Remote sensing applications using satellite sensors rely heavily on the quantitative estimation of geo-physical parameters, which are contingent upon the radiometric performance of the in-orbit sensor. There are multiple measures and mechanisms that assess the in-orbit radiometric performance of the sensor and eventually lead to the correction of the estimated deviation in calibration. The correction in calibration is performed through the fine tuning or recalculation of the parameters used in calibration. The estimation of deviations in calibration and accurate calibration parameters requires the radiance values of a reference target, which is supposed to be spatially and temporally invariant and stable, along with a physically consistent standardized approach for the comparison of reference and target radiances. To monitor the calibration accuracies, the moon, terrestrial pseudo invariant targets, and deep convective clouds (DCCs) are widely used as reference targets. Cyclonic systems are largely an ensemble of DCC and a large pool of reference observations that cover the variability of DCC radiances can be easily obtained. This study proposes the use of cyclones as the aggregator of DCC observations to create a pool of reference DCC observations that could be used for calibration monitoring and for the accurate estimation of calibration parameters for the visible channels of the INSAT-3D and INSAT-3DR imagers. To carry out this study, all major cyclonic events from 2021 to 2022 over Indian regions were used to assess the quantification and radiometric stability of the INSAT-3D and INSAT-3DR imager visible channels. To compute the gain (“slope” in the linear calibration equation) of the visible channel of the INSAT-3D/3DR imager, the theoretical computation of the DCC of cyclonic system radiances at the top of the atmosphere was performed using a radiative transfer simulation. The Santa Barbara DISORT Atmospheric Radiative Transfer model was used for the simulation exercise. An analysis of DCC observations aggregated through different cyclones and radiative transfer simulations showed that the gain for the INSAT-3D and INSAT-3DR imager visible channels varied from 1.45 to 1.48 and from 1.36 to 1.39, respectively, for all cyclones in the Indian region during 2021–2022.
Hussain et al. (Mon,) studied this question.