## 4 Atmospheric Correction Results and Analysis
### Comparison and Validation with Sentinel-2 Surface Reflectance Products
This approach enables a comprehensive assessment of surface reflectance accuracy across diverse land cover types. The surface reflectance from the three images in Section 4.1 were compared with the corresponding reflectance products from Sentinel-2 satellite for the same locations, including land cover types such as airport aprons, bare soil, water bodies, buildings, and vegetation. Areas with similar times and locations were selected, and the mean values were calculated for each, along with their absolute errors. The results are presented in Table 5. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Sentinel-2**} & \multicolumn{3}{c}{**HJ-2A/B**} \\ \hline \multirow{2}{*}{**Name**} & **Center** & **Spectral** & \multirow{2}{*}{**SR (m)**} & **Name** & **Center** & **Band** & \multirow{2}{*}{**SR (m)**} \\  & **(nm)** & **Width (nm)** & & & **(nm)** & **Width (nm)** & \\ \hline B2 & 490 & 65 & 10 & B1 & 485 & 70 & 16 \\ B3 & 560 & 35 & 10 & B2 & 555 & 70 & 16 \\ B4 & 665 & 30 & 10 & B3 & 660 & 60 & 16 \\ B5 & 705 & 15 & 20 & B4 & 710 & 40 & 16 \\ B8 & 842 & 115 & 10 & B5 & 830 & 120 & 16 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Spectral bandwidths and spatial resolutions of the Sentinel-2 and HJ-2A/B satellites. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Imaging** & & & & **SR of** & **SR of** & **Absolute** \\
**Location** & **Imaging Time** & **Band** & **Sentinel-2** & **HJ-2A/B** & **Error** \\ \hline \multirow{2}{*}{\begin{tabular}{c} Indian Plains \\ bare soil \\ (21.0601\({}^{\circ}\)N, \\ 80.0198\({}^{\circ}\)E) \\ \end{tabular} } & 25 November 2022 & B1 & 0.0981 & 0.0941 & 0.0040 \\  & (HJ-2B) & B2 & 0.1346 & 0.1219 & 0.0127 \\  & and & B3 & 0.1966 & 0.1689 & 0.0277 \\  & (S2A) & B4 & 0.2288 & 0.2144 & 0.0144 \\ \hline \multirow{2}{*}{\
\begin{tabular}{c} Indian Plains \\ water body \\ (21.0527\({}^{\circ}\)N, \\ 80.0262\({}^{\circ}\)E) \\ \end{tabular} } & 24 November 2022 & B4 & 0.2950 & 0.2734 & 0.0216 \\  & (HJ-2A) & B2 & 0.3521 & 0.3200 & 0.0321 \\  & and & B3 & 0.3822 & 0.3541 & 0.0279 \\  & 15 November 2022 & B4 & 0.3917 & 0.3493 & 0.0424 \\  & (S2A) & B5 & 0.3886 & 0.3563 & 0.0323 \\ \hline \multirow{2}{*}{
\begin{tabular}{c} Airport apron \\ (39.4986\({}^{\circ}\)N, \\ 116.4341\({}^{\circ}\)E) \\ \end{tabular} } & 14 November 2022 & B1 & 0.0659 & 0.0844 & 0.0185 \\  & (HJ-2A) & B2 & 0.0994 & 0.0982 & 0.0012 \\  & and & B3 & 0.1546 & 0.1347 & 0.0199 \\  & 15 November 2022 & B4 & 0.1791 & 0.1753 & 0.0038 \\  & (S2A) & B5 & 0.2315 & 0.2252 & 0.0063 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison of surface reflectance products between HJ-2A/B and Sentinel-2 satellites. As shown in Table 5, the surface reflectance from the HJ-2A/B MSC closely matches Sentinel-2 SR across different land cover types, including bare soil, water bodies, vegetation, aprons, and buildings, with most absolute errors being smaller than 0.04. Higher errors (>0.04) were observed for aprons and buildings, attributed to their higher reflectance values and differences in spectral response between HJ-2A/B band 4 and Sentinel-2 band 5. These discrepancies are further influenced by differences in the central wavelengths and bandwidths between the two satellites. Overall, the results demonstrate that the atmospheric correction applied to HJ-2A/B MSC data achieves high consistency with Sentinel-2 SR products, validating the correction's effectiveness across diverse surface types. Sentinel-2 validation complements the in situ measurements by providing broader geographic coverage and additional land cover variability.