INSAT-3D and -3DR relative radiometric performance over bright target

The Geostationary Earth Orbit (GEO) satellite INSAT-3D and -3DR derived data products plays a vital role in helping to develop long-term trends of many climate variables and to improve our understanding of earth's environmental processes and dynamics occurring on the land, in the oceans and in the atmosphere. In order to provide high quality, consistent and well calibrated measurements, the radiometric calibration of the sensor must be consistent, stable and traceable to its calibration standards. Due to absence of an onboard calibration mechanism for the visible (VIS) and Short Wave Infra-Red (SWIR) bands, accuracy of Top-of-atmospheric radiance of these bands data depend on vicarious calibration methods to provide post launch calibration coefficients to compensate for the degradation responsivity. In this study, a methodology has been implemented to perform degradation assessment and seasonal variation pattern for the visible and SWIR band of INSAT-3D and -3DR satellite based on clear-sky observations. The current methodology relies on clear sky apparent reflectance which are derived from level-1 images over pseudo-invariant desert target. In this study, authors are focused to investigate visible and SWIR band performance based on clear-sky observations over desert target and the instrument longterm drift traced by a polynomial fitting function. A trending algorithm is proposed here, which consists of a polynomial function (for the description of instrument degradation performance) with two sine terms for quashing the impact of the seasonal variations of the solar zenith angle and atmospheric constituents, is applied to fit the time series of apparent reflectance.