Monitoring MODIS calibration stability of visible and near-IR bands from observed top-of-atmosphere BRDF-normalized reflectances over Libyan Desert and Antarctic surfaces

MODIS is one of the major instruments for the National Aeronautics and Space Administration (NASA) Earth Observing System (EOS) missions. It is on-board both the EOS Terra and Aqua spacecrafts, launched in December 1999 and May 2002, respectively. Each MODIS provides spectral observations in multiple angular views of reflectance at the top of atmosphere (TOA) around the globe. This study focuses on one visible (0.65μm) band and one near-IR (0.84μm) band to examine the variations of observed TOA reflectances due to the impact of the bi-directional reflectance distribution function (BRDF). Two highly uniform ground sites in the Libyan Desert and Antarctica are selected. The variation of reflectance as a function of view zenith angle at a fixed solar zenith angle is studied based on reflectances obtained from multiple granules over passing each site. The variation of reflectance as a function of solar zenith angle at a fixed view zenith angle is studied based on reflectances collected from 16-day repeatable orbits, which have the same view geometry relative to each site. Results show that variations of the near nadir reflectances at the desert site are close to the Lambertian pattern while those at the Dome C. site are strongly anisotropic. Comparison of a simple Lambertian and a BRDF correction is made in terms of their effectiveness in removing reflectance trending variations. The BRDF correction is based on a semi-empirical model consisting of two kernel-driven components. Results show that both corrections are able to significantly remove trending variations at the desert site, which produce a remarkably low variability of less than 2% relative to the linear fit. The normalized reflectance trends show that from year 2000 to 2007, the visible and near-IR bands dropped only 1.7 and 1.2% in total, respectively.