|
MODIS has 20 reflective solar bands (RSB) that are calibrated on-orbit using a solar diffuser (SD) and a solar diffuser
stability monitor (SDSM). The MODIS SD bi-directional reflectance factor (BRF) was characterized pre-launch. Its on-orbit
degradation is regularly monitored by the SDSM at wavelengths ranging from 0.41 to 0.94μm. During each
SD/SDSM calibration event, the SDSM views alternately the sunlight directly through a fixed attenuation screen and the
sunlight diffusely reflected from the SD panel. The time series of SDSM measurements (ratios of the SD view response
to the Sun view response) is used to determine the SD BRF degradation at SDSM wavelengths. Since launch Terra
MODIS has operated for more than seven years and Aqua for over five years. The SD panel on each MODIS instrument
has experienced noticeable degradation with the largest changes observed in the VIS spectral region. This paper provides
a brief description of MODIS RSB calibration methodology and SD/SDSM operational activities, and illustrates the SD
on-orbit degradation results for both Terra and Aqua MODIS. It also discusses the impact on the SD degradation due to
sensor operational activities and SD solar exposure time. Aqua MODIS has been operated under nearly the same
condition for more than five years. Its SD annual degradation rate is estimated to be 2.7% at 0.41μm, 1.7% at 0.47μm,
and less than 1.0% at wavelengths above 0.53μm. Terra MODIS, on the other hand, has experienced two different SD
solar exposure conditions due to an SD door (SDD) operation related anomaly that occurred in May 2003 that had led to
a decision to keep the SDD permanently at its "open" position. Prior to this event, Terra MODIS SD degradation rates
were very similar to Aqua MODIS. Since then its SD has experienced much faster degradation rates due to more
frequent solar exposure.
|
