MODIS has 20 reflective solar bands (RSB), covering the VIS, NIR, and SWIR spectral regions. They are calibrated on-orbit
using a solar diffuser (SD) panel, made of space-grade Spectralon. The SD bi-directional reflectance factor (BRF)
was characterized pre-launch by the instrument vendor with reference to the NIST reflectance standard. Its on-orbit
degradation is tracked by an on-board solar diffuser stability monitor (SDSM). The SeaWiFS on-orbit calibration
strategy uses monthly lunar observations to monitor the long-term radiometric stability of the instrument and applies
daily observations of its solar diffuser (an aluminum plate coated with YB71 paint) to track the short-term changes in the
instrument response. This paper provides an overview of MODIS and SeaWiFS SD observations, applications, and
approaches used to track their on-orbit degradations. Results from both sensors are presented with emphasis on the
spectral dependence and temporal trends of the SD degradation. Lessons and challenges from the use of SD for sensor
on-orbit calibration are also discussed.
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