Angular signatures of surface reflectances from airborne POLDER data

Thanks to its optical design, the airborne POLDER instrument allows a multidirectional measurement of the surface reflectance. During the “La Crau 91” experiment, the radiometer has been flying over a study area of 10 km x 10 km composed of various cultivated fields. The measurements have been registered on a regular grid of resolution 100 m. For each grid point, up to 50 directional measurements provide an adequate description of the surface reflectance directional signature. These signatures are discussed for a few selected targets representative of the area. The measurements show typical features of the reflectance angular signature, including the “hot spot” and specular reflection. The quality of the measurements is shown to be satisfactory. A simple directional model is then applied to the measurements. It provides, for each of the l04 grid points, a normalized reflectance corrected for the angular effects, together with two parameters that give a quantitative assessment of the directional signature. The article discusses these parameters and the ability of the model to represent the measurements. Some spatial structure of directional signature is apparent in the study area and is consistent with a land use map of this area. The squared correlation coefficient R2 of the fit between model and observations is larger than 0.8 for 50% of the area and smaller than 0.5 for about 20% of it at 550 nm and 850 nm. The part of the area for which the model is not satisfactory corresponds primarily to pixels with a larger specular reflection signal or with few directional measurements. Angular signatures of reflectances are measured here at the regional scale, a key step for the preparation of the operational processing of spaceborne POLDER measurements at the global scale.