Reduction of skylight reflection effects in the above-water measurement of diffuse marine reflectance.

Reflected skylight in above-water measurements of diffuse marine reflectance can be reduced substantially by viewing the surface through an analyzer transmitting the vertically polarized component of incident radiance. For maximum reduction of effects, radiometric measurements should be made at a viewing zenith angle of approximately 45 degrees (near the Brewster angle) and a relative azimuth angle between solar and viewing directions greater than 90 degrees (backscattering), preferably 135 degrees. In this case the residual reflected skylight in the polarized signal exhibits minimum sensitivity to the sea state and can be corrected to within a few 10(-4) in reflectance units. For most oceanic waters the resulting relative error on the diffuse marine reflectance in the blue and green is less than 1%. Since the water body polarizes incident skylight, the measured polarized reflectance differs from the total reflectance. The difference, however, is small for the considered geometry. Measurements made at the Scripps Institution of Oceanography pier in La Jolla, Calif., with a specifically designed scanning polarization radiometer, confirm the theoretical findings and demonstrate the usefulness of polarization radiometry for measuring diffuse marine reflectance.

Modeling of the atmospheric effects and its application to the remote sensing of ocean color.

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.

Influence of the atmosphere on space measurements of directional properties.

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.

Influence of the background contribution upon space measurements of ground reflectance.

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.

Atmospheric modeling for space measurements of ground reflectances, including bidirectional properties.

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.

Améliorations apportées à la mesure de la température de surface de l'océan par l'utilisation d'un radiomètre infrarouge polarisant.

When the water surface temperature is measured with an infrared radiometer (8-13 microm), there is an error due to reflection on the water surface which is about 0.5 degrees C and depends on sky radiance. This reflection error can be divided by more than four if the infrared radiometer is fitted with a polarizer and sights the water surface at Brewsterian incidence. Two successive measurements give an estimate of this error, the first one with the polarizer axis parallel to the incidence plane and the other one with this axis perpendicular to it. Experiments have been performed on a laboratory water tank to check that point. The results are little modified by taking into account the surface roughness induced by wind.