An on-board Solar Diffuser (SD) is used for the MODIS reflective solar bands (RSB) calibration. Its on-orbit bidirectional
reflectance factor (BRF) degradation is tracked using an on-board Solar Diffuser Stability Monitor (SDSM).
The SDSM is a ratioing radiometer with nine detectors, covering wavelengths from 412 nm to 936 nm. During each
scheduled SD calibration event, the SDSM makes alternate observations of the Sun and the sunlight reflected by the SD.
To best match the SDSM detector signals from its Sun view and SD view, a fix attenuation screen is placed in its Sun
view path. This paper provides a brief description of MODIS RSB on-orbit calibration and the use of its on-board SD
and SDSM subsystem, including different approaches developed and used to track MODIS SD on-orbit degradation. It
reports recent progress made to better characterize MODIS SD on-orbit degradation and to support MODIS Level 1B
(L1B) calibration look-up table (LUT) updates for the upcoming collection 6 (C6) reprocessing. Results of both Terra
and Aqua SD on-orbit degradation derived from newly improved SDSM Sun view screen vignetting function and
response fitting strategy, and their impact on RSB calibration uncertainties are also presented.
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