Airborne Bidirectional Reflectance Study during HAPEX-Sahel

Airborne Bidirectional Reflectance Study
during HAPEX-Sahel
Hydrology and Remote Sensing Lab

	In 1992, over 170 scientists from many parts of the world collaborated on the Hydrologic and Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel) near Niamey, Niger. The objectives of the study were to improve the predictions and estimation of the impacts of climate change in regions of high interannual variability in rainfall such as the Sahel region of Africa. As a part of HAPEX-Sahel, an airborne imaging sensor, the Advanced Solid-state Array Spectroradiometer (ASAS), was used to collect digital image data from a variety of viewing angles and to estimate hemispherical reflectance from the bidirectional data. By viewing the surface from ten different viewing angles the ASAS data provide information on surface structure that is not available from traditional nadir viewing instruments.
	The NASA C-130 was one of three remote sensing aircraft used during HAPEX-Sahel and served as the platform for the ASAS instrument. A sequence of view angles, both off-nadir and nadir, were collected as the aircraft followed a flight line over a specific study site.
	The principal land cover types of the region were mostly millet fields in various stages of growth, fallow grasses and bushland, tiger bush vegetation, and bare soils. Tiger Bush are dense bushes typically found on plateaus and receive their name from their characteristic banding when seen from the air.

For more information on this study go to http://www.orstom.fr/hapex or
contact Dr. C. L. Walthall, Remote Sensing and Modeling Laboratory (cwalthal@asrr.arsusda.gov) or
Eric Brown de Colstoun, Department of Geography, University of Maryland(ebrownde@geog.umd.edu).