# On-Orbit Absolute Radiometric Calibration and Validation of ZY3-02 Satellite Multispectral Sensor
## 4 Results
### Validation of Three Radiometric Calibration Coefficients  
For the automated radiometric calibration and validation of moderate and high-resolution satellite sensors, a desert area (300 m \(\times\) 300 m) was established in October 2015. This is 1.8 km away from the permanent target region to the north-west and has been flattened. Figure 12 shows the desert area at Baotou site. The distance between the gray-scale permanent artificial targets and the desert area is approximately 3 km. In order to validate the reliability of the three radiometric calibration results, the desert area (300 \(\times\) 300 m\({}^{2}\)) was taken as a validation target. We obtain the surface reflectance of the desert and the atmospheric parameters (AOD and CWV). The TOA predicted radiance of the target \(L_{predicted}\) can be calculated with the radiative transfer model MODTRAN. \[L_{predicted}=\frac{\rho_{\lambda}\cdot ESUN\cdot\cos\theta_{SZA}}{\pi\cdot d ^{2}} \tag{5}\] \begin{table}
\begin{tabular}{c c c} \hline \hline
**Source of Uncertainty** & **Accuracy \%** &
\begin{tabular}{c} **TOA Radiance** \\ **Uncertainty \%** \\ \end{tabular} \\ \hline Surface reflectance measurement & 2\% & 2.0\% \\ Lambertian assumption of targets & 1.5\% & 1.5\% \\ Aerosol optical depth & & 0.5\% \\ Total columnar water vapor & & 0.4\% \\ Aerosol size distribution & & 2.0\% \\ Aerosol complex index of refraction & & 2.5\% \\ MODTRAN 6.0 Radiative transfer & 2.0\% & 2.0\% \\ Overall Uncertainty & & 4.57\% \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Sources of uncertainties in TOA radiance in the reflectance-based approach. Figure 12: The desert area at Baotou calibration site. The average DN values of the desert area were extracted from ZY3-02 MUX imagery. The measured radiance \(L_{measured}\) was radiometrically calibrated from DN with three different coefficients, and was compared with the TOA predicted radiance. \[L_{measured}=Gain_{i}\times DN+Bias_{i} \tag{6}\]  
\[\Delta L\%=\left|\frac{L_{measured}-L_{prelated}}{L_{predicted}}\right|\times 1 00\% \tag{7}\]  
where \(L_{measured}\) is the TOA measured radiance, and \(\Delta L\%\) is the relative difference between the TOA predicted radiance and the TOA measured radiance. With regard to the comparison between the measured radiance and TOA predicted radiance in Table 7, the relative difference between measured radiance C and TOA predicted radiance were lowest for the blue band (2.25%), followed by the green band (2.52%), red band (2.91%), and NIR band (2.98%), less than 3% in all bands (Figure 13). The calibration result of combining the permanent artificial targets and the radiometric calibration tarps (coefficient C) shows the best agreement between the measured radiance and TOA predicted radiance. Largest relative differences occurred in the laboratory radiometric calibration results, which indicated that the on-orbit radiometric calibration is a critical activity that must be regularly performed. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{2}{c}{**Band**} & **Blue** & **Green** & **Red** & **NIR** \\ \hline \multicolumn{2}{c}{TOA Predicted Radiance} & \multicolumn{1}{c}{97.233} & 106.576 & 113.235 & 96.328 \\ \multicolumn{2}{c}{by MODTRAN 6.0 \(L_{predicted}\)} & \multicolumn{1}{c}{} & & & \\ \hline \multirow{3}{*}{ZY3-02} & Laboratory & \multirow{3}{*}{105.642} & \multirow{3}{*}{113.173} & \multirow{3}{*}{121.068} & \multirow{3}{*}{89.449} \\  & Coefficients & & & & \\ \cline{2-2} \cline{5-6}  & Coefficients A & 101.327 & 112.289 & 116.940 & 90.686 \\ \cline{2-6}  & Coefficients B & 102.243 & 112.587 & 118.436 & 92.375 \\ \cline{2-6}  & Coefficients C & 99.419 & 109.267 & 116.530 & 93.456 \\ \hline \multirow{3}{*}{Relative difference} & Diff\_Lab & 8.65\% & 6.19\% & 6.92\% & 7.14\% \\ \cline{2-6}  & Diff\_A & 4.21\% & 5.36\% & 3.27\% & 5.86\% \\ \cline{1-1} \cline{2-6}  & Diff\_B & 5.15\% & 5.64\% & 4.59\% & 4.10\% \\ \cline{1-1} \cline{2-6}  & Diff\_C & 2.25\% & 2.52\% & 2.91\% & 2.98\% \\ \hline \hline \end{tabular}
\end{table}