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Carbon Cycle & Ecosystems

The goal of this research is to examine the expected accuracy of vegetation height derived from space based laser altimetry for future NASA missions including DESDynI. In particular this proposal will address to fundamental questions: 1. How accurately can space-based laser altimetry determine vertical structure of vegetation and what are the anticipated height retrieval errors for different biomes? Retrieval of canopy height is based on the premise of differencing the ground elevation from the canopy elevation. Accurate canopy height measurements from laser altimetry are confounded by two fundamental factors, canopy closure and ground topography. Canopy closure or the presence of understory vegetation makes the identification of the ground signal difficult if the laser energy does not penetrate through the vegetation. Second, in areas of significant topography, the ground signal is convolved with the vegetation signal. While laser altimetry can provide critical information regarding vegetation height, especially over forested regions, there remains great potential to utilize the waveform data to characterize other biomes. This research will quantify the expected height retrieval errors associated with various footprint configurations and biomes. 2. How does returned energy from vegetation and topography interact such that the accuracy of retrieved canopy height and structure is improved? Standard waveform processing consists of ranging to specific times in the return pulse which correspond to centroid of energy or mode identified through decomposition. Waveform shapes are dependent upon complex relationships between several factors including laser properties (i.e. laser pointing angle, laser energy, footprint size, shape and orientation) and surface properties (i.e. topography, reflectance, and vegetation height and position within the footprint). Can waveform metrics be used to describe vegetation conditions within each footprint. Metrics and correction factors based upon returned waveforms reported in the literature will be assessed for DESDynI. To address the aforementioned research questions, small-footprint full-waveform laser altimetry data will be used to quantify the error assessment. The University of Texas Center for Space Research (UTCSR) operates and processes full-waveform data from an airborne small-footprint lidar system and these datasets will be available for the research presented here. As such, by synthesizing space-based waveforms from various footprint sizes and biomes from the airborne waveforms, the height retrieval errors can be estimated which will provide a foundation to develop new algorithms for the DESDynI mission and future NASA laser altimetry missions. The preliminary research on this topic indicates that there are fundamental relationships between vegetation and topography whose impacts on lidar waveforms are still unknown. This research will examine the expected error budget of vegetation structure as determined through laser altimetry. However, the received waveforms are affected by many complexities including laser energy, surface topography and reflectivity, satellite attitude and laser pointing, and distribution of elements within the laser footprint. Among the objectives discussed here, results from this proposed research will investigate the accuracies associated with vegetation height retrievals and potentially provide new methodologies for processing of terrestrial waveforms for DESDynI or other NASA missions.

Project PI: Amy Neuenschwander/University of Texas at Austin

113D Knowles Engineering Building University of Massachusetts 151 Holdsworth Way Amherst MA 01003-9284

Phone: (413)577-0623

Email: amy@csr.utexas.edu

http://www.csr.utexas.edu/personal/amy/index.html

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Started: Aug 10, 2010

Last Activity: Dec 16, 2010

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