RESUMO
A method of obtaining the spectral aerosol optical thickness that uses a multispectral instrument and a crude instrument with one spectral band only, here the blue at 450 nm, is presented. The instrument in the blue range is calibrated from a stratospheric balloon. The method is shown to be efficient by using experimental data collected in West Africa, where the daily variations of the aerosol optical thicknesses are large because of frequent dust storms and where the classic Langley plot method could not be used.
RESUMO
The effect of atmospheric scattering on ocean color measurements from space is considered. It is shown that modeling of the atmospheric effects can be improved by taking into account not only the direct but also the diffuse component of atmospheric transmittance and by a more precise formulation of the interaction between molecular and aerosol scattering in the calculation of atmospheric reflectance. This method, necessitating two near-infrared channels, should be used in future ocean color experiments to better correct for variable aerosol reflectance. The relative accuracy of the aerosol reflectance correction would then be to within 5%, as opposed to the more than 10% obtained with previous modelings.
RESUMO
To compare measurements performed in different geometrical conditions, one must take into account the angular anisotropy of the reflection properties of natural surfaces. As use of the exact boundary conditions in the radiative transfer codes seems prohibitive, a simple but accurate formulation of the problem has been sought. In this paper, two average angular reflectances are defined from which the reflected radiance may be deduced for any distribution of the downward radiance. Calculations made for different atmospheric models show that the solar directionality is partly preserved in the downward radiation field, so that the average reflectances can be written as a linear combination of actual reflectance and spherical albedo of the surface. Finally, the feasibility of detecting directional properties from space measurements is discussed.
RESUMO
The influence of Thebackground contamination on the apparent reflectance of a target as viewed from space has been studied as a function of the characteristics of atmospheric aerosols and the simple geometry of the target. For relatively common aerosol characteristics, the main features of the environment effect may be accounted for by simple correction terms, which depend only upon the optical thicknesses of aerosols and the molecular scattering of the atmosphere.
RESUMO
A fairly accurate analytical expression of the measured reflectance is established for the general case of a non-Lambertian and nonuniform ground by separating the atmospheric and surface effects. The signal is nearly linear in the function of intrinsic atmospheric reflectance, the actual target reflectance, and two average ground reflectances, angular and spatial, to be defined. Contrast reduction by the atmosphere, defined in the cases of Lambertian and directional ground reflectances, has been evaluated using this formulation.
