## 2 Materials and Methods
### RADARSAT-2 Data Preprocessing  
Four fine quad-polarized single-look complex (SLC) RADARSAT-2 SAR images were acquired between May and July of 2016 (Table 2). The revisit period was 24 days. The images had a nominal spatial resolution of 5.4 m in the slant range direction and 7.9 m in the azimuth direction. The swath width of the image was about 25 km. The incidence angle ranged from 31\({}^{\circ}\) at near range to 33\({}^{\circ}\) at far  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & \multicolumn{5}{c}{**Acquisition Dates of SAR Datasets**} \\ \cline{3-6}  & & **15 May** & **8 June** & **2 July** & **26 July** \\ \hline \multirow{4}{*}{Fields has} & Transplanting & 17 & 0 & 0 & 0 \\  & Vegetative & 10 & 30 & 16 & 0 \\ \cline{1-1}  & Reproductive & 0 & 0 & 14 & 10 \\ \cline{1-1}  & Maturity & 0 & 0 & 0 & 20 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The number of sample sites in different phenological phases when SAR datasets were acquired. On 15 May 2016, fields where three sample sites were located were not planted yet.  
Figure 3: In situ rice phenology at 30 sample sites in 2016. Blue, green, yellow, and red dots, respectively, represent the transplanting, vegetative, reproductive, and maturity phase. Four horizontal lines denote the acquisition dates of four RADARSAT-2 datasets.  
range. The Next ESA SAR Toolbox (NEST) software ([[https://earth.esa.int/web/nest/downloads/](https://earth.esa.int/web/nest/downloads/)]([https://earth.esa.int/web/nest/downloads/](https://earth.esa.int/web/nest/downloads/))) was utilized for data preprocessing including radiometric correction, multilooking, geometry reprojection, speckle filtering, and multi-temporal coregistration. A color composite is shown in Figure 4. Backscattering coefficients (linear scale) of all images on four polarizations (VH, VV, HH, and HV) were extracted for 30 sample sites. As described previously, each sample site is at least 50 m wide and 50 m long and the preprocessed images have a pixel size of 11.6 m \(\times\) 11.6 m, which means that each sample site has at least 4 by 4 or 16 pixels. Therefore, the mean value of the backscattering coefficients within a sample site was calculated to represent the backscatter properties of that site.  
Cross-polarization VH data and HV data were the same. Thus, only VH, VV, and HH data were used. Previous studies have found that combinations (e.g., addition, subtraction, multiplication, and ratioing) of different polarized backscattering coefficients were more sensitive to the morphological structures of rice plants [20; 31; 39]. These operations calculated in a linear scale first, and then the results expressed in a logarithmic scale were explored.  
All backscattering coefficients and combinations, acquired at different sample sites and on different dates, were divided into four groups based on the four phenological phases identified from ground observations (Table 2) for further analyses. Three-quarters of each group (i.e., 13 of 17 transplanting datasets, 42 of 56 vegetative datasets, 18 of 24 reproductive datasets, and 15 of 20 maturity datasets) were randomly chosen for training. The rest in each group was used for validation, respectively.