Four years of TRMM/VIRS on-orbit calibrations and characterization using lunar models and data from Terra/MODIS

Four years of on-orbit solar calibration data have been used to quantify the temporal degradation of the two reflected solar bands of the Visible and Infrared Scanner (VIRS) aboard the Tropical Rainfall Measuring Mission (TRMM) satellite. Solar calibrations, performed using an onboard solar diffuser, show no significant changes in the VIRS 1.61-mum data. The responsivity (digital counts per radiance unit) of the 0.62-mum band has changed and, as a function of days since 1998, is given by R-1 (day) 5 69.782 - 0.0022 x day, with an annual degradation of 1.15%. Four years of on-orbit lunar measurements were also examined in an attempt to quantify any degradation in the solar diffuser. The integrated lunar reflectance with phase angles ranging from 1.6degrees to 106degrees were fit with the lunar photometric model of Hapke. A good fit was made to the 0.62-mum data, but overall, the quality of the data did not support efforts to quantify any diffuser degradation. The quality of the VIRS radiometry was also examined by comparing data from the five VIRS spectral bands with similar Terra/Moderate Resolution Imaging Spectroradiometer (MODIS) bands while viewing near-coincident nadir scenes. The median reflectance differences, VIRS - MODIS, are -1.4% and -12.1% for the solar bands (VIRS 0.62 and 1.61 mum; MODIS 0.65 and 1.64 mum). The median brightness temperature differences for the thermal bands at 3.78, 10.83, and 12.03 mum, respectively, are -0.53, 0.04, and -0.76 K. These values compare well with the values from Minnis et al. The large difference in the 1.61-mum bands and day-night differences in the thermal emissive bands are discussed.