Title: Airborne Lidar Simulator for the Lidar Surface Topography (LIST) Mission
Author: Anthony Yu
Organization: NASA Goddard Space Flight Center
Co-Authors: Michael A. Krainak, David J. Harding, James B. Abshire, Xiaoli Sun,
John Cavanaugh, Susan Valett, Luis Ramos-Izquierdo, Tom Winkert, Michael Plants, Timothy Filemyr, Brian Kamamia, William Hasselbrack
In 2009 we started a three-year Instrument Incubator Program (IIP) project, funded by NASAís Earth Science Technology Office (ESTO), for early technology development for the Lidar Surface Topography (LIST) mission. The purpose is to develop and demonstrate technologies for a next-generation, efficient, swath-mapping space laser altimeter. Our approach allows simultaneous measurements of 5-m spatial resolution topography and vegetation vertical structure with decimeter vertical precision in an elevation-imaging swath several km wide from a 400 km altitude Earth orbit. This capability meets the goals of the LIST mission recommended in the Earth Science Decadal Survey by the NRC Committee on Earth Science and Applications from Space. To advance and demonstrate needed technologies for the LIST mission, we are developing the Airborne LIST Simulator (A-LISTS) pathfinder instrument. A-LISTS is a micropulse, photon-sensitive waveform recording system that is based on a new, highly efficient laser measurement approach utilizing emerging laser transmitter and detector technologies.
The A-LISTS instrument uses a single laser to generate sixteen beams for high-resolution mapping. Backscatter from the surface is collected with a telescope and the spots from the swath are imaged onto a photon sensitive detector array. The output from each detector element is histogrammed and analyzed to determine ranges to the surface and derive echo waveforms that characterize the vertical structure of the surface. This signal processing technique allows for through-foliage interrogation in order to observe the ground surface beneath vegetation cover and to characterize vegetation vertical structure.
The returned signal from the ground is captured by a 20 cm diameter ruggedized, athermal telescope. The image of the 4x4 ground pattern is captured by a 4x4 fiber bundle and routed to a 16-element photon-sensitive detector array. The output of this array will be sent to a 16-channel, 1.5 GSamples/s, 8-bit digitizer to record an echo pulse waveform. The detection rate will be controlled to be a few 10's of photons per laser fire per beam by attenuation of the outgoing pulse energy in order to emulate the expected rate of our LIST measurement concept. This approach has the advantage of single photon receiver sensitivity but multiple photon dynamic range and no detector dead-time. In post processing, the waveform output from multiple laser fires along a profile will be aggregated at 5 m spatial scale to yield a composite waveform used for ground detection and canopy structure characterization. The over-sampling along profiles will be used to conduct sensitivity studies to guide refinement of our LIST mission measurement approach.
Engineering flights of this instrument are scheduled for April 2011; we will be demonstrating the performance of the A-LISTS instrument by acquiring data over a variety of topographic and land cover conditions. A focus will be demonstration of the ground detection through dense, closed-canopy forest cover. The component technologies, algorithms and measurement approach developed under the IIP effort will provide a path forward to the capabilities needed to meet the challenging requirements of the LIST mission.