Tracking VIRS/TRMM on-orbit calibration with MODIS

The Visible and Infrared Scanner (VIRS) aboard the Tropical Rainfall Measuring Mission (TRMM), launched on 28 November 1997, has two reflected solar bands and three thermal infrared bands. The solar bands are calibrated using an onboard solar diffuser (SD) and the thermal bands are calibrated using an onboard blackbody (BB). Since launch, VIRS has provided more than eight years of on-orbit observations. The five VIRS bands have a close spectral match with corresponding Moderate Resolution Imaging Spectroradiometer (MODIS) bands. MODIS was launched on 18 December 1999 and 4 May 2002 aboard the NASA EOS Terra and Aqua spacecrafts, respectively. In this study, six years of VIRS and MODIS overlapping data are used to examine VIRS long-term calibration stability and consistency. This is particularly useful for the VIRS solar band calibration due to a lack of capability to track the on-orbit SD degradation. To reduce impacts due to scene variations, measurements from simultaneous nadir overpasses (SNOs) for VIRS and MODIS are co-located and aggregated to 30-by-30km areas for inter-comparison. Results show that the VIRS thermal bands maintain a stable calibration. For the two VIRS solar bands at 0.62 mu m and 1.62 mu m, the calibrated reflectance values gradually drift higher over the six-year period. The 0.62 mu m band increases at a rate of 1.1%/yr over the period, compared to an increase of 0.4%/yr for the 1.62 mu m band.