# Evaluation on Radiometric Capability of Chinese Optical Satellite Sensors
## 4 Conclusions  
Since an excellent radiometric capability of satellite optical sensors is prerequisite, the radiometric capability needs to be evaluated first. Most of the new satellite optical sensors have onboard calibrators, which have the ability of real-time monitoring; therefore, an excellent radiometric capability can be expected. However, most of the Chinese satellite optical sensors lack onboard calibrators and the radiometric capability of these sensors needs to be evaluated often to ensure the quality of the data. Thus, this paper proposed an alternative procedure for monitoring the radiometric capability of Chinese optical satellite sensors using long time series data. In this procedure, two major indicators including radiometric stability and accuracy are employed for evaluation. Following an evaluation of the eight Chinese satellite optical sensors, the following conclusions can be made. First of all, almost all of the Chinese satellite optical sensors are less stable than the Moderate Resolution Imaging Spectroradiometers (MODISs), and their radiometric accuracy is less than that of the MODISs. Secondly, among all the evaluated sensors, the Visible and Infrared Radiometer (VIRRs) thus far have the best stability throughout their lifetime, although they have an obvious decreasing trend probably induced by instrument degradation; the Medium-Resolution Spectral Imagers (MERSIs) have the best radiometric performance in terms of both stability and accuracy at their later stages. Thirdly, a vicarious calibration procedure carried out once a year or once every several years has been operated by the surveillance departments of the sensors, which has greatly contributed to improving the lower-quality radiometric capability of Chinese satellite optical sensors. However, a more frequent calibration procedure urgently needs to be developed and fulfilled in the future due to the lack of onboard calibrator. Moreover, in order to fully take advantage of the ample Chinese satellite data, the recalibration of the historical data also needs to be carried out in the future. Fourthly, with the ongoing integrated global observations, more and more optical sensors onboard different satellites with global observing capability have been placed into orbit. Quantitative \begin{table}
\begin{tabular}{l l l l l l l l l} \hline \hline \multicolumn{1}{c}{**Sensor**} & \multicolumn{1}{c}{**Band**} & \multicolumn{1}{c}{**Slope**} & \multicolumn{1}{c}{**Maximum**} & \multicolumn{1}{c}{**Minimum**} & \multicolumn{1}{c}{**Mean**} & \multicolumn{1}{c}{**Mean\_a**} & \multicolumn{1}{c}{**Mean\_a**\({}^{*}\)**} & \multicolumn{1}{c}{**Standard**} \\  & Blue & \(-1.33\times 10^{-6}\) & 0.1951 & 0.1331 & 0.1573 & 0.1569 & 0.0169 & 0.0164 \\ FX-3A/VIRR & Green & \(1.44\times 10^{-4}\) & 0.1939 & 0.1530 & 0.1671 & 0.1709 & 0.0049 & 0.0118 \\  & Red & \(1.88\times 10^{-4}\) & 0.2353 & 0.1965 & 0.2086 & 0.2082 & \(-0.0148\) & 0.0109 \\  & NIR & \(-7.06\times 10^{-4}\) & 0.2767 & 0.2385 & 0.2545 & 0.2544 & \(-0.0196\) & 0.0116 \\ \hline  & Blue & \(-1.67\times 10^{-4}\) & 0.2010 & 0.1316 & 0.1604 & 0.1594 & 0.0004 & 0.0168 \\ FY-3B/VIRR & Green & \(-3.93\times 10^{-4}\) & 0.2018 & 0.1486 & 0.1681 & 0.1717 & \(-0.0233\) & 0.0127 \\  & Red & \(-4.36\times 10^{-4}\) & 0.2247 & 0.1822 & 0.1960 & 0.2057 & \(-0.0223\) & 0.0103 \\  & NIR & \(-5.04\times 10^{-4}\) & 0.2862 & 0.2262 & 0.2510 & 0.2508 & \(-0.0282\) & 0.0128 \\ \hline \hline \end{tabular}
\end{table} 
Table 13: The statistics from the time series of the TOA reflectance of FY-3/VIRRs. Figure 10: Time series of TOA reflectance of VIRRs in reflective bands: **(a)** FY-3A/VIRR from 2008 to 2015; **(b)** FY-3B/VIRR from 2010 to 2015. The color lines separate different calibration stages. applications through multi-sensors require continued and consistent measurements from different instruments. This study provides reliable reference for the sensors' applications, and as such will promote the development of Chinese satellite data.