MODIS Solar Diffuser Earthshine Modeling and Analysis

摘要

MODIS's solar diffuser is one of the key calibration sources for its reflective bands. Geometric optical modeling shows that Earthshine illuminating the solar diffuser contaminates measurements of the direct solar irradiance. Before launch, a simple model was used that did not consider the non-diffuse component and the atmospheric transfer of the Earthshine contamination. Recently, a more detailed Earthshine model has been recently developed to better determine the magnitude and characteristics of Earthshine contamination. The model includes a geometric optical model of the instrument, a model of the Earth/Sun/instrument geometry during the calibration interval, an atmospheric model, and various bi-directional models of Earth surface types. Several types of vegetation and open-ocean with different wind speeds are modeled. Analysis was performed of the solar diffuser data with particular emphasis on the surface type at the Earth locations where specular reflections (glint) might occur, i.e., where the solar and view zenith angles are almost the same and the relative azimuth angle is near 180°. The new model compares well with detailed analysis of the solar diffuser data, both over open-ocean with glint, and over vegetation. Both the modeling and analysis show a spectral dependence in the non-diffuse radiation that increases with wavelength. The modeling and analysis give lower and upper bounds on the Earthshine contamination and suggest approaches for minimizing its impact on the MODIS calibration.