Differences in grass-shrub transition zone canopy composition from CHRIS/proba multi-angle data

The surface spectral-directional reflectance of a grass-shrub transition zone at 631 nm was estimated from spectral radiance images in a narrow (∼10 nm wide) band recorded by the Compact High Resolution Imaging Spectrometer (CHRIS) flown on the European Space Agency's Proba micro-satellite. An experimental satellite-borne sensor developed by Sira Electro-Optics Ltd. (UK), CHRIS is one of the few sources of multi-angular reflectance data on kilometer scales, providing up to five looks in different directions at a given target within the space of a few minutes. For a selected December 23, 2003 overpass, orbital ephemeris were used to obtain the viewing geometry for the set of four available images. The images were resampled to a consistent 25 m grid and used to invert a simple geometric bidirectional reflectance distribution (BRDF) model for plant number density, plant width, and plant height. The retrieved parameters were compared to subsets of high resolution aerial photographs. The inversions did not always result in the expected parameter behavior and further work is required to determine the underlying reasons, which may include decoupling model parameter response to brightness and anisotropy.