Electromagnetic Modeling of Forests on Mountainsides
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This dissertation is about the electromagnetic modeling of forests situated on mountainous terrains, with a concentrated focus on single scattering theory and direct (volume) scattering contributions. The study is driven by the need to improve models of wave propagation through forested areas, particularly in complex environments where the forest canopy overlays a sloping terrain. The research is underpinned by a series of radar experiments conducted in the White Mountains of New Hampshire, USA. This region was selected due to its unique characteristics, including a sloping forest layer and optimal accessibility, making it ideal for studying electromagnetic interactions. The experiments were carried out during three field trips in 2018, 2019, and 2021. These expeditions were conducted in collaboration with researchers from the Cold Regions Research and Engineering Laboratory (CRREL) of the US Army. The fieldwork involved forest data collection, including parameters such as tree types, trunk, branch, and needle properties (e.g., density, height, radius, orientation, dielectric constant). This ground truth data served as the foundation for the electromagnetic models developed in this study. The research focused on the P and L frequency bands, centered around 437 MHz and 1270 MHz, respectively, with the dissertation concentrating on the L Band data due to its higher relevance and data quality. One of the primary objectives of this study was to examine the contributions of different types of scatterers—trunks, branches, and needles—to the overall attenuation of radar signals. For instance, large trunks and horizontally aligned primary branches were identified as major contributors to attenuation, particularly at steep incident angles. In contrast, secondary branches and needles, being more vertically oriented and smaller in size, contributed less to attenuation. The theoretical framework of this dissertation is based on the Foldy-Lax and Distorted Born Approximations. These approaches were employed to calculate the mean electromagnetic fields in a layer of random scatterers and to derive the bistatic scattering coefficients. The study introduced an assumption of negligible surface contributions due to the high attenuation of this forest on mountainsides. This simplification allowed the analysis to focus on direct (volume) scattering from the forest canopy. The research findings demonstrate that the slope of the terrain and the associated changes in scatterer orientation lead to significant variations in the polarization and attenuation of the scattered waves, especially for steep angles. This polarization conversion effect, caused by the anisotropic nature of the tree trunks and branches, was particularly evident in forests with an evident slope. The study also showed that the mean field within the forest layer is affected by the density and distribution of scatterers, which in turn influences the overall scattering behavior. In addition to theoretical analysis, the dissertation includes detailed comparisons between the acquired experimental data and the model results. The radar measurements, which included both backscatter and bistatic scattering geometries, provided valuable data to compare and explain the interactions between the wave propagation and forested landscapes. This study contributes to the development of more accurate remote sensing techniques, by improving the understanding of electromagnetic wave propagation through forested mountainsides. These advancements can aid in better forest management practices, environmental monitoring, and potentially improve communication systems in forested and mountainous regions. In conclusion, this dissertation advances the theoretical understanding of electromagnetic interactions with forested landscapes on mountainous terrains. The combination of theoretical analysis, field data, and simulations provides a comprehensive framework for studying wave propagation in these complex environments.
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