Degradation of MODIS optics and its reflective solar bands calibration

The MODerate Resolution Imaging Spectroradiometer (MODIS) has 36 spectral bands with wavelength ranging from 0.41mu to 14.5mu and spatial resolution of 0.25km (2 bands), 0.5km (5 bands), and 1.0 km (29 bands) at Nadir. Its ProtoFlight Model (PFM) on the NASA EOS Terra spacecraft has been providing global coverage of the Land, Ocean, and Atmosphere for the science community since the instrument opened its Nadir door on 24 February 2000. The MODIS optical system includes a 2-sided paddle wheel scan mirror, a fold mirror, and a primary mirror. The sensor's 20 reflective solar bands (RSB) from 0.41mu to 2.1mu are calibrated on-orbit by a solar diffuser (SD) and a solar diffuser stability monitor (SDSM). In addition to the SD, degradation of the MODIS optics in the reflective solar bands has been observed, including variations in degradation between the two sides of the MODIS scan mirror. During MODIS first year of on-orbit operation, the overall degradation at the shortest wavelength of 0.41mu is about 2.5% for the SD, and in excess of 8% for the MODIS system. In this paper, we present our degradation analysis results and discuss their impact on the RSB on-orbit calibration.