Assessing Radiance Contributions Above Near-Space over the Ocean Using Radiative Transfer Simulation

Using the near-space platform to conduct radiometric calibrations of ocean color sensors is a promising method for refining calibration precision, but there is knowledge gap about the radiance contributions above near-space over the open ocean. We used the radiative transfer (RT) model (PCOART) to assess the contributions (LR) of the upwelling radiance received at the near-space balloons to the total radiance (Lt) measured at the top of the atmosphere (TOA). The results indicated that the LR displayed distinct geometric dependencies with exceeding 2% across most observation geometries. Moreover, the LR increased with wavelengths under the various solar zenith angles, and the LR values fell below 1% only for the two near-infrared bands. Additionally, the influences of variations in oceanic constituents on LR were negligible across various azimuth angles and spectral bands, except in nonalgal particle (NAP)-dominated waters. Furthermore, the influences of aerosol optical thicknesses (AOTs) and atmospheric vertical distributions on LR were examined. Outside glint-contaminated areas, the atmosphere-associated LR variations could exceed 2% but declined substantially as AOTs increased under most observation geometries. The mean height of the vertically inhomogeneous layer (hm) significantly influenced LR, and the differences in Lt could exceed 5% when comparing atmospheric vertical distributions following homogeneous versus Gaussian-like distributions. Finally, the transformability from near-space radiance to Lt was examined based on a multiple layer perceptron (MLP) model, which exhibited high agreement with the RT simulations. The MAPD averaged 0.420% across the eight bands, ranging from 0.218% to 0.497%. Overall, the radiometric calibration utilizing near-space represents a significant innovation method for satellite-borne ocean color sensors.