Electromagnetic scattering model of a laser-scanned forest

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Examensarbete för masterexamen
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2019
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Gustavsson, Dennis
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There exists a lot of research on how to use radar to measure forest properties, like the forest’s biomass. Knowledge about a forest’s biomass can then be used to determine whenever or not it is time to cut down the trees in the forest. There are also environmental reasons for knowing the biomass of a forest on a global scale, where the total biomass in the forest relates to the carbon cycle. However, there are multipleforestparametersbesidesthebiomassthataffectstheforestbackscattering andthismaygiveinconsistencieswhenestimatingthebiomassatdifferentlocations. The goal of this thesis was to study how different forest parameters affect the radar backscattering by creating an electromagnetic model of a forest area, and perform computations of the backscattering of incident radar waves for frequencies in the VHF and UHF bands. To accomplish this, LIDAR data from SLU (The Swedish University of Agricultural Sciences) was used. The trees in the forest were modelled as a sum of cylinders placed on top of one another to approximate the trees and the forest. The scattering for a single cylinder can be calculated either with the truncated infinite cylinder approximation or Generalized Rayleigh-Gans approximation. By also considering possible ground reflections in combination with the scattering at the cylinders, the total backscattering of the incident wave can be computed for a single cylinder placed above the ground. The total scattering from a tree or a forest, which is built up of multiple cylinders, can then be calculated as a sum of the backscattering amplitude from each cylinder. As expected, the computed radar cross section (RCS) is shown to be different depending on the polarization of interest for the incident and reflected radar waves. A clear connection between the size of the trees and the RCS was found. The RCS for a single tree in the forest is also shown to be sensitive to small changes in the ground slope or the tree’s inclination. It could also be seen that when the number of cylinders used to approximate a tree increases, the more the RCS fluctuates with the frequency. However, the fluctuations are around the same RCS values as the calculated RCS for tree models with fewer cylinders. For the forest it is observed that small changes in antenna position and small changes to the approximation of the tree geometry barely changes the averaged backscattering. The computations on the forest model gives consistent results but there is currently not possible to judge how well the computed backscattering matches reality. Some differences compared to the real forest backscattering is expected due to simplifications and approximations. An example is that the backscattering from the needles and branches have been neglected.
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Electromagnetic scattering model of a laser-scanned forest
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