MODIS is currently onboard NASA’s EOS Terra and Aqua spacecraft launched on December 18, 1999 and May 4, 2002, respectively. MODIS reflective solar bands (RSB) are calibrated on-orbit using solar illumination reflected from its onboard solar diffuser (SD). The solar diffuser stability monitor (SDSM) is designed to track the on-orbit degradation of the SD via alternate observations of the Sun and SD. A wavelength-dependent degradation pattern is observed for both MODIS instruments with a faster degradation rate observed at shorter wavelengths. The UV exposure of the SD to sunlight and the scattered light (the sunlight reflected from top of the atmosphere) through the instrument nadir port contributes to its reflectance degradation. The scatter off the diffuser onto the scan mirror is in the forward direction, whereas the scatter off the diffuser onto the SDSM scan mirror is in the backward direction. Since the outgoing angles (viewed by MODIS detectors) are the same as the scheduled SD calibration, the gain derived from scattering light facilitates monitoring the dependence on SD degradation on incident angles. A methodology is formulated to track the MODIS SD degradation using scattered light through the nadir-port and comparing the result with the SD degradation as measured by the SDSM. In this study, multiple orbits from a given day of each month are processed to obtain a SD response to the nadir-port illumination. Results show that a reasonable agreement is observed between the SD degradation estimates derived from both view-angles.

Date Published: 26 September 2014
PDF: 11 pages
Proc. SPIE 9218, Earth Observing Systems XIX, 92181J (26 September 2014); doi: 10.1117/12.2061358

Published in SPIE Proceedings Vol. 9218:
Earth Observing Systems XIX
James J. Butler; Xiaoxiong (Jack) Xiong; Xingfa Gu, Editor(s)