越地平天波通訊是將電波斜向入射電離層並透過電離層反射實現，所以為了建立越地平天波通訊技術，須先了解電波傳播之情形。電波的傳播方向會受折射指數梯度影響；其中，主要影響高頻電波傳播方向的因素為電離層內折射指數的梯度。本篇論文將在不同背景假設下進行電波軌跡模擬(電波覓跡)，包含：(1)直角坐標僅考慮電子濃度；(2)球坐標僅考慮電子濃度；(3)球坐標考慮電子濃度、磁場，其中以考慮磁場、電子濃度與梯度的球坐標模擬為主要探討對象。模擬中，不考慮磁場部分可透過理論方程式與解析解進行驗證，論文的模擬結果與理論值比對後十分接近，可認定模擬結果有極高的準確性；而考慮磁場模擬部分，電離層中的磁場資訊將參考國際地磁參考場模式(International Geomagnetic Reference Field model,IGRF model)；而電子濃度則是參考國際電離層參考模式(International Reference Ionosphere model ,IRI model)，電波模擬之時間為太陽活動週期極大期(2015)與太陽活動周期極小期(2008)的正午與凌晨。因考慮磁場作用，電波將分為正常波與異常波，因傳播性質與特性差異甚大，所以對於模擬結果將分開做討論。模擬結果包含對影響正異常波傳播特性因素比較、太陽活動周期與日夜變化對傳播路徑與頻率的影響，最後模擬在三維球坐標下，正異常波傳播軌跡、落點與極化面旋轉估算。 本篇論文藉由在不同假設下的電波模擬比較電波傳播性質的差異進而驗證電波傳播特性，以建立越地平系統發射電波軌跡模擬模型，提供使用者快速、準確的發射參數選擇依據。 ;Over the horizon (OTH) communications is obliquely transmitting the electromagnetic wave (EM wave) through the atmosphere and reflecting by ionosphere. To establish OTH sky wave communications system, understanding the EM wave properties is the most important. The propagation directions are influenced by the gradient of refractive indexes. Because transmission frequency of OTH communications is HF band (3~ 30MHz), the major affected factor is ionospheric refractive index gradient. In the thesis, the simulations apart into two kinds: including magnetic field and not including magnetic field. When the simulations don’t include the magnetic field, the result can verify by theoretical formula. By comparing results and theoretical value, the simulation results are much precise. When the simulations include the magnetic field, the background information of ionosphere refer to IRI and IGRF and the dates are minimal solar activity period (2008) and maximal solar activity period (2015). Under the influence of magnetic field, the EM wave divides into ordinary wave (O-wave) and extraordinary wave (X-wave) when the wave propagates in ionosphere. The results include the major influence of the difference between O-wave and X-wave propagation property and the impact of solar activity and daily varying on transmission frequency. In the end, the thesis shows the results about EM wave propagation paths, footprints and polarized plane variations in three-dimensional spherical coordinates. The thesis concludes the propagation properties in different situations and coordinates by numerical simulations. By results, establishing the ray trace model to provide much fast and accurate basis about OTH communication system.
