# On-Orbit Absolute Radiometric Calibration and Validation of ZY3-02 Satellite Multispectral Sensor
## 4 Results
### Cross-Validation with Landsat-8 OLI Using the Three Determined Radiometric Calibration Coefficients  
To further validate the reliability of the abovementioned three radiometric calibration results, the calibrated ZY3-02 MUX was compared with Landsat-8 Operational Land Imager (OLI), which is considered as a reference sensor of ZY3-02. The radiometric accuracy of Landsat-8 OLI is typically within 3% for all solar-reflective bands [24]. The Dunhuang test site was carefully selected as reference standard test site for cross-validation of ZY3-02 MUX with Landsat-8 OLI. The Dunhuang test site was China's national radiometric calibration site, which had been used to calibrate the Chinese satellites such as Feng-Yun series of satellites and the Gao-Fen series of satellites [25]. Figure 14 shows the Dunhuang test site in Landsat-8 OLI and ZY3-02 MUX imagery. The Dunhuang test site was used for cross-validation based on the stability and homogeneity of the Gobi Desert (Figure 15). Figure 16 shows the flow chart of cross-validation with Landsat-8 OLI. The corresponding subset areas with 900 \(\times\) 900 m from ZY3-02 and Landsat-8 were selected to acquire  
Figure 14: The Dunhuang test site in Landsat-8 OLI and ZY3-02 MUX imagery. Figure 15: Surface reflectance and standard deviation of Gobi in Dunhuang test site. the homogenous areas within the Dunhuang Gobi site. In the case of ZY3-02 MUX, a total of 150 \(\times\) 150 pixels were used, while Landsat-8 OLI was 30 \(\times\) 30 pixels. Table 8 shows geometric conditions of ZY3-02 and Landsat-8 OLI for cross-calibration. The TOA radiance of the ZY3-02 sensor is calculated by Formula (8). \[L_{measured}^{MUX}=Gain\times DN+Bias \tag{8}\]  
where \(L_{measured}^{MUX}\) is the TOA radiance at ZY3-02 MUX band, expressed in units of \(W\cdot m^{-2}.sr^{-1}\cdot\mu m^{-1}\). \(DN\) is the digital number from the reference satellite imagery. \(Gain\) and \(Bias\) are the official calibration coefficients in units of \(W\cdot m^{-2}.sr^{-1}\cdot\mu m^{-1}\). As the relative spectral responses (RSR) of each band from \"reference sensor\" Landsat-8 OLI and \"sensor to calibrate\" ZY3-02 MUX were strictly different, the TOA radiance of Landsat-8 OLI imagery needed to be transferred to ZY3-02 sensor imagery with a spectral band adjustment factor (SBAF). The differences in the RSR between different satellite sensor were compensated by SBAF, as in Formula (9). \[SBAF_{MUX/OLI}^{Radiance}=\frac{\frac{\int L_{MUX}(\lambda)\cdot RSR_{MIX}( \lambda)\lambda\lambda}{\int RS_{MUX}(\lambda)\lambda\lambda}}{\frac{\int L_{ OLI}(\lambda)\cdot RSR_{OLI}(\lambda)\lambda\lambda}{\int RS_{OLI}(\lambda) \lambda\lambda}} \tag{9}\]  
The TOA radiances of the Landsat-8 OLI imagery were transferred to ZY3-02 sensor imagery by the formula which follows, in which \(L_{OLI}^{SBAF}\) is the TOA radiance from \"reference sensor\" Landsat-8 OLI after SBAF correction. \[L_{OLI}^{SBAF}=L_{OLI}\times SBAF_{MUX/OLI}^{Radiance} \tag{10}\]  
Figure 16: Flow chart of cross-validation with Landsat-8 OLI. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Sensor** & **Date** & \begin{tabular}{c} **Overpass** \\ **Time** \\ **(UTC)** \\ \end{tabular} & \begin{tabular}{c} **Solar** \\ **Zenith** \\ \end{tabular} & \begin{tabular}{c} **Solar** \\ **Azimuth** \\ \end{tabular} & \begin{tabular}{c} **Viewing** \\ **Zenith** \\ \end{tabular} & \begin{tabular}{c} **Viewing** \\ **Azimuth** \\ \end{tabular} & \
\begin{tabular}{c} **AOD@** \\ **550 nm** \\ \end{tabular} &
\begin{tabular}{c} **Cloud** \\ **Condition** \\ \end{tabular} \\ \hline
**ZY3-02** & 22 July 2016 & 04:31:48 & 24.966 & 137.774 & 0.899 & 186.249 & 0.0892 & no cloud \\ \hline
**Landsat-8** & 25 July 2016 & 04:20:20 & 27.759 & 132.077 & 0 & 51.462 & 0.1022 & no cloud \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Geometric conditions of ZY3-02 and Landsat-8 OLI for cross-calibration.