## 2 Data and Methodology
### Data and Preprocessing
#### 2.2.2 GF-3 Data and Preprocessing  
The GF-3 SAR sensor is able to image in 12 modes. For instance, the spot light (SL) mode image has the highest spatial resolution, of up to 1 m, among all of the imaging modes, and the Quad-Polarization Strip I (QPSI) mode image has the most polarization channels (HH/HV/VH/VV), with a nominal spatial resolution of 8 m. In this investigation, 14 Ultra Fine Strip (UFS) mode GF-3 images were collected (Table 3), which have a relatively high spatial resolution (3 m) and wider swath (30 km), in HH polarization.  
A series of preprocessing procedures was performed on the single-look complex images using PIE-SAR 6.3 software [24] by PIESAT Information Technology Co., Ltd. First of all, the multilook module was applied to improve the quality of the single-look SAR image at the L1A processing level. Next, the enhanced frost filter, with a window size of 3 by 3 and damping ratio of 1, was applied to reduce the speckle noise. In addition, the geocoded terrain correction (GTC) module was implemented by using a DEM map around the study area. Finally, radiometric calibration was conducted to convert the DN value of the pixel into the normalized radar cross section (NRCS).  
The overall preprocessing procedure was drawn in Figure 3.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Platform** & **Sensor** & \begin{tabular}{c} **Date of** \\ **Acquisition** \\ \end{tabular} & \
\begin{tabular}{c} **Number of** \\ **Scenes** \\ \end{tabular} &
\begin{tabular}{c} **Spatial** \\ **Resolution** \\ \end{tabular} \\ \hline GF-2 & PMS1 & 28 January 2017 & 1 & 1 m/4 m \\ GF-2 & PMS1 & 3 August 2017 & 1 & 1 m/4 m \\ GF-2 & PMS1 & 26 October 2017 & 2 & 1 m/4 m \\ GF-2 & PMS2 & 31 October 2017 & 1 & 1 m/4 m \\ GF-2 & PMS1 & 9 December 2018 & 1 & 1 m/4 m \\ GF-2 & PMS2 & 9 December 2018 & 1 & 1 m/4 m \\ GF-2 & PMS2 & 16 September 2019 & 1 & 1 m/4 m \\ GF-2 & PMS1 & 19 November 2019 & 2 & 1 m/4 m \\ GF-2 & PMS2 & 15 February 2021 & 1 & 1 m/4 m \\ GF-2 & PMS1 & 24 May 2021 & 2 & 1 m/4 m \\ GF-2 & PMS2 & 14 October 2021 & 1 & 1 m/4 m \\ GF-2 & PMS2 & 22 December 2021 & 1 & 1 m/4 m \\ GF-2 & PMS2 & 1 March 2022 & 1 & 1 m/4 m \\ \hline \hline \end{tabular}
\end{table}
Table 2: GF-2 data collected in this paper.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Platform/Sensor** & \
\begin{tabular}{c} **Date of** \\ **Acquisition** \\ \end{tabular} & **Imaging Mode** &
\begin{tabular}{c} **Spatial** \\ **Resolution** \\ \end{tabular} & **Polarization** \\ \hline GF-3/SAR & 8 September 2018 & UFS & 3 m & HH \\ GF-3/SAR & 8 April 2019 & UFS & 3 m & HH \\ GF-3/SAR & 3 August 2019 & UFS & 3 m & HH \\ GF-3/SAR & 16 June 2020 & UFS & 3 m & HH \\ GF-3/SAR & 31 August 2020 & UFS & 3 m & HH \\ GF-3/SAR & 30 September 2020 & UFS & 3 m & HH \\ GF-3/SAR & 5 January 2021 & UFS & 3 m & HH \\ GF-3/SAR & 4 March 2021 & UFS & 3 m & HH \\ GF-3/SAR & 1 May 2021 & UFS & 3 m & HH \\ GF-3/SAR & 28 June 2021 & UFS & 3 m & HH \\ GF-3/SAR & 23 July 2021 & UFS & 3 m & HH \\ GF-3/SAR & 18 October 2021 & UFS & 3 m & HH \\ GF-3/SAR & 16 November 2021 & UFS & 3 m & HH \\ GF-3/SAR & 10 April 2022 & UFS & 3 m & HH \\ \hline \hline \end{tabular}
\end{table}
Table 3: GF-3 data collected in this paper.