Influence of data acquisition geometry on soybean spectral response simulated by the prosail model Influência da geometria de aquisi o de dados na resposta espectral da soja simulada pelo modelo prosail

View angle and directional effects significantly affect reflectance and vegetation indices, especially when daily images collected by large field-of-view (FOV) sensors like the Moderate Resolution Imaging Spectroradiometer (MODIS) are used. In this study, the PROSAIL radiative transfer model was chosen to evaluate the impact of the geometry of data acquisition on soybean reflectance and two vegetation indices (Normalized Difference Vegetation Index - NDVI and Enhanced Vegetation Index -EVI) by varying biochemical and biophysical parameters of the crop. Input values for PROSAIL simulation were based on the literature and were adjusted by the comparison between simulated and real satellite soybean spectra acquired by the MODIS/Terra and hyperspectral Hyperion/Earth Observing-One (EO-1). Results showed that the influence of the view angle and view direction on reflectance was stronger with decreasing leaf area index (LAI) and chlorophyll concentration. Because of the greater dependence on the near-infrared reflectance, the EVI was much more sensitive to viewing geometry than NDVI presenting larger values in the backscattering direction. The contrary was observed for NDVI in the forward scattering direction. In relation to the LAI, NDVI was much more isotropic for closed soybean canopies than for incomplete canopies and a contrary behavior was verified for EVI. Os efeitos direcionais e de angulo de visada afetam significativamente a reflectancia e os índices de vegeta o, especialmente quando s o usadas imagens diárias adquiridas por sensores com amplo campo de visada como o Moderate Resolution Imaging Spectroradiometer (MODIS). No presente estudo, o modelo de transferência radiativa PROSAIL foi escolhido para avaliar o impacto da geometria de aquisi o de dados na reflectancia da soja e no cálculo de dois índices de vegeta o (Normalized Difference Vegetation Index -NDVI e Enhanced Vegetation Index -EVI), variando-se parametros bioquímicos e biofísicos do dossel. Os valores usados para a simula o PROSAIL foram definidos com base na literatura e foram ajustados pela compara o entre espectros de soja simulados e adquiridos pelos sensores MODIS/Terra e hiperespectral Hyperion/Earth Observing-One (EO-1). Os resultados mostraram que os efeitos direcionais e de angulo de visada na reflectancia foram mais fortes com a diminui o do índice de área foliar (IAF) e da concentra o de clorofila. Por ser mais dependente da banda do infravermelho próximo (IVP), o EVI foi muito mais sensível à geometria de visada do que o NDVI, apresentando maiores valores na dire o d