# Evaluation on Radiometric Capability of Chinese Optical Satellite Sensors
## 2 Materials and Methods
### Methods
#### 2.3.1. Top-of-Atmosphere (TOA) Reflectance Calculation  
The HJ-1/CCD data used in this study are derived from the Level 2 products, and the ground observation station of Chinese resources satellites has conducted radiation correction on these data to correct for the radiation deviation of the sensors. The TOA reflectance of the CCD can be calculated using Equation (1). \[\rho_{\lambda}=\frac{\pi L_{\lambda}d^{2}}{ESUN_{\lambda}\cdot\sin(\theta_{SE})} \tag{1}\]  
where \(\rho_{\lambda}\) is the TOA reflectance; \(L_{\lambda}\) is the TOA radiance; \(d\) is the Earth-Sun distance in astronomical units; \(\theta_{SE}\) is the solar elevation; \(ESUN_{\lambda}\) is the solar irradiance at the top of the atmosphere; and \(L_{\lambda}\) can be calculated using Equation (2)  
\[L=DN/gain+offset \tag{2}\]  
Table 2 presents the gains and offsets of HJ-1/CCDs. Of those, the gains of 2008 are obtained by prelaunch Lab calibration, and the gains of 2009-2012 are obtained by post launch site calibration. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**HJ-1A/CCD1**} \\ \hline \multirow{2}{*}{**Years**} & \multicolumn{2}{c}{**Blue Band**} & \multicolumn{2}{c}{**Green Band**} & \multicolumn{2}{c}{**Red Band**} & \multicolumn{2}{c}{**NIR Band**} \\ \cline{2-10}  & **Gain** & **Offset** & **Gain** & **Offset** & **Gain** & **Offset** & **Gain** & **Offset** \\ \hline
2008 & 0.916 & 7.325 & 0.9228 & 6.0737 & 1.1277 & 3.6123 & 1.0753 & 1.903 \\
2009 & 0.6925 & 7.325 & 0.7438 & 6.0737 & 0.9636 & 3.6123 & 1.0545 & 1.903 \\
2010 & 0.7768 & 7.325 & 0.7796 & 6.0737 & 1.0312 & 3.6123 & 1.0049 & 1.903 \\
2011 & 0.7696 & 7.325 & 0.7815 & 6.0737 & 1.0914 & 3.6123 & 1.0281 & 1.903 \\
2012 & 0.7069 & 7.325 & 0.7497 & 6.0737 & 1.0673 & 3.6123 & 1.0429 & 1.903 \\ \hline \multicolumn{10}{c}{**HJ-1A/CCD2**} \\ \hline \multirow{2}{*}{**Years**} & \multicolumn{2}{c}{**Blue Band**} & \multicolumn{2}{c}{**Green Band**} & \multicolumn{2}{c}{**Red Band**} & \multicolumn{2}{c}{**NIR Band**} \\ \cline{2-10}  & **Gain** & **Offset** & **Gain** & **Offset** & **Gain** & **Offset** & **Gain** & **Offset** \\ \hline
2008 & 0.9997 & 4.6344 & 1.0016 & 4.0982 & 1.3777 & 3.736 & 1.3043 & 0.739 \\
2009 & 0.923 & 4.6344 & 0.9399 & 4.0982 & 1.3093 & 3.736 & 1.3178 & 0.739 \\
2010 & 0.7892 & 4.6344 & 0.7831 & 4.0982 & 1.1635 & 3.736 & 1.1995 & 0.739 \\
2011 & 0.7435 & 4.6344 & 0.7379 & 4.0982 & 1.0899 & 3.736 & 1.0852 & 0.739 \\
2012 & 0.7257 & 4.6344 & 0.7291 & 4.0982 & 1.0464 & 3.736 & 1.0519 & 0.739 \\ \hline \multicolumn{10}{c}{**HJ-1B/CCD1**} \\ \hline \multirow{2}{*}{**Years**} & \multicolumn{2}{c}{**Blue Band**} & \multicolumn{2}{c}{**Green Band**} & \multicolumn{2}{c}{**Red Band**} & \multicolumn{2}{c}{**NIR Band**} \\ \cline{2-10}  & **Gain** & **Offset** & **Gain** & **Offset** & **Gain** & **Offset** & **Gain** & **Offset** \\ \hline
2008 & 0.8685 & 3.0089 & 0.9367 & 4.4487 & 1.2443 & 3.2144 & 1.3002 & 2.561 \\
2009 & 0.7726 & 3.0089 & 0.8092 & 4.4487 & 1.117 & 3.2144 & 1.1337 & 2.561 \\
2010 & 0.761 & 3.0089 & 0.7727 & 4.4487 & 1.0827 & 3.2144 & 1.1181 & 2.561 \\
2011 & 0.706 & 3.0089 & 0.696 & 4.4487 & 1.0082 & 3.2144 & 1.0068 & 2.561 \\
2012 & 0.6697 & 3.0089 & 0.7118 & 4.4487 & 1.0555 & 3.2144 & 1.1042 & 2.561 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The calibration coefficients of CCDs. Figure 2: The block diagram of the framework for radiometric capability evaluation. The TOA reflectance of the FY-3A/MERSI can be calculated using Equation (3). \[\rho_{\lambda}=\frac{(k_{0}+k_{1}\cdot DN+k_{2}\cdot DN^{2})\cdot d^{2}}{100\cdot \cos(\theta_{SZ})} \tag{3}\]  
where \(k_{0}\), \(k_{1}\) and \(k_{2}\) is the calibration coefficients, which can be read from the data source file.