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Improvements in the On-Orbit Response Versus Scan Angle Characterization of the Aqua MODIS Reflective Solar Bands

TitleImprovements in the On-Orbit Response Versus Scan Angle Characterization of the Aqua MODIS Reflective Solar Bands
Publication TypeJournal Publication
Year of Publication2018
AuthorsAngal, A., X. Xiong, A. Wu, X. Geng, and H. Chen
JournalIEEE Transactions on Geoscience and Remote Sensing
KeywordsAqua; calibration; MODIS; RSB

The Aqua Moderate Resolution Imaging Spectrora-
diometer (MODIS) has been chosen by the Global Space-based
Inter-Calibration System operational community as the reference
sensor in cross-sensor calibration. A number of geostationary
orbit and low-earth orbit sensors use the 0.64-μm band from
Aqua MODIS as a calibration reference. After over 15 years
on-orbit, the performance characteristics of the MODIS instru-
ment have changed, with effects evident at short wavelengths.
MODIS employs a reflectance-based calibration using the solar
diffuser measurements with the monthly lunar observations facil-
itating a response versus scan angle characterization on-orbit.
As the instrument continues to operate beyond its design lifetime
of 6 years, the on-board calibrators alone are insufficient to
accurately characterize the instrument’s response at all scan
angles. This results in a long-term reflectance drift, particularly
observed in the 0.64- and 0.85-μm bands, while observing the
temporally invariant desert sites. Long-term reflectance drifts
of up to 2% and 3% are observed at the beginning of scan
for the 0.64- and 0.85-μm bands, respectively. An approach
using earth-view response to supplement the on-board calibrator
measurements has been shown to overcome these inadequacies
and is now implemented for the 0.64-, 0.85-, 0.46-, and 0.55-μm
land bands of Aqua MODIS. This paper presents the details
related to the algorithm implementation and an independent
evaluation using the Dome Concordia site and deep-convective
clouds. This approach has been reviewed, tested, and approved
by the MODIS science team and has been implemented in the
forward production of the MODIS L1B Collection 6 starting
July 9, 2016. This enhanced approach has also been adopted in
the MODIS L1B Collection 6.1 reprocess for the entire mission
to facilitate an improved quality of downstream science products.
The results with the enhanced approach reduce the reflectance
drifts to within 0.5% for most cases.