## 2 Materials and Methods
### Extraction and Analysis of Rice Coherence in the Sentinel-1A Image  
The purpose of SAR image interference processing is to obtain the index representing the similarity between two images or among many SAR complex images, called coherence [28]. The similarity of target scatterers is reflected by radar echo. The similarity information and intensity information of ground objects provide the information related to target scatterers. The study on the surface activities in a specific region should be realized through decoherence in the interference analysis. The coherence coefficient is an important index of coherence. It is introduced to measure the similarity of measured ground objects. On the basis of many SAR images at different times in the study area, quantitative analysis was adopted to monitor the change of rice-planting area in different growth periods so as to extract the rice-planting area. The rice in the study area is cultivated from March to April, including seeding and seedling raising. The rice is transplanted in May, matures in August, and is harvested in September. During the whole period, the rice field exhibits obvious changes. Therefore, this study was based on InSAR technology, and the changes in the rice field were monitored by using Sentinel-1A data in the process of transplanting, heading, ripening, and harvesting of rice so as to provide a reference for the identification of rice-planting areas. The coherence of VV and VH polarization in Sentinel-1 time series was calculated on the basis of the SARscape module, and then the coherence coefficient is obtained. The data processing for calculating the coherence (Figure 5) included baseline estimate, interferogram generation, self-adaptive filtering, coherence calculation, registration, geocoding and calibration, and cutting the study areas. The baseline is an important part in coherence generation. The baseline and slope distance of two SAR images form a triangle so as to produce the coherence. The baseline estimate is used to evaluate the quality of interference image pair. Thus, interference can happen. The SAR images generally used for change monitoring must meet the following conditions: the same sensor, the same ascending track (descending track), and incident angle. Taking the SAR image in rice field cultivation monitoring as an example, the sensor, ascending track, and incident angle meet the use conditions [29]. When at least two antennas are overlapped in obtaining the ground reflection, interference can be produced. The baseline-estimated relevant parameters of this study are shown in Figure 6. The accuracy of elevation and deformation increases with the increase of coherence. The baseline is 73.348 m long, which is far less than the critical baseline (486.486 m). The  
Figure 4: Synthetic map of the research area after Sentinel-1A (**VV**) and (**VH**) polarization (R (05/19), G (06/12), B (07/18)). minimum change in the elevation that can be detected is 211.737 m, and the minimum deformation is 0.028 m. The baseline estimate result (Figures 7 and 8) shows that the quality of interference image pair used in the research satisfies the requirement of InSAR technology [30]. Figure 5: InSAR processing flow. Figure 6: Baseline estimate result. On the premise that the baseline estimate meets the interference processing, the interferogram was generated. The interferogram generation results include interferogram (Figure 9) and flattened interferogram (Figure 10). On the basis of the interferogram, self-adaptive filtering and coherence calculation were performed, and the Sentinel-1 coherence graph (Figures 11 and 12) was obtained. The coherence coefficient of the study area was extracted.