Science Service System

Summary of Proposal MTH1603

TitleCompeting effects of vegetation and tectonics on SAR data for Pacific NW
Investigator Lohman, Rowena - Cornell University, Earth and Atmospheric Sciences
Team Member
Asst Professor Lohman, Rowena - Cornell University, Earth and Atmospheric Sciences
SummaryForest canopy structure is one of the keypieces of information that can feed into efforts to manage forests, modelecosystem processes and constrain the amount of carbon stored locally andworldwide. Deforestation, particularly in tropical regions, accounts forapproximately 20% of anthropogenic greenhouse gas emissions, second only to thecombustion of fossil fuels for energy production. Remote sensing techniquessuch as light detection and ranging (LiDAR) and polarimetric synthetic apertureradar (SAR), both of which can be performed using either airplane orsatellite-based platforms, have the potential to assess forest biomass lossrapidly over larger areas than can be monitored through ground studies. Toreach a meaningful understanding of the world’s carbon budget and itsuncertainties, we need temporally repeatable measurements.With current data, terrestrial sinks haveuncertainties as large as their magnitude.Remote-sensingapproachesTraditional methods for measuring treeheights involve fieldwork and the assessment of individual trees.These approaches are relatively inexpensivewhen performed in small areas and are critical for benchmarking any remotesensing approach.However, they are notscalable to world-wide or national forest studies and often require that thetop of the tree is visible, which can be difficult in dense forests. Interferometric phase is a function of anyperturbations to the propagation path between the satellite and the ground,such as variations in the ionosphere or atmospheric water vapor, changes in thereflective properties of the ground surface, and actual deformation of theground surface such as might occur during an earthquake, landslide or otherevent.The measured phase also has acontribution from any topographic relief, with a magnitude that depends on thespatial baseline between the satellites. The effect of topography on the interferogramallows its use in determining canopy heights. InSAR, because of the dependenceon topography, can be used to generate DEMs – this is what was done during theShuttle Radar Topography Mission (SRTM).Theoretically, every coherent interferogram produces a DEM that could beused in tree height investigations and other research.However, in practice the aforementioned errorterms such as atmospheric water vapor and errors in our knowledge of the exactsatellite geometry result in effects that can be as large or larger than the phasedifference due to elevation changes between the clearcuts and surroundingforest. We can show that this is an effect of a “DEM error” due to the fact thatthis phase difference scales linearly with baseline in interferograms withinthe Pacific Northwest. By only focusing on the change in phaseacross the boundary between trees and clearcuts, not on the absolute value ofthe phase on either side, we remove the effect of all noise sources at spatialscales larger than that of the clearcuts themselves.And by using many interferograms with a widerange of B^, we can more effectively unwrap the phase change across theclearcuts.Our initial work involved 22 L-bandinterferograms, with a range of temporal and spatial baselines between theimage acquisitions, and produced reasonable estimates of tree canopy height. C-bandinterferograms, as expected, produce higher estimates of canopy height, sinceL-band data interacts with a lower elevation portion of the trees. TerraSAR-X data will produce a third,independent estimate of tree height, allowing us to characterize the differingsensitivity to wavelength.Coherence inthe Pacific Northwest at L-band is high relative to other forested regions, sowe anticipate that the short repeat time of TerraSAR-X may result in goodcoherence as well.

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