# Evaluation on Radiometric Capability of Chinese Optical Satellite Sensors
## 2 Materials and Methods
### Methods
#### 2.3.3 Evaluation Method  
Radiometric stability and accuracy are the two most important indicators to evaluate a sensor's radiometric capability. Radiometric stability is defined as follows: given a calibration site with very stable radiometric characterizations, such as Badain Jaran Desert in this study, the radiance measured by the sensor to be evaluated is stable temporally. Ideally, it would be permanent, but it is usually defined as the sensor's lifetime. In this study, the radiometric stability of a sensor is analyzed by using the tendency of long-term top-of-atmosphere (TOA) reflectance at the calibration \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sensor** & **a (Blue)** & **a (Green)** & **a (Red)** & **a (NIR)** \\ \hline HJ-1A/CCD1 & 0.8786 & 0.9065 & 0.9772 & 1.0002 \\ HJ-1A/CCD2 & 0.9220 & 0.9186 & 0.9792 & 0.9990 \\ HJ-1B/CCD1 & 0.8457 & 0.9193 & 0.9856 & 0.9988 \\ HJ-1B/CCD2 & 0.8778 & 0.8749 & 0.9806 & 0.9988 \\ FY3A/MERSI & 1.0331 & 1.0486 & 1.0085 & 1.0016 \\ FY3A/VIRR & 0.9648 & 1.0009 & 0.9779 & 0.9808 \\ FY3B/MERSI & 1.0224 & 0.9775 & 1.0020 & 1.0003 \\ FY3B/VIRR & 1.0062 & 0.9791 & 0.9529 & 1.0009 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The spectral matching factors between CCDs, MERSIs, VIRRs and MODIS, respectively. Figure 5: The relative spectral responses of the MERSIs and MODIS in VNIR bands. site. Radiometric accuracy is defined as how the radiance measured from the sensor is close to the actual radiance measured using ground instruments with high precision. In this study, the MODIS is used as the standard instruments with excellent performance on radiometric capability and the radiometric accuracy of a sensor is determined by comparing with the TOA reflectance from MODIS after spectrally matching. In order to evaluate the radiometric stability and accuracy for a given sensor, a procedure including three steps is worked out and the procedure is as follows:  
Firstly, the long-term TOA reflectance of MODIS is plotted and the stability of MODIS is analyzed. Furthermore, it can be compared to the to-be-evaluated Chinese sensors visually. Secondly, the long-term TOA reflectance of each reflective band for every sensor is plotted, which can illustrate the radiometric stability very intuitively. If the stability is good enough, a linear fitting of the long-term radiometric tendency can be done and the stability can be quantitatively described using the slope of the fitted line. If the stability is not good, the possible reasons can be analyzed based on the plot. Thirdly, compare the TOA reflectance between the sensor to be evaluated and the MODIS to get the radiometric accuracy of the to-be-evaluated sensors. By employing some indices including maximum, minimum, mean, and standard deviation of TOA reflectance, the accuracy of the sensor can be quantified. The indices for evaluating a sensor's radiometric capability are summarized as follows:  
1. Slope: the slope of the fitted line for long-term tendency of the TOA reflectance at the calibration site indicates the radiometric stability of a sensor. When a slope value is close to 0, it means the TOA reflectance remains stable. A positive slope means an increasing tendency of TOA reflectance and a negative one means a decreasing tendency. 2. Maximum and minimum values: the maximum value and minimum value indicate the largest and smallest TOA reflectance within the long time TOA reflectance, respectively. If the maximum value is within the proper range, it is usually induced by some self-factors, such as intra-annual variation, observation angles, solar angles, aerosols, and so on; however, if the maximum value is out of the proper range, it indicates that some external factors are working. 3. Mean value: the mean value indicates the average of the TOA reflectance. 4. Standard deviation. The standard deviation is a measure that is used to quantify the amount of variation or dispersion of a set.