An Overview of the Visible and Infrared Scanner Radiometric Calibration Algorithm

Abstract

The Visible and Infrared Scanner (VIRS) is one of the principal instruments on board the Tropical Rainfall Measuring Mission (TRMM) satellite. VIRS measurements provide cloud-top temperatures and structures that complement the rainfall rates derived from other TRMM sensors. The VIRS radiometric calibration algorithm converts the digital data downlinked from the instrument into spectral radiances. VIRS has five bands: one in the visible, one in the shortwave infrared, and three in the thermal infrared. The calibration algorithm treats each band in the same manner except that the visible and shortwave infrared bands do not respond to the thermal radiation emitted by the instrument, and these bands do not have the nonlinear responses with input radiance found in the thermal bands. The calibration coefficients for the visible and shortwave infrared bands were determined in the laboratory before launch. VIRS carries a reference blackbody that is used to update the calibration coefficients for the thermal bands for each scan of the instrument on orbit. In addition, VIRS uses an onboard diffuser to view the Sun approximately once per month. The VIRS radiometric algorithm uses measurements of these reference sources to provide calibrated spectral radiances for each Earth pixel that it views.