Abstract
The impact of target bidirectional reflectance in dual field of view spectroscopy was described and quantified using field measurements and ray-tracing simulations. A data-driven normalization method was developed to convert reflectance factors under cloud obscured conditions into clear sky reflectance by decomposing the target bidirectional reflectance into an isotropic target-specific component and a group-specific bidirectional component. An evaluation on tree, grass and gravel targets suggests a reduction in relative reflectance error obtained by normalization from 15% to less than 5% between 400 and 1800 nm. At higher wavelengths a decreased signal-to-noise ratio increases the errors.
Highlights
During the last decades, field spectroscopy has become an established technique for characterizing reflectance of surfaces in situ, for supporting calibration of airborne and satellite sensors and for providing a means of up-scaling data from small surfaces such as leaves, branches and Remote Sens. 2009, 1 stones to composite scenes such as canopies and to pixels [1]
Since single field of view reflectance spectroscopy is based on taking the ratio of sequentially measured target radiance to whitepanel radiance, stable illumination conditions are imperative, which can only be guaranteed under cloudless conditions [2, 3]
Ray-tracing simulations combined with atmospheric radiative transfer models are used to reproduce bidirectional reflectance distribution function (BRDF) effects of different targets for varying illumination conditions
Summary
Field spectroscopy has become an established technique for characterizing reflectance of surfaces in situ, for supporting calibration of airborne and satellite sensors (vicarious calibration) and for providing a means of up-scaling data from small surfaces such as leaves, branches and Remote Sens. 2009, 1 stones to composite scenes such as canopies and to pixels [1]. The comparably small footprint of spectroradiometers combined with their high spectral resolution enables a precise characterization of an object’s reflectance, without signal deterioration due to atmospheric interactions of the reflected radiance This higher detail comes at the expense of significantly larger efforts required to establish field datasets. In single field of view spectroscopy, often (linear) interpolations are made of the target radiance before and after a set of target scans to account for smoothly varying irradiance such as that due to varying solar zenith angle at clear sky conditions [5] This interpolation technique, can hardly be applied to account for the typically rapid, unpredictable and nonlinear changes in irradiance caused by cloud obscuration. The significant additional expense of a second spectroradiometer for DFOV measurements has lead to the development of techniques for estimating the irradiance spectrum by only sampling a limited number of fixed wavelength bands using a much simpler filter-based radiometer [5, 7]
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