An overview of the visible and infrared scanner radiometric calibration algorithm

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