Analysis of the POLDER (POLarization and directionality of earth's reflectances) airborne instrument observations over land surfaces

Abstract The POLDER (POLarization and Directionality of the Earth's Reflectances) instrument provides polarized reflectance measurements that can be used to distinguish atmospheric and surface contributions to reflectance. Polarized reflectance measurements can then be used for an accurate aerosol estimation over land. The POLDER instrument was flown for the first time on 17 June 1990 over the “La Crau” site, in southern France and the results are present in this article. The POLDER instrument is scheduled for launch in 1995 on the Japanese ADEOS (ADvanced Earth Observing System) platform. Surface based atmospheric optic measurements (spectral optical thickness, sky radiance) are used to estimate the aerosol refractive index and size distribution. The corresponding aerosol model is then used in a radiative transfer model to simulate POLDER polarized measurements. The correlation between the observations and the simulations is good for the 550 nm and 650 nm wavelengths, but the simulation is biased for the 850 nm wavelength. These results indicate that, compared with the atmospheric contribution to the polarized reflectance, the surface contribution can be neglected at shorter wavelengths, but not so in near infrared wavelengths. The POLDER instrument allows multidirectional reflectance measurements. A surface target bidirectional reflectance, therefore, can be sampled at various viewing angles. In this article, we investigate the angular variations of the surface reflectance of various surface covers. The main observed variations are: i) a limb brightening; ii) a larger reflectance in the backscattering direction; iii) a local maximum in the forward direction for the shorter wavelengths, indicating specular reflection by the leaves. A very simple empirical model is proposed to quantify the main reflectance angular variations.

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