# Evaluation on Radiometric Capability of Chinese Optical Satellite Sensors
## 3 Results and Discussion
### Radiometric Capability Evaluation of Huan Jing 1/Charge-Coupled Device (H)-1/CCD  
The same plots of the HJ-1/CCDs as the MODIS are presented in Figure 8. The temporal range of HJ-1/CCDs is 2009-2012 when the sensors are operational and in excellent condition. In Figure 8, it can be observed that the stability of the four CCDs is not as good as the MODIS for pronounced calibration drifts. The TOA reflectance of HJ-1/CCD varies periodically and the turning points are exactly the same as the calibration time, which is done once a year based on a vicarious calibration, thereby demonstrating that the procedure of the calibration may have some issues. However, as we have not involved the calibration, this is just an assumption. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Sensor** & **Band** & **Slope** & **Intercept** & **Maximum** & **Minimum** & **Mean** & **Standard Deviation** \\ \hline \multirow{4}{*}{Terra/MODIS} & Blue & \(4.70\times 10^{-5}\) & 0.144 & 0.185 & 0.106 & 0.140 & 0.019 \\  & Green & \(5.22\times 10^{-7}\) & 0.166 & 0.206 & 0.138 & 0.166 & 0.013 \\  & Red & \(2.81\times 10^{-5}\) & 0.220 & 0.260 & 0.198 & 0.223 & 0.013 \\  & NIR & \(5.33\times 10^{-5}\) & 0.270 & 0.312 & 0.247 & 0.274 & 0.015 \\ \hline \multirow{4}{*}{Aqua/MODIS} & Blue & \(9.87\times 10^{-7}\) & 0.159 & 0.207 & 0.122 & 0.159 & 0.020 \\  & Green & \(5.30\times 10^{-5}\) & 0.194 & 0.248 & 0.160 & 0.195 & 0.020 \\ \cline{1-1}  & Red & \(-5.29\times 10^{-6}\) & 0.230 & 0.263 & 0.207 & 0.228 & 0.012 \\ \cline{1-1}  & NIR & \(-1.41\times 10^{-5}\) & 0.282 & 0.313 & 0.256 & 0.279 & 0.014 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: The statistics from the time series of the TOA reflectance of MODIS from both Terra and Aqua. Figure 8: _Cont_. Figure 7: The calibration site’s directional characterization of the red band of Terra/MODIS. The reflectance varies with relative azimuth and solar zenith angle for each bin of view zenith angle, which shows systematic variation that is due to directional effects. The directional effect is about 15%. Tables 8-11 summarize the statistics of the CCDs; the radiometric characteristics are as follows:  
1. For HJ-1A/CCD1 (Table 8), the TOA reflectance in 2009 is the most stable, especially the blue band with a slope at the 10\({}^{-7}\) level. A gradual decreasing tendency is observed in 2012 for all bands (slopes all less than 0). The TOA reflectance tendency in 2011 in blue and NIR bands do not agree with the linear rule; consequently, their slopes are not calculated. The highest variation of TOA reflectance occurs in 2011 in blue band, up to (0.2193 \(-\) 0.1585)/0.1795 = 33.87%, while the lowest occurs in 2010 in the green band: 6.32%. The mean TOA reflectances of HJ-1A/CCD1 after spectral matching are all higher than that of MODIS in the blue and green bands from 2009-2012, while lower than that of MODIS in the NIR bands of the four years. All the standard deviations of the TOA reflectance are within 0.02, and the lowest standard deviations occur in 2009 (0.0054 for blue band, 0.0052 for green band, 0.0067 for red band, and 0.0062 for NIR band, respectively). 2. For HJ-1A/CCD2 (Table 9), the TOA reflectance in 2011 is the most stable, especially the blue band with a slope at the 10\({}^{-6}\) level. A gradual decreasing tendency is observed in 2009 and 2012 for all bands, while the increasing tendency is observed in 2010 and 2011 for all bands. The highest variation of TOA reflectance occurs in 2011 in the NIR band, up to 25.30%, while the lowest occurs in 2009 in the red band: 7.91%. Blue bands of HJ-1A/CCD2 in 2009, 2010 and 2012 have the largest variation over the other three bands. The mean TOA reflectances of HJ-1A/CCD2 after spectral matching are higher than those of MODIS except the red and NIR bands in 2009 and 2010, and the NIR band in 2011. All the standard deviations of the TOA reflectances are within 0.02, and the lowest standard deviations occur in 2009 (0.0069 for blue band, 0.0052 for green band, 0.0057 for red band, and 0.0063 for NIR band, respectively). 3.