First results from the on-orbit calibrations of the visible and infrared scanner for the tropical rainfall measuring mission

This work presents the first on-orbit calibration results using the Moon, the Sun, and cold deep space as inputs to the Visible and Infrared Scanner (VIRS) on board the Tropical Rainfall Measuring Mission (TRMM) satellite. The authors have developed lunar reflectance curves using VIRS data for phase angles ranging from 1.6 degrees to 82 degrees. Comparisons with modeled reflectance curves show that the VIRS lunar data are as predicted. Specifically, the six-parameter model of Helfenstein and Veverka provides a good description of the VIRS 0.62-mu m data. The lunar reflectance data will be used to discern long-term changes in VIRS response. Solar calibrations, performed using an onboard solar diffuser, show fluctuations of the VIRS responsivity of less than 1.3% and no indication of any systematic change during 11 months. Using the deep-space calibration implemented via a spacecraft maneuver on three different dates-7 January, 8 January, and 2 September 1998-the authors have measured the dependence of the scan mirror reflectance with angle of incidence for the VIRS thermal bands. These results have replaced the scan modulation curves adopted from the prelaunch measurements. From the on-orbit calibration results, it appears that the TRMM/VIRS is functioning as predicted by the prelaunch calibration and characterization tests and that there has been no discernable change in sensor performance during the first 11 months of operation.