## 2 Materials and Methods
### Data Sources and Preprocessing
The data were processed as follows: (1) apply the orbit file, download the accurate orbit file, and update the S-1 satellite orbit state information in the metadata file of S-1 satellite data; (2) perform S-1 thermal noise removal, reducing the influence of thermal noise on the accuracy of radar backscatter signals; (3) perform calibration--the received backscattering signal is converted into a backscattering coefficient; (4) perform single product speckle filtering, using the refined Lee filter to remove speckle noise; (5) perform range-Doppler terrain correction--the SRTM 3 Sec digital elevation model is used for geocoding to give the actual coordinate information of the image; (6) converts bands to/from DB, which is convenient for visualization and data analysis [39]. The Sentinel-2A Level-1C multispectral data are orthophotos that have undergone geometric correction but have not yet been radiometrically calibrated or atmospherically corrected. Therefore, radiometric calibration and atmospheric correction are necessary. The Level-1C multispectral data were downloaded and then radiometrically calibrated and atmospherically corrected using the SNAP 9.0.0 (European Space Agency, Paris, France) provided by the European Space Agency. The data in the utilized bands were then resampled to a 10 m resolution [40]. A total of 1156 field sampling sites were investigated, comprising 724 sampling sites for litchi and 432 sampling sites for non-litchi (Figure 1). The investigation was carried out in townships and areas surrounding large citrus cultivation areas within the study area. The non-litchi sampling sites consisted of five main features, i.e., citrus, longan, cropland, and other forest and vegetation. One third of the field sampling sites were selected for the \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Satellite** & **Acquisition Date** & **Number of Scenes** & **Growth Stage** \\ \hline S-1A & 11 January 2018 (0111) & 6 & The branch growth period of litchi and citrus \\ \hline S-1A & 14 January 2022 (0114), 19 February 2022 (0219) & 4 & The bud differentiation and antihesis of lithchi and citrus. The branch growth period of longan \\ \hline S-1A & 15 March 2022 (0315), 8 April 2022 (0408) & 4 & The fruit growth and ripening of litchi. The bud differentiation and antihesis of longan \\ \hline S-1A & 11 May 2018 (0511), 23 May 2018 (0523), 4 June 2018 & & The fruit growth and ripening of litchi. The bud differentiation and antihesis of longan \\ \hline S-1A & 2 May 2022 (0502), 14 May 2022 (0514) & 14 & The bud differentiation and antihesis of longan \\ \hline S-1A & 26 May 2022 (0526), 7 June 2022 (0607) & & \\ \hline S-1A & 10 June 2018 (0610), 22 June 2018 (0622) & & The harvest period of litchi. The fruit growth and ripening of citrus and longan \\ \hline S-1A & 7 June 2022 (0607), 19 June 2022 (0619) & 12 & The harvest period of litchi. The fruit growth and ripening of citrus and longan \\ \hline S-1A & 13 July 2022 (0713), 25 July 2022 (0725) & & \\ \hline S-1A & 6 August 2018 (0806), 15 August 2018 (0815) & 8 & The fruit growth and ripening of citrus. The harvest period of longan \\ \hline S-1A & 6 August 2022 (0806), 30 August 2022 (0830) & 8 & The harvest period of longan \\ \hline S-1A & 20 September 2018 (0920), 14 October 2018 (1014) & & \\ S-1A & 11 September 2022 (0911), 23 September 2022 (0923), & 10 & The harvest period of longan \\ \hline S-1A & 17 October 2022 (1017) & 13 December 2018 (1213) & \\ \hline S-1A & 22 November 2022 (1122), 16 December 2022 (1216) & 8 & / \\ \hline S-2A & 30 September 2018, 9 September 2022, & 6 & / \\ \hline \hline \end{tabular}
\end{table} 
Table 3: A summary of the remote sensing data. construction of the growth stage characteristics of orchard vegetation, while the remaining sites were used for accuracy evaluation. Because of the large area of agricultural land and the many types of crops in the central and southern plains, there were more field sampling sites in and around the townships in the central and southern parts of the study area.