## 2 GEO Grid ASTER EVI Products
### Radiometric Calibration and Atmospheric Correction Modules  
The ASTER Grid system stores all of Level 0 (L0) ASTER scenes acquired since its launch in December 1999 on its cluster system which is mounted as a single storage volume by Gfarm [4]. Newly acquired ASTER scenes are automatically pushed to the ASTER Grid system from the National Aeronautics and Space Administration (NASA) Earth Observation System Data and OperationsSystem (EDOS) [4]. One of the characteristics of this system is that researchers contracted with the GEO Grid committee can implement their own algorithms into this system. Upon a user's request, selected L0 ASTER scenes are radiometrically and geometrically rectified into Level 3A (L3A) orthorectified radiance scenes. The radiometric calibration module is built into this processing, which converts rectified digital numbers (DN) to top-of-the-atmosphere (TOA) reflectances [21, 22, 23]. \[\rho^{TOA}_{ASTER,b}=\frac{\pi[(DN^{*}_{ASTER,b}-1)\times C_{b}]}{F_{0,b}\cos \theta/d^{2}} \tag{2}\]  
and  
\[d=1-0.1672\cdot\cos\left[0.9856\cdot(DOY-4)\frac{\pi}{180}\right] \tag{3}\]  
where  
\(\rho^{TOA}_{ASTER,b}\)TOA reflectance for ASTER band \(b\);  
\(DN^{*}_{ASTER,b}\)rectified DN for ASTER band \(b\);  
\(C_{b}\)unit conversion coefficient (UCC) for ASTER band \(b\) [W\(\cdot\)m\({}^{-2}\cdot\)sr\({}^{-1}\cdot\)um\({}^{-1}\cdot\)DN\({}^{-1}\)];  
\(F_{0,b}\)exoatmospheric solar irradiance for ASTER band \(b\) [W\(\cdot\)m\({}^{-2}\cdot\)um\({}^{-1}\)];  
\(\theta_{s}\)solar zenith angle for the ASTER scene being processed;  
\(d\)Earth-Sun distance in astronomical unit;  
\(DOY\)day of year for the ASTER scene being processed. The square bracket in the numerator of Equation (2) converts \(DN^{*}_{ASTER,b}\) to the at-sensor radiance [20]. Changes in the radiometric response of the sensor since launch as well as detector-to-detector relative response differences are taken into account when deriving \(DN^{*}_{ASTER,b}\), and, thus, \(C_{b}\)'s are held constant over time [19]. The ASTER Grid system uses the onboard calibration for the radiometric rectification  
Figure 1: Normalized spectral response functions of select Advanced Spaceborne Thermal Emission and Reflection Radometer (ASTER) and Moderate Resolution Imaging Spectroradiometer (MODIS) bands. The sample reflectance spectra of vegetation and soils in the figure are obtained from the ASTER spectral library [20]. by default, but other calibration methods developed by ASTER vicarious calibration groups [24, 25] have also been incorporated into the system, which are selectable upon ordering via the ASTER Grid portal. In this study, the onboard calibration was used to rectify ASTER data. The solar model (\(F_{0,b}\) in Equation (2)) adopted in this radiometric calibration module is based on the World Radiation Center (WRC) model [26]. When the user requests atmospheric correction via the ASTER Grid portal, the atmospheric correction module is activated and processes the orthorectified, radiometerically calibrated L3A ASTER scenes. Currently, the atmospheric correction module can correct for the effects of molecular scattering, and ozone and water vapor absorptions.