MODIS solar diffuser stability monitor: function and applications

Abstract

MODIS reflective solar bands (RSB) are calibrated on-orbit using a solar diffuser (SD) with its degradation tracked by an on-board solar diffuser stability monitor (SDSM). The SDSM has 9 detectors with wavelengths from 0.41 to 0.94μm. It is operated during each scheduled SD calibration event, making alternate observations of the Sun and the SD. Due to a design defect in the SDSM there are significant ripples in the Sun view responses. Because of this, an alternative approach was developed by normalizing SDSM detector responses to detector 9 at 0.94μm. It has been very effective since the responses of all SDSM detectors have the same ripples and SD degradation at detector 9 wavelength is extremely small. After many years of on-orbit operations, the accumulated degradation at detector 9 can have some impact on the RSB calibration quality. As a result, a new approach is developed and implemented to characterize the SD degradation directly. This approach reduces the ripples via a look-up table (LUT) constructed with data carefully selected from the existing SDSM observations made in a short period. In this paper, we provide an overview of different approaches applied over the years by the MODIS Characterization Support Team (MCST) to track the on-board SD degradation. Results of both Terra and Aqua MODIS SD degradation derived from different approaches are presented and evaluated. Lessons learned from MODIS SD on-orbit operation and recent improvements for the RSB calibration are also discussed.