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, t he Sun, and cold deep space as inputs to the Visible and Infrared Scanner ( VIRS) on board the Tropical Rainfall Measuring Mission (TRMM) satellite. Th e authors have developed lunar reflectance curves using VIRS data for phase angles ranging from 1.6 degrees to 82 degrees. Comparisons with modeled re flectance curves show that the VIRS lunar data are as predicted. Specifical ly, the six-parameter model of Helfenstein and Veverka provides a good desc ription 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, perform ed using an onboard solar diffuser, show fluctuations of the VIRS responsiv ity of less than 1.3% and no indication of any systematic change during 11 months. Using the deep-space calibration implemented via a spacecraft maneu ver on three different dates-7 January, 8 January, and 2 September 1998-the authors have measured the dependence of the scan mirror reflectance with a ngle of incidence for the VIRS thermal bands. These results have replaced t he 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 t hat there has been no discernable change in sensor performance during the f irst 11 months of operation.