The Swiss field-goniometer system (FIGOS)

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Abstract
The reflectance characteristics of most natural objects vary with illumination and viewing geometry, i.e. expose a non-Lambertian behaviour. New sensor systems are capable of viewing targets quasi-simultaneously from nadir and different off-nadir positions. In regard to radiometric corrections the Lambertian assumption has to be overcome by detailed knowledge of the bidirectional reflectance distribution function (BRDF) of targets at the Earth's surface. In order to obtain bidirectional reflectance factor (BRF) data of naturally illuminated targets a transportable field-goniometer system (FIGOS) has been developed. It is operated together with a GER-3700 spectroradiometer. The goniometer consists of an azimuth full-circle and a zenith semi-arc of 2 m radius each. It enables one to observe a target from any desired viewing direction. First measurements are taken from a plane meadow under different solar zenith angles over the hemisphere in a resolution of 15/spl deg/ and 30/spl deg/ in zenith and azimuth direction, respectively. A Spectralon panel is measured at the beginning and end of a hemispherical data set. As the position of the Sun and the atmospheric conditions cannot be assumed constant over the measurement period of about 18 minutes, the global solar irradiance is monitored simultaneously to the BRF-data acquisition. The obtained results clearly show the non-Lambertian reflectance characteristics of the meadow.

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