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OCEAN COLOR REMOTE SENSING
USING POLARIZATION PROPERTIES OF REFLECTED SUNLIGHT
R. FROUIN 1 , E. POULIQUEN 1 , F.-M. BREON 2
U Scripps Institution of Oceanography,
La Jolla, California 92093-0221, USA
2 : Laboratoire de Modélisation du Climat et de l’Environnement
CEA/DSM, 91191 Gif sur Yvette, France
ABSTRACT:
The effects of the atmosphere and surface on sunlight backscatterred to space by the ocean may be
substantially reduced by using the unpolarized component of reflectance instead of total
reflectance. At 450 nm, a wavelength of interest in ocean color remote sensing, and for typical
conditions, 45% of the unpolarized reflectance may originate from the water body instead of 20%
of the total reflectance, which represents a gain of a factor 2.2 in useful signal for water
composition retrieval. The best viewing geometries are adjacent to the glitter region; they
correspond to scattering angles around 100°, but they may change slightly depending on the
polarization characteristics of the aerosols. As aerosol optical thickness increases, the atmosphere
becomes less efficient at polarizing sunlight, and the enhancement of the water body contribution
to unpolarized reflectance is reduced. Since the perturbing effects are smaller on unpolarized
reflectance, at least for some viewing geometries, they may be more easily corrected, leading to a
more accurate water-leaving signal and, therefore, more accurate estimates of phytoplankton
pigment concentration.
KEY WORDS: Polarization, Ocean Color, Atmospheric Effects, Aerosols
1- INTRODUCTION
A major problem in ocean color remote sensing from space is the correction of top-of-atmosphere
reflectance, the signal measured, for atmospheric and surface interference. At the wavelengths of
interest, located in the blue-green region of the solar spectrum, as much as 90% of the photons
reflected and/or backscattered to space may not have interacted with the water body and, thus, do
not contain information on water composition. Since the perturbing effects are large, extraction of
the useful signal requires accurate computation of those effects, which must be known to within a
few percent to yield reasonably accurate estimates of phytoplankton pigment concentration.
Uncertainties in radiometric calibration, aerosol characteristics, and sunglint and foam effects
make the task arduous and, in some cases, impossible.
To reduce the influence of the atmosphere and surface on the top-of-atmosphere signal, one
may exploit the polarization properties of reflected sunlight. Because molecules, aerosols, and
hydrosols polarize sunlight differentially, there may be viewing geometries for which the
contribution of the water body to the polarized or unpolarized components of total reflectance may
be enhanced. Exploiting those properties would obviously require measurements of total
reflectance and polarization rate. These measurements will be made by (or may be envisioned on)
future sensors, such as the Polarization and Directionality of the Earth Reflectance (POLDER)
imager (Deschamps et al., 1994). If the enhancement is substantial, correction of the perturbing
effects becomes easier since accuracy constraints may not be as severe, leading to a more accurate
water-leaving signal (the signal of interest).
Since the signal backscattered by the water body is polarized weakly after transmission accross
the air-sea interface and through the atmosphere (see section 3), we focus our study on unpolarized
reflectance. Using a Monte Carlo code we simulate the relative contribution of the water body to
top-of-atmosphere unpolarized reflectance for varied conditions. We compare this contribution
with that of the water body to total reflectance, identifying the viewing geometries that minimize
atmospheric and surface effects. We quantify, for those geometries, the increase in the ratio of
useful to total signal when using unpolarized reflectance instead of total reflectance.
