Improvement of MODIS RSB Calibration by Minimizing the Earthshine Impact on Solar Diffuser Observations

摘要

The MODerate Resolution Imaging Spectroraiometer (MODIS) reflective solar bands (RSB) are calibrated on-orbit using solar illuminations reflected from its onboard solar diffuser (SD) plate. The specified calibration uncertainty requirements for MODIS RSB are ±2% in reflectance and ±5% in radiance at their typical top of atmosphere (TOA) radiances. The onboard SD bi-directional reflectance factor (BRF) was characterized pre-launch by the instrument vendor using reference samples traceable to NIST reflectance standard. The SD on-orbit degradation is monitored using a solar diffuser stability monitor (SDSM). One of contributors to the RSB calibration uncertainty is the earthshine (ES) illumination on the SD plate during SD calibration. This effect was estimated pre-launch by the instrument vendor to be of 0.5% for all RSB bands. Analyses of on-orbit observations show that some of the SD calibration data sets have indeed been contaminated due to extra ES illumination and the degree of ES impact on the SD calibration is spectrally dependent and varies with geo-location and atmospheric conditions (ground surface type and cloudiness). This paper illustrates the observed ES impacts on the MODIS RSB calibration quality and compare them with the effects derived from an ES model based on the viewing geometry of MODIS SD aperture door and likelihood atmospheric conditions. It also describes an approach developed to minimize the ES impact on MODIS RSB calibration.