Abstract

There is interest in the prediction of the top-of-the-atmosphere (TOA) reflectance of the ocean–atmosphere system for in-orbit calibration of ocean color sensors. With the use of simulations, we examine the accuracy one could expect in estimating the reflectance ρ_T of the ocean–atmosphere system based on a measurement suite carried out at the sea surface, i.e., a measurement of the normalized sky radiance ρ_B and the aerosol optical thickness (τ_a), under ideal conditions—a cloud-free, horizontally homogeneous atmosphere. Briefly, ρ_B and τ_a are inserted into a multiple-scattering inversion algorithm to retrieve the aerosol optical properties—the single-scattering albedo and the scattering phase function. These retrieved quantities are then inserted into the radiative transfer equation to predict ρ_T. Most of the simulations were carried out in the near infrared (865 nm), where a larger fraction of ρ_T is contributed by aerosol scattering compared with molecular scattering, than in the visible, and where the water-leaving radiance can be neglected. The simulations suggest that ρ_T can be predicted with an uncertainty typically ≲1% when the ρ_B and τ_a measurements are error free. We investigated the influence of the simplifying assumptions that were made in the inversion-prediction process, such as modeling the atmosphere as a plane-parallel medium, using a smooth sea surface in the inversion algorithm, using the scalar radiative transfer theory, and assuming that the aerosol was confined to a thin layer just above the sea surface. In most cases, these assumptions did not increase the error beyond ±1%. An exception was the use of the scalar radiative transfer theory, for which the error grew to as much as ~2.5%, suggesting that the use of ρ_B inversion and ρ_T prediction codes that include polarization would be more appropriate. However, their use would necessitate measurement of the polarization associated with ρ_B. We also investigated the uncertainty introduced by an unknown aerosol vertical structure and found it to be negligible if the aerosols were nonabsorbing or weakly absorbing. An extension of the analysis to the blue, which requires measurement of the water-leaving radiance, showed significantly better predictions of ρ_T because the major portion of ρ_T is the result of molecular scattering, which is known precisely. We also simulated the influence of calibration errors in both the Sun photometer and the ρ_B radiometer. The results suggest that the relative error in the predicted ρ_T is similar in magnitude to that in ρ_B (actually it was somewhat less). However, the relative error in ρ_T induced by error in τ_a is usually much less than the relative error in τ_a. Currently, it appears that radiometers can be calibrated with an uncertainty of ~±2.5%, therefore it is reasonable to conclude that, at present, the most important error source in the prediction of ρ_T from ρ_B is likely to be error in the ρ_B measurement.

© 1996 Optical Society of America

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