Abstract
The Moderate Resolution Imaging Spectroradiometer (MODIS) has successfully operated onboard the Terra spacecraft for more than 12 years and the Aqua spacecraft for more than ten years. It has 20 reflective solar bands covering the visible (VIS), near infrared (NIR), and short-wave infrared (SWIR) spectral regions. They are calibrated on orbit using regularly scheduled solar diffuser measurements and lunar observations. In recent years, observations over selected ground targets are also used to monitor detector responses at different angles of incidence. This paper provides a brief description of MODIS on-orbit calibration and characterization methodologies and examines the calibration stability of the VIS, NIR, and SWIR spectral bands over the entire missions of both instruments. Results obtained from four different vicarious approaches (deserts, Dome Concordia, deep convective cloud, and simultaneous nadir overpass) show that Terra MODIS VIS and NIR spectral bands have a wavelength-dependent drift in reflectance with a drop up to 8 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\%$</tex></formula> in the shortest wavelength region. All four approaches have a relative agreement to within 2.0 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$ \%$</tex></formula> with an uncertainty of less than 1.5 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\%$</tex></formula> for most bands. It is anticipated that the improvements made in the MODIS Collection 6, with additional corrections based on the desert reflectance trending results, will significantly reduce, if not completely remove, some of the trending drifts identified in the Collection-5 data product.
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