## 4 Results
### Different Remote Sensing Indices Temporal Contribution and Variation Characteristics  
Crops have unique growth and seasonal phase-change characteristics. Multi-temporal spectral characteristics can provide effective crop identification information and reveal their changes over time [67]. Usually, the changes in these characteristics are stable and can be used to distinguish between crops over a period of time. This study calculated the importance of all multi-temporal difference features for each period to reflect the relative importance of the features in different periods. Tables 12-12 summarizes the top 10 optimal feature sets of each best combination type of three crops in each period. Features with a higher proportion and ranking are better for crop identification [54]. The key spectral and temporal characteristics of the three crops are analyzed as follows. (1) Early identifying characteristics and temporal changes characteristics of rice  
Figure 8 shows the proportions of feature types in the top10 optimal rice feature sets of all best combination types in each period. Rice is a typical paddy crop, which behaves as a mixture of water and paddy in the early stage. The LSWI, SR, B11, and NDTI can effectively reflect the water changes in the canopy and canopy background during the growth and development of rice in this period so that rice crops can be effectively identified. Table 10 shows the top10 optimal feature sets of the best combination types for rice in each period. It can be seen that on 8 May, only one combination of differential temporal phases (A) was able to achieve 81% recognition accuracy. On 13 May, two combinations of BF achieved the maximum classification accuracy (94.6%). In addition, the maximum recognition accuracy of each combination in that period was able to meet the requirement for early recognition of rice (\(\geq\)91%). Figure 9 shows the frequency of key temporal changes in the top10 feature sets of all the best combination types for rice on 13 May. The frequency of F was significantly greater than those of the other different temporal phases, indicating that the difference in remote sensing information between 13 May and 28 April is key information for early rice crop identification. (2) Early identifying characteristics and temporal changes characteristics of corn  
Figure 10 shows the proportion of different feature types in the top-10 optimal corn feature sets of all best combination types at each period. By early June, corn had undergone the sowing and emergence stages, and the images acquired at that time showed low vegetation cover \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Date**} & \multicolumn{2}{c}{**Best Com-**} & \multirow{2}{*}{**\
\begin{tabular}{c} **\#1** \\ **\#Type** \\ \end{tabular}} & \multicolumn{2}{c}{**
\begin{tabular}{c} **\#1** \\ **Accuracy** \\ \end{tabular} } & \multicolumn{2}{c}{**Optimum Features**} \\ \hline
8 May & A & 83.8\% & A\_NDTI & A\_LSWI & A\_B11 & A\_SR & A\_B2 & A\_RENDVI & A\_B5 & A\_NDRE2 & A\_OSAVI \\
13 May & BF & 94.6\% & F\_NDTI & F\_B12 & B\_B12 & B\_LSWI & F\_B11 & F\_LSWI & B\_RESI & F\_NDVI & F\_SR & F\_B6 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: The top-10 optimal feature sets of best rice combination types for each period in 2020. Figure 8: Percentage of different character types of rice in 2020 different periods. information. The NDTI, SR, LSWI, and B11 accounted for a relatively large proportion during this period. As the crop grows, it gradually enters its peak growth period. In early July, when corn is at the seventh leaf stage, the vegetation cover is greater compared to the previous months. Features such as short-wave infrared bands, some vegetation red-edge bands, and indices of vegetation cover (B11, NDRE3, LSWI, VJgreen, RESI, NERED2) become more prominent and can effectively capture corn growth information. Figure 10: Percentage of different character types of corn in 2020 different periods. Figure 9: Frequency of the key change temporal on 13 May 2020.