Ten Years of TRMM/VIRS On-Orbit Calibrations and Multiyear Comparisons of VIRS and MODIS

After 10 years of successful operation of the Tropical Rainfall Measuring Mission ( TRMM)/Visible Infrared Scanner ( VIRS), based on sensor performance, the authors have reexamined the calibration algorithms and identified several ways to improve the current VIRS level-1B radiometric calibration software. This study examines the trends in VIRS on-orbit calibration results by using lunar measurements to enable separation of the solar diffuser degradation from that of the VIRS Earth-viewing sensor and by comparing the radiometric data with two nearly identical Moderate Resolution Imaging Spectroradiometer ( MODIS) instruments on board the NASA Earth Observing System ( EOS) Terra and Aqua satellites. For the VIRS, with spectral bands quite similar to several of the MODIS bands, the integrated lunar reflectance data were measured, from January 1998 to March 2007, at phase angles ranging from 0.94 degrees to 121.8 degrees. The authors present trending of the lunar data over periods of 4 yr ( Aqua/MODIS), 6 yr ( Terra/MODIS), and 10 yr ( TRMM/VIRS) and use these observations to examine instrument radiometric stability. The VIRS-measured lunar irradiances are compared with the MODIS-measured lunar irradiances at phase angles around 54 degrees-56 degrees. With the upcoming modified VIRS level-1B version 7 calibration algorithm, the VIRS, along with MODIS, should provide better references for intercalibrating multiple Earth-observing sensors.