# Evaluation on Radiometric Capability of Chinese Optical Satellite Sensors
## 3 Results and Discussion
### Radiometric Capability Evaluation of Feng Yun 3/Medium-Resolution Spectral Imager (FY-3/Merssi)
\begin{tabular}{c} **Mean\_a** \\ **Mean\_M** \# \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Standard** \\ **Deviation** \\ \end{tabular}  } \\ \hline \multirow{8}{*}{HJ-IB/CCD2} & \multirow{8}{*}{2009} & Blue & \(-2.91\times 10^{-5}\) & 0.1721 & 0.1449 & 0.1543 & 0.1758 & 0.0358 & 0.0081 \\  & Green & \(-2.80\times 10^{-5}\) & 0.1918 & 0.1666 & 0.1763 & 0.2015 & 0.0355 & 0.0078 \\  & Red & \(-4.75\times 10^{-5}\) & 0.2388 & 0.2144 & 0.2242 & 0.2286 & 0.0056 & 0.0075 \\  & NIR & \(-1.70\times 10^{-5}\) & 0.2671 & 0.2506 & 0.2600 & 0.2607 & \(-0.0133\) & 0.0055 \\ \cline{2-10}  & \multirow{8}{*}{2010} & Blue & \(-4.66\times 10^{-5}\) & 0.1681 & 0.1486 & 0.1603 & 0.1826 & 0.0426 & 0.0064 \\  & Green & \(-1.45\times 10^{-5}\) & 0.1753 & 0.1557 & 0.1687 & 0.1928 & 0.0268 & 0.0062 \\  & Red & \(-1.85\times 10^{-6}\) & 0.2168 & 0.1990 & 0.2100 & 0.2142 & \(-0.0088\) & 0.0056 \\  & NIR & \(-1.99\times 10^{-5}\) & 0.2509 & 0.2244 & 0.2371 & 0.2377 & \(-0.0363\) & 0.0085 \\ \hline \multirow{8}{*}{HJ-IB/CCD2} & \multirow{8}{*}{2011} & Blue & \(-2.23\times 10^{-6}\) & 0.1785 & 0.1642 & 0.1684 & 0.1918 & 0.0518 & 0.0044 \\  & Green & \(1.44\times 10^{-5}\) & 0.2044 & 0.1801 & 0.1925 & 0.2200 & 0.0540 & 0.0062 \\ \cline{1-1}  & Red & - & 0.2334 & 0.1994 & 0.2199 & 0.2243 & 0.0013 & 0.0113 \\ \cline{1-1}  & NIR & \(-\) & 0.2569 & 0.2244 & 0.2393 & 0.2399 & -0.0341 & 0.0101 \\ \cline{1-1} \cline{2-10}  & \multirow{8}{*}{2012} & Blue & \(-2.67\times 10^{-5}\) & 0.1 of the TOA reflectance are within 0.02. After the malfunction duration is removed, the stability and accuracy are very close to that of MODIS. 2. The stability of FY-3B/MERSI has a two-stage mode. At the first stage, three sub-stages are very obvious. An obvious over-estimation of the TOA reflectance appears at the first sub-stage; the second sub-stage has a close estimation; the third sub-stage has an under-estimation. The three sub-stages changed suddenly, so manual adjustments of the instrument were executed by the surveillance department. However, the radiometric stability and accuracy have become better and very close to the MODIS's. Only the statistics of the second stage are calculated and listed in Table 12. The TOA reflectance in green and red bands has a tendency towards lightly decreasing because all the slopes are at the \(10^{-4}\) level and greater than 0. The highest variation of TOA reflectance occurs in the blue band, up to 41.75%. The variations of the other three bands are 28.52%, 20.75%, and 18.39% respectively. The differences in four bands between the mean TOA reflectance after spectral matching and corresponding bands of MODIS are less than 0.015 except the green band (0.023). All the standard deviations of the TOA reflectance are within 0.02. The stability and accuracy of FY-3B/MERSI in the second stage are very good. 3. Although both the two MERSIs have very low frequency of calibration, the stability remains very good; however, with the degradation of the instruments after a long operation, it is difficult to retain very good radiometric capability with very low frequent calibrations. Moreover, high frequent calibrations can find the instruments' malfunction timely. Figure 9: Time series of TOA reflectance of MERSIs in reflective bands: (**a**) FY-3A/MERSI from 2008 to 2015; (**b**) FY-3B/MERSI from 2010 to 2015.