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The MODIS Protoflight Model (PFM), on-board the NASA EOS Terra spacecraft, has been in operation for more than two years. Its 20 reflective solar bands (RSB) from 0.412μ to 2.13μ are calibrated on-orbit by a solar diffuser (SD) with its degradation tracked by a solar diffuser stability monitor (SDSM). The results derived from the SD/SDSM calibration data have shown that SD degradation is wavelength dependent. After nearly 2.5 years, the SD has degraded about 7.0% at 0.412μ, 4.0% at 0.466μ, 2.1% at 0.530μ, and the degradation is smaller at other longer wavelengths. The MODIS optical system includes a rotating scan mirror and other fixed aft optics. Overall system response in the visible spectral range has also shown wavelength dependent degradation over time. This degradation varies with the angle of incidence (AOI) to the scan mirror and the degradation rate is different between two sides of the scan mirror. During the first 20 months of instrument on-orbit operation, the system degradation (mirror side 1) at SD calibration AOI (50.2β) is about 11% at 0.412μ (MODIS Band 8), 6.5% at 0.443μ (Band 9), 5.0% at 0.469μ (Band 3), and 4.0% at 0.488μ (Band 10). Again the degradation is smaller for other bands with longer wavelengths. At other smaller AOIs, our results show that the degradation rate is higher. Since Oct./Nov. 2001, the system response degradation has essentially stopped. In this paper, we present MODIS RSB degradation analyses and the associated trending results including degradation at different AOIs to the scan mirror. We also address their impact on and application to the RSB on-orbit calibration.
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