Photon Counting Airborne Laser Swath Mapping (PC-ALSM)
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Commercially marketed airborne laser swath mapping (ALSM) instruments currently use lasers with sufficient energy per pulse, in combination with optics of sufficient aperture, to work with return signals of thousands of photons per shot. The resulting high signal to noise level virtually eliminates spurious range values caused by noise, such as background solar radiation and sensor thermal noise. However, the high signal level approach requires laser repetition rates of hundreds of thousands of pulses per second to obtain contiguous coverage of the terrain at submeter spatial resolution, and with currently available technology, affords little scalability for significantly downsizing the hardware, or reducing the costs.
University of Florida (UF) researchers are developing an ALSM unit based on a different paradigm, referred to as photon counting ALSM, or PC-ALSM. In the PC-ALSM approach, relatively low energy laser pulses are transmitted, and are used to illuminate a surface ‘patch’ of terrain a few meters in extent. The returning photons are detected by a multichannel photomultiplier tube, which separately senses the returns from an array of groundals comprising each patch, providing high (few decimeter) resolution contiguous coverage of the terrain. A multi-channel multi-stop timing unit records both noise and signal events within a range gated window, which enables noise to be filtered out of the data during post flight processing.
Researchers at NASA GSFC have already tested a first generation system based on this new paradigm. The NASA system operated from a high altitude aircraft, to obtain proof of concept data, prior to the development of a satellite based instrument. Details of the preliminary UF design for a second generation system that will operate from a light aircraft flying less than 1000 meters above local ground level and providing contiguous coverage of the terrain with 30 cm spatial resolution are reviewed.
KeywordsAirborne laser terrain mapping
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