# Evaluation on Radiometric Capability of Chinese Optical Satellite Sensors
## 3 Results and Discussion  
### Radiometric Capability Evaluation of Moderate Resolution Imaging Spectroradiometer (MODIS)  
The Terra/MODIS acquired from 2008 to 2015 and the Aqua/MODIS acquired from 2010 to 2015 have been collected in order to evaluate the temporal radiometric capability of the two sensors. Figure 6 shows that the stability of MODIS from both Terra and Aqua are visually excellent. Subsequently, in order to evaluate the radiometric capability of MODIS quantitatively, a linear fitting to the TOA reflectance data was carried out and all the statistics are listed in Table 7. Table 7 summarizes the slopes and intercepts of the trend lines, and the maximum, minimum, mean and standard deviation values of the TOA reflectances in reflective bands including blue, green, red, and NIR. Several things about the table need to be clarified.  
1. The difference between maximum and minimum values indicates the range of TOA reflectance variation. The blue band usually has a larger variation than the other three bands and the variation becomes smaller and smaller with an increasing wavelength, because the TOA reflectance at a shorter wavelength incorporates more scattering radiation from the atmosphere, and the atmospheric effect becomes stronger with the increasing of aerosols. For example, the difference of TOA reflectance at the blue band for Terra is \(0.185-0.106=0.079\); compared to the mean TOA reflectance of 0.14, the variation reaches \(0.079/0.14=56.43\%\). The variations of the other three bands are \(40.96\%\), \(27.80\%\), and \(23.72\%\) respectively. In addition, the minimum, maximum, and the values close to the minimum and the maximum appear for most of the years, which clarifies that the maximum and minimum are not abnormal values but rather the normal ones, and that the aerosol condition varies day by day, though the variation is very similar year by year.
2. Besides the atmospheric effect, the BRDF also contributes to the variation of the TOA reflectance. Figure 7 gives an example of the directional effect 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 \(15\%\).
3. The mean of TOA reflectance at the same band are usually a little bit different, especially at the blue and red bands. The radiometric accuracy of Aqua is slightly better than that of Terra [28].  
Although several factors can induce the variation of the TOA reflectance, the long-term tendency of the TOA reflectance remains consistent and the slope values of the fitted lines for different bands range from \(10^{-7}\) to \(10^{-5}\), which indicates a very small variation trending. The intercepts of the fitted lines are close to the corresponding means of TOA reflectance. The standard deviations of the TOA reflectance are within 0.02. All of the above show that the radiometric capability of both Terra/MODIS and the Aqua/MODIS are stable, and can be used to evaluate other sensors as the reference data.  
Figure 6: Time series of TOA reflectance of MODIS in reflective bands. The color lines are the trend lines of the bands. (**a**) Terra/MODIS; (**b**) Aqua/MODIS.