## 3 Results and Analysis  
### Comparative Analysis of Spectral Reflectance of Two Sensors  
Figure 3 displays the canopy reflectance curves of the two sensors throughout various fertility periods when traditional nitrogen treatment (N4) was used. Both sensors exhibited comparable patterns in the visible light-red edge (454-750 nm) region. The \"green peak\" feature, observed at approximately 550 nm, is primarily caused by the intense reflection of green light and the absorption of red and blue light by chlorophyll in winter wheat  
\begin{table}
\begin{tabular}{c c c c} \hline
**Spectral Index** & \
\begin{tabular}{c} **Based on Original** \\ **Spectra** \\ \end{tabular} &
\begin{tabular}{c} **Based on** \\ **Derivative Spectra** \\ \end{tabular} & **References** \\ \hline Normalized Difference & & & \\ Spectral Index (NDSI) & \(\frac{R_{31}-R_{32}}{R_{31}+R_{32}}\) & \(\frac{D_{31}-D_{32}}{D_{31}+D_{32}}\) & [27] \\ Ratio Spectral Index (RSI) & \(\frac{R_{31}}{R_{32}}\) & \(\frac{D_{31}}{D_{32}}\) & [28] \\ Difference Spectral Index (DSI) & \(R_{31}-R_{32}\) & \(D_{31}-D_{32}\) & [29] \\ \hline \end{tabular} \(R_{31}\) and \(R_{32}\) are the reflectances at any two wavelengths, and \(D_{31}\) and \(D_{32}\) are their corresponding derivative spectra.  
\end{table}
Table 1: Nomenclature and algorithm for calculating the spectral index.  
leaves. The reflectance experiences a significant increase near 680 nm, resulting in the appearance of the characteristic \"red valley\" feature commonly found in green vegetation. Consequently, at 680 nm, the reflectance trend of the two sensors is similar. Hence, within the wavelength range of 680-750 nm, known as the red edge, the spectral reflectance of ASD and UHD185 sensors exhibit a close resemblance. However, during the flowering stage of winter wheat, characterized by robust growth and high pigment content, the spectral curves of the two sensors display varying degrees of the phenomenon known as \"red shift\". The spectral curve of ASD exhibited a smooth and high reflectance platform in the near-infrared range of 790-920 nm. In contrast, UHD185 had a considerably lower near-infrared reflectance compared to ASD. The spectral curve of UHD185 showed a noticeable decline after 830 nm, which could be attributed to the variation in geometric position caused by the bidirectional reflectance distribution function (BRDF). The BRDF is the primary factor influencing the spectral curve of UHD185. It has been demonstrated that the effect of BRDF on UAV hyperspectral data imagery is significant [30]. Additionally, the UHD185 sensor has a lot of noise when detecting the range boundary, which may also be the reason for this phenomenon.  
By analyzing the correlation between the reflectance of the two sensors in the 454-830 nm region, as shown in Figure 4, it is evident that the coefficients of determination between the sample data at the pulling and flowering stages of winter wheat fall within the range of 0.98-1.00, with the average value exceeding 0.98. The correlation analysis of the canopy spectral data obtained by two sensors during the nodulation and flowering stages of winter wheat reveals that the canopy spectral reflectance of wheat images captured by UHD185 is of excellent quality within the 450-830 nm band range. This high-quality data can be utilized for future investigations on the estimation of ag  
Figure 3: Comparison between UHD185 spectral curve and resampled ASD spectral curve at different stages.