Vegetation Canopy  Lidar (VCL)
-Instrument Description-
 

VCL is a laser altimeter system built around the Multi-Beam Laser Altimeter (MBLA), a three-beam instrument with 25 m contiguous along track resolution. The three beams are in a circular configuration 8 km across and each beam traces a separate ground track spaced 4 km apart, eventually producing 4 km-grid coverage between 67° N and S, with orbit crossovers producing a denser grid away from the equator.
 

Each laser beam operates at the 1064 nm fundamental wavelength of the neodymium-doped yttrium aluminum garnet (Nd:YAG) solid-state laser and are arranged in a pentagon inside a 20 mrad telescope circular field-of-view that is centered on nadir as illustrated below.




VCL mission concept.

The optical telescope is 0.9 m in diameter composed of beryllium. For the VCL orbital altitude of 400 km the across-track separation between adjacent tracks is 4 km. VCL makes simultaneous measurements of range to the surface by synchronous triggering of the 3 laser pulse transmitters and detection with a single telescope that is staring at nadir and is equipped with multiple silicon avalanche photo diode detectors in its focal plane. Individual laser footprints are 25 m in diameter and are contiguous along-track, commensurate with the best DTED Level 2 topography mapping resolution and LANDSAT Thematic Mapper pixel resolution. Surface echoes from the 3 beams are digitized in the MBLA electronics at 250 Megasamples per sec to achieve the required sub-meter vertical resolution in the vegetation canopy and permit pulse centroid correction of the range measurement.

The MBLA pulsed laser transmitter modules are based on high power Nd:YAG and employ the Q-switching technique to concentrate laser energy in a short pulse. Each of these laser modules produces a laser pulse of 5 nsec duration at the rate of 290 pps. Laser pulse energy of 10 mJ per pulse will be sufficient to establish a link performance for the MBLA instrument that results in 95% probability of detection of the Earth's surface under clear atmospheric conditions and permits surface lidar investigations.


  For more information see the VCL instrument page at GSFC.