Effects of Sun and view geometries on cotton bidirectional reflectance. Test of a geometrical model

During the MAC-VI campaign, bidirectional reflectance measurements were taken on a cotton crop in order to analyze the effects of sun and view geometries in the wavebands and in the viewing vertical plane of the SPOT High Resolution Visible (HRV) instrument. The bidirectional reflectance was measured using a CIMEL SPOT simulation radiometer fixed on a goniometric apparatus 2 m above the top of the crop. Data are presented in terms of bidirectional reflectance factors relative to nadir reflectance for 15 view angles (− 70° to + 70°) and for 12 solar zenith (73-26°) angles. Results show major variations according to Sun position and backscattering or forward scattering observations. The relative reflectance factor of the cotton varies from 0.7 to 1.9 throughout one day. Measurements are compared to the output of a geometrical model where cotton rows are simulated by spheroidal cylinders sitting on an horizontal surface. The relative reflectance factor is calculated according to the global structure of the rows which is divided into illuminated and shaded facets viewed by the radiometer for each view and solar angle. The analysis of comparative results shows a good agreement between predicted and measured data in the three SPOT HRV wavebands.