# Evaluation on Radiometric Capability of Chinese Optical Satellite Sensors
## 2 Materials and Methods
### Methods
#### 2.3.2 Spectral Matching  
Figures 3-5 show the relative spectral responses of the CCDs, MERSIs, VIRRs and MODIS in blue, green, red, and NIR bands. These relative spectral response profiles differ in shape, bandwidth, peak value, location of the central wavelength, and degree of overlap between channels. Since the similar principle of design, the relative spectral response profiles of the sensors are almost unanimous.  
Figure 3: \(Cont.\)  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{2}{c}{**Sensor**} & \multicolumn{2}{c}{**St: 2008.11.11–2015.2.5**} & \multicolumn{2}{c}{**Sensor**} & \multicolumn{2}{c}{**St: 2010.10.18–2015.2.5**} \\ \hline \multirow{8}{*}{FY3A/VIRR} & **Band** & **S** & **I** & **Band** & **S** & **I** \\ \cline{2-7}  & Blue & 0.0894 & \(-\)1.1622 & & Blue & 0.0748 & \(-\)0.9098 \\  & Green & 0.0687 & \(-\)0.8236 & & Green & 0.0746 & \(-\)0.8953 \\  & Red & 0.1457 & \(-\)1.7484 & & Red & 0.1264 & \(-\)1.4320 \\  & NIR & 0.1435 & \(-\)1.7348 & & NIR & 0.1353 & \(-\)1.6236 \\ \cline{2-7}  & \multicolumn{2}{c}{**S2: 2015.2.5–Current**} & \multicolumn{2}{c}{FY3B/VIRR} & \multicolumn{2}{c}{**S2: 2015.2.5–Current**} \\ \cline{2-7}  & **Band** & **S** & **I** & **Band** & **S** & **I** \\ \cline{2-7}  & Blue & 0.102 & \(-\)1.3283 & & Blue & 0.0938 & \(-\)1.1494 \\  & Green & 0.08 & \(-\)0.9576 & & Green & 0.0803 & \(-\)0.9636 \\  & Red & 0.1586 & \(-\)1.892 & & Red & 0.1264 & \(-\)1.432 \\  & NIR & 0.1435 & \(-\)1.7348 & & NIR & 0.1353 & \(-\)1.6236 \\ \hline \hline \end{tabular}
\end{table}
Table 5: The calibration coefficients of VIRRs.  
Figure 4: The relative spectral responses of the VIRRs and MODIS in VNIR bands.  
Figure 5: _Cont_.  
Figure 3: The relative spectral responses of the CCDs and MODIS in VNIR bands.  
Due to the atmospheric transmittance and the atmosphere outside solar irradiance varying with wavelength, different responses will affect the sensors' reflectances at the top of the atmosphere. Thus, spectral matching is performed to compare radiation characteristics between the sensors to be evaluated and MODIS. Given the spectral behavior of the site (the ground feature is desert) and the spectral response functions of the instruments, the spectral difference between MODIS and other sensors can be expressed as a linear translation equation of the form \(\rho_{sensor,i}=a_{i}\cdot\rho_{modis,j}\), where \(i\) represents band number of the other sensor and \(j\) is the corresponding band number of MODIS. The spectral difference coefficient \(a_{i}\) is calculated based on the Equation (7) [14; 26; 27].  
\[a=\int\limits_{\lambda_{1}}^{\lambda_{2}}\rho_{\lambda}\times f_{sensor}( \lambda)d\lambda/\int\limits_{\lambda_{3}}^{\lambda_{4}}\rho_{\lambda}\times f_ {\text{modis}}(\lambda)d\lambda \tag{7}\]  
where \(f_{sensor}(\lambda)\) and \(f_{modis}(\lambda)\) are the relative spectral response functions for other sensors and MODIS, respectively. \(\lambda_{1}\!-\!\lambda_{2}\) is the spectral range of other sensor; \(\lambda_{3}\!-\!\lambda_{4}\) is the spectral range of MODIS. The ground-measured spectrum of the calibration site we used in this paper comes from the measurement in the Badain Jaran Desert [14]. Based on the definition of the spectral matching factor, the spectral matching factors between the sensors and the MODIS in reflective bands are calculated and listed in Table 6.