# On-Orbit Absolute Radiometric Calibration and Validation of ZY3-02 Satellite Multispectral Sensor
## 5 Conclusions  
In this study, the methods and results of a reflectance-based vicarious calibration campaign were described. The relationship between the DN and TOA radiance exploiting radiometric calibration coefficients was obtained via an absolute radiometric calibration campaign at the RadCalNet Baotou site. We obtained three radiometrically calibrated coefficients of ZY3-02 MUX using different targets and tarps. The calibration results show that linear correlations between DN and TOA radiances of ZY3-02 MUX are very high, with a correlation coefficient \(R^{2}\) value of up to 0.99 for each band. A detailed discussion on the uncertainty analysis of the radiometric calibration is presented in this paper, and the overall uncertainty of this reflectance-based approach is 4.57%.  
Figure 17: The relative difference between the Landsat-8 OLI TOA radiance after SBAF correction and the measured radiance with different coefficients.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Calibration**} & \multicolumn{4}{c}{**Average Relative Difference (\%)**} \\ \cline{2-5}  & **Blue** & **Green** & **Red** & **NIR** \\ \hline \multirow{2}{*}{**Laboratory**} & \multirow{2}{*}{8.86\%} & \multirow{2}{*}{6.63\%} & \multirow{2}{*}{6.06\%} & \multirow{2}{*}{7.35\%} \\
**Coefficients** & & & & \\ \hline
**Coefficients A** & 6.14\% & 6.67\% & 5.30\% & 5.68\% \\ \hline
**Coefficients B** & 5.34\% & 5.73\% & 5.17\% & 4.43\% \\ \hline
**Coefficients C** & 2.76\% & 2.77\% & 2.43\% & 3.11\% \\ \hline \hline \end{tabular}
\end{table}
Table 9: Comparison of results between ZY3-02 MUX Measured TOA Radiance using different calibration coefficients and Landsat-8 OLI TOA Radiance after SBAF correction.  
The aim of our study was to obtain a reliable and high-accuracy radiometric calibration coefficient for ZY3-02 MUX. In order to consolidate the ZY3-02 MUX absolute radiometric calibration, the three calibration coefficients were validated by the synchronous measurement of surface reflectance and atmospheric characterization in the RadCalNet Baotou desert site, and also cross-validated with Landsat-8 OLI in the Dunhuang Gobi site. A detailed discussion on the validation analysis of the comparison results between the different radiometric calibration coefficients is presented in this paper. The reasonably good agreement of the radiometrically calibrated coefficients of the ZY3-02 MUX is encouraging, which shows that the percentage difference in coefficient C was within 4% in the MUX bands. The radiometric coefficient C was determined by combining permanent artificial targets and radiometric calibration tarps, which means that there would be seven gray-scale targets and two color-scaled targets. It should be noted that absolute radiometric calibration incorporating the permanent gray-scale target and calibration tarps significantly enhances the accuracy and quality. The results also indicate that radiometric characteristics of ZY3-02 are reliable and high-accuracy quantitative applications. The radiometric calibration coefficients would be useful for users when utilizing ZY3-02 MUX imagery.  
[PERSON] developed the research plan and supervised the work, and [PERSON], [PERSON], [PERSON] and [PERSON] participated in the literature review, method selection, data acquisition, and discussions. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Key R&D Program of China, 2018 YFB0504800, 2018 YFB0504801.  
Not applicable.  
The authors declare no conflict of interest.