients of rice, vegetation, and water area. However, the VV and VH polarization coherence coefficients can be used to better distinguish artificial buildings from other ground objects. The VV polarization coherence coefficient can be used to distinguish rice from water area and vegetation in August and September, whereas the VH polarization coherence coefficient can also be used to well distinguish rice from water area and vegetation in August and October. However, after harvesting the rice in October, the SAR images show two intensities of soil and water in the rice field. In addition, the base time in this study is May. The difference between rice planting and harvest is little. Therefore, the coherence coefficient in October is not highly available. It may be more effective to distinguish rice by using the coherence coefficients of two time phases, namely, rice maturity and harvest. On the basis of the comprehensive analysis on the polarization coherence coefficients, this study shows that the polarization interferogram introduced by InSAR technology can support rice identification.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **12-June** & **18-July** & **11-August** & **16-September** & **10-October** \\ \hline Artificial buildings & 0.50 & 0.43 & 0.45 & 0.41 & 0.37 \\ Rice & 0.04 & 0.07 & 0.04 & 0.06 & 0.04 \\ Water area & 0.07 & 0.08 & 0.08 & 0.07 & 0.08 \\ Vegetation & 0.10 & 0.08 & 0.08 & 0.09 & 0.10 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Coherence coefficient of VH polarization.  
Figure 14: Sentinel-1 VH polarization coherence coefficient map.