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MODIS solar diffuser degradation at short-wave infrared band wavelengths

TitleMODIS solar diffuser degradation at short-wave infrared band wavelengths
Publication TypePresentation
Year of Publication2017
AuthorsTwedt, K. A., A. Angal, X. Xiong, X. Geng, and H. Chen
Proceedings Proc. SPIE Earth Observing Systems XXII
MCST ConferenceSPIE - Earth Observing Systems XXII

The MODIS instruments aboard the Terra and Aqua spacecraft have 20 reflective solar bands (RSB) with wavelengths
spanning 412 nm to 2130 nm. The primary on-board calibration source for the RSB is a sunlit solar diffuser (SD), with
its degradation tracked by a SD stability monitor (SDSM). The SDSM measurements show that the decrease in SD
reflectance over time has a strong wavelength dependence, with longer wavelengths showing less degradation. The
SDSM has 9 detectors to track the SD degradation at wavelengths from 412 nm to 936 nm, but is not designed to track
the degradation at short-wave infrared (SWIR) wavelengths. In recent years, the SDSM has measured non-negligible
degradation in the SD reflectance at 936 nm for both Terra (>2%) and Aqua (>0.5%) MODIS. In addition, comparison
of SD calibration results to earth view targets suggests that smaller but non-negligible SD degradation also exists at the
SWIR band wavelengths. In this paper, we review the current status of the MODIS SD degradation as measured by the
SDSM. We present efforts to extend the SD degradation measurements to the SWIR band wavelengths (1240 nm to
2130 nm) by fitting a wavelength-dependent model to the SDSM results from the visible and NIR wavelength detectors.
The predicted degradation results are used to correct the MODIS SWIR band degradation, and comparisons are made
with trends from pseudo-invariant earth targets. Results are presented for both Terra and Aqua MODIS.