# On-Orbit Absolute Radiometric Calibration and Validation of ZY3-02 Satellite Multispectral Sensor
## 4 Results
### Uncertainty Analysis of the Reflectance-Based Approach  
Early work with Landsat-7 ETM+, Landsat-8 OLI, SPOT HRV, IKONOS, RapidEye, and KOMPSAT showed that the reflectance-based approach had an absolute radiometric uncertainty of \(\sim\)5% [6; 20; 21; 22; 23]. The reflectance-based uncertainty table listed the major source of radiometric uncertainty of the reflectance-based approach. Detailed descriptions of the uncertainty were discussed as follows. There are several basic areas of uncertainty in the method: (1) Surface reflectance measurement, (2) Atmospheric characterization, and (3) Inherent accuracy of the MODTRAN 6.0 radiative transfer code.  
The uncertainty in a single measurement of the surface reflectance with SVC HR1024 spectroradiometer is <2%. Based on the laboratory calibration tarp BRDF measurements, we can assign a 1.5% error accuracy for assuming a Lambertian calibration tarp. The view zenith angle of ZY3-02 MUX was less than 3 degrees, and the BRDF effects of the artificial calibration targets and calibration tarp were not considered in this study.  
The second primary source of uncertainty in the reflectance-based approach is atmospheric characterization, which includes the aerosol optical depth measurement, total  
Figure 11: Calibration result of combining the permanent artificial targets and the radiometric calibration tarps (coefficient C).  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Coefficients** & **A** & \multicolumn{3}{c}{**B**} & \multicolumn{3}{c}{**C**} & \multicolumn{3}{c}{**Laboratory**} \\ \hline
**Band** & _Gain_ & _Bias_ & _Gain_ & _Bias_ & _Gain_ & _Bias_ & _Gain_ & _Bias_ \\ \hline MUX-B1 & 0.2495 & \(-18.36\) & 0.2342 & \(-17.07\) & 0.2291 & \(-11.62\) & 0.2004 & \(-0.1794\) \\ \hline MUX-B2 & 0.2282 & \(-10.55\) & 0.2333 & \(-13.60\) & 0.2213 & \(-6.241\) & 0.2031 & \(-0.3262\) \\ \hline MUX-B3 & 0.2542 & \(-15.48\) & 0.2454 & \(-12.95\) & 0.2432 & \(-10.29\) & 0.2205 & \(-0.1812\) \\ \hline MUX-B4 & 0.2466 & \(-23.33\) & 0.2261 & \(-16.40\) & 0.2110 & \(-9.552\) & 0.2044 & \(-0.4489\) \\ \hline \hline \end{tabular}
\end{table}
Table 5: Different radiometric calibration coefficients (in units of W\(\cdot\)m\({}^{-2}\cdot\)sr\({}^{-1}\cdot\)um\({}^{-1}\)).  
columnar water vapor, aerosol size distribution, and aerosol complex index of refraction. The AOD and CWV were retrieved by the AERONET data process center at the AOE Baotou site, and they led to the TOA radiance uncertainties of 0.5% for AOD and 0.4% for CWV. A standard rural aerosol type in the MODTRAN 6.0 radiative transfer code was used in this study. The two parameters of aerosol (the aerosol size distribution and the aerosol complex index of refraction) resulted in uncertainties of TOA radiance less than 2.0% for aerosol size distribution and 2.5% for aerosol complex index of refraction, respectively. The inherent accuracy of the MODTRAN 6.0 radiative transfer code is <1%.  
All of the analyzed uncertainty sources above were assumed independent, and the overall uncertainty in the TOA radiance in this reflectance-based approach is 4.57%, less than 5%, as shown in Table 6.