(thumb1.jpeg here: Helicopter picture) 

Helicopter remote sensing during the BOREAS experiment in Canada.
RS&ML

(thumb2.jpeg here: ASAS String)

Airborne bidirectional reflectance studies during the HAPEX-Sahel experiment in Niger, Africa.
RS&ML

Boreal Forest Reflectance Study Using a Helicopter Platform

(fig.1.jpeg here: BOREAS logo)

	The Boreal Ecosystem Atmosphere Study (BOREAS) [Link to http://boreas.gsfc.nasa.gov] was conducted between the dates of May 24 and September 19, 1994 in Canada near Prince Albert, Saskatchewan and Thompson, Manitoba. The study was conducted in an attempt to reach a greater understanding of the land/atmosphere interactions in the Northern Boreal forest regions. With this greater understanding, scientists will be able to better estimate and predict how global change will affect us in the future. NASA's Global Change Master Directory [Link to http://gcmd.gsfc.nasa.gov] has more information on global change research/issues.

(fig.3.jpeg here: Helicopter picture)

	During BOREAS, remote sensing measurements were made on the ground, from a helicopter and other aircraft, and from satellites. Some of the instruments used on the helicopter were a Barnes Modular Multiband Radiometer (MMR) and a Spectron Engineering SE-590 spectrometer.

	Because the remote sensing signal even at low altitudes is affected by the atmosphere, a sun photometer was also used to allow for atmospheric corrections. Sun photometers [Link to fig4.jpeg: Sunphotometer picture] measure the direct solar irradiance at the level they are operating at and their signal is used in converting at-sensor radiances to at-surface reflectances. 

(fig.5.jpeg here: Boreal forest picture)

	The Boreal Forest in September 1994.

(NIR_LAI.tiff here)

	Near Infrared reflectance as a function of Leaf Area Index (LAI) for three Boreal Forest tree species. The influence of change in LAI on near-infrared reflectance for these three tree species is markedly different. Notice that the three groups occupy mutually-exclusive areas in cartesian space.

For more information about this study go to http://boreas.gsfc.nasa.gov [Link] or contact Dr. C. L. Walthall, Remote Sensing and Modeling Laboratory (cwalthal@asrr.arsusda.gov).
Airborne Bidirectional Reflectance Study during HAPEX-Sahel

(fig.2.jpeg here: HAPEX logo)

	In 1992, over 170 scientists from many parts of the world collaborated on the Hydrologic and Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel) [Link to http://www.orstom.fr/hapex] 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) [Link to http://asas.gsfc.nasa.gov/asashome.html], 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.

(C130C.jpeg: Diskette #2)

	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.

(fig.6 here: Veg. Types)

	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.

	Below is a sequence of images collected as the plane flew over a tiger bush site. Differences in brightness in the images are due mainly to viewing different amounts of shadows as the view angle of ASAS changes.

(tiger.jpeg here: String of Tiger Bush)

	Below is a land cover classification of the nadir image of ASAS for the tiger bush subsite. 1) Bare Sandy Soil, 2) Laterite with Sand, 3) Laterite with Silt, 4) Bare Laterite, 5) Vegetation on Sandy Soil, 6) Tiger Bush Vegetation.

(class.jpeg here: classification of nadir scene)

	The radiances collected for each cover types are then corrected for atmospheric effects to obtain at-ground reflectance at each view angle. View some results here... [Link to t6_ref.jpef: Spectra for cover types]

Now view these results as a function of viewing angle for two ASAS spectral bands... [Link to view.jpeg: Ref. vs. view angle]

For more information on this study go to http://www.orstom.fr/hapex [Link] 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 [Link to http://www.geog.umd.edu] (ebrownde@geog.umd.edu).