Lidar Satellite Will Analyze Forests
Dr. James P. Smith
GAITHERSBURG, Md. -- In September 2000, the Vegetation Canopy Lidar satellite is scheduled for liftoff on an Athena I rocket -- the first scheduled orbital launch from the Kodiak Launch Complex in Alaska. This satellite will provide direct measurements of forest canopy height, canopy vertical and spatial structure, as well as subcanopy topography, all of which are critical for land-surface and climate modeling.
Bryan Blair, one of the designers of the satellite's instrument package, said that the aim of the 18- to 24-month mission is to build an image of the Earth's ground surface at a 2-km resolution. Although the mission will actually illuminate less than 2 to 3 percent of the land, his team will put a sufficient number of laser samples in each 2-km2 cell of the Earth's surface to characterize the vegetation; i.e., the carbon content, the biomass and the aerodynamic roughness, of more than 99 percent of the Earth's forests.
Vegetation Canopy Lidar makes simultaneous measurements of range to the surface by synchronous triggering of the five laser pulse transmitters and by detection with a single telescope equipped with multiple silicon avalanche photodiodes. Individual laser footprints are 25 m in diameter and are contiguous along track. Surface echoes from the five beams are digitized at 250 megasamples per second to achieve the required submeter vertical resolution in the vegetation canopy.
Vegetation Canopy Lidar was selected in 1997 as one of the first missions in NASA's Earth System Science Pathfinder program, which funds unique science-based missions of high value. "The whole mission, including everything from development, launch, obtaining and processing the data, and distribution and archiving the information, is only $60 million," said Blair. "This was such a quick turnaround mission that it has allowed us to use the latest technology in terms of lasers, detectors and telescopes."
The laser transmitter system incorporates five Nd:YAG lasers, each operating at 15 mJ with 10-ns pulse widths. They will sample five 25-m-diameter spots around the perimeter of an 8-km-diameter target area. The telescope receiving the reflections is segmented into five areas; each observes the reflection of only one laser, with no crosstalk. Five silicon avalanche photodiodes with a bandwidth of 70 to 80 MHz detect the reflected pulses.
From orbit, the laser beams will illuminate five 25-m-wide stripes that are 2 km apart on the ground as the satellite passes over the equator. A stripe will consist of a series of 25-m-diameter spot measurements, 30 m apart. As the satellite moves over 1 km of the Earth's surface, approximately 167 individual lidar measurements will be made in an 8-km2 area. At higher latitudes, the stripes will be a bit closer together. Except during laser-pointing-bias calibration procedures, the Vegetation Canopy Lidar will power down over the oceans.
The mission's principal director is Ralph Dubayah of the University of Maryland. The instrumentation and information technology have been cultivated at NASA's Goddard Space Flight Center, where the instrument design was led by Jack Bufton of the Laboratory for Terrestrial Physics.
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