| Very Low Frequency(VLF:3-30 kHz)waves are extensively utilized in wireless communication and navigation technology research in automatic navigation systems due to their advantages of low attenuation,strong penetration,and relatively stable propagation characteristics.However,using VLF waves for long-range navigation in the Earth-ionosphere waveguide is subject to various factors.These include insufficient or inaccurate electron density data in available lower ionospheric models,complicated interference during mode transitions among different VLF wave propagation modes due to the presence of the Earth’s magnetic field,etc.All of these factors significantly impact the predicted accuracy of the VLF wave propagation.Therefore,improving the predicted accuracy of the VLF wave amplitude and phase in the Earth-ionosphere waveguide by technological methods holds crucial academic research and engineering application significance for the VLF communication,positioning,navigation,and timing systems.A two-dimensional frequency-domain finite-element method(FDFEM)is first proposed in this dissertation for simulating the propagation of VLF waves in the Earth-ionosphere waveguide.Subsequently,a combination of theoretical modeling,numerical simulations,artificial intelligence,and experimental data is mainly used to study the improvement of the calculation accuracy of the amplitude and phase of the VLF wave in the waveguide.The main work and innovations are as follows:1.The highly efficient FDFEM is proposed to predict the propagation characteristics of VLF waves in the Earth-ionosphere waveguide.Initially,a numerical computation model for the VLF wave propagation in the waveguide based on the FDFEM is established,and each step of the computation process is elaborately explained.Subsequently,numerical simulations based on the FDFEM and the International Reference Ionosphere(IRI)model are conducted.The results demonstrate that the simulations based on the FDFEM combined with the IRI model show good agreement with experimental data reported in references,and outperform simulations based on the Long Wave Propagation Capability(LWPC)combined with the exponential ionosphere profile.Moreover,the computational speed is faster than that of the finite-difference timedomain(FDTD)method.Thus,the correctness and efficiency of using the FDFEM to simulate VLF wave propagation are confirmed.2.VLF propagable modes coupling and interference in the anisotropic Earthionosphere waveguide are studied.Initially,the coupled-mode theory is incorporated into the improved FDFEM to simulate VLF propagable modes coupling and interference in the anisotropic Earth-ionosphere waveguide.Subsequently,the method for determining propagation constants of different VLF propagable modes is presented.Then,based on the obtained propagation constants,the attenuation rates,relative phase velocities,and electromagnetic field distributions of different VLF propagable modes in the fine anisotropic Earth-ionosphere waveguide are simulated.Finally,the impact of the location-varying geomagnetic information on the attenuation rates,relative phase velocities,mode coupling,and multi-mode interference of the VLF wave in the anisotropic waveguide are discussed.3.Two methods for fitting and revising the IRI model electron density are proposed,and a more fine and accurate Earth-ionosphere waveguide model based on the revised electron density is constructed to study VLF wave propagation characteristics.Firstly,different electron density models are compared.Secondly,the measured data and FDFEM simulation results on different VLF wave propagation paths are analyzed and discussed.Subsequently,two electron density revision methods are proposed based on FDFEM and IRI model.Finally,based on the IRI model and the revised IRI model,the amplitude and phase of the VLF wave propagation are analyzed and then compared with experimental results.The prediction accuracy of the VLF wave based on the FDFEM combined with the revised IRI model is significantly improved.4.A new method that is based on the FDFEM and artificial intelligence(FDFEMAI)to invert the electron density in the lower ionosphere is proposed.The FDFEM is used to construct dataset for training and testing the neural network.The Elman neural network(ENN)is improved by optimizing the connection weights,connection thresholds,and the neural number.The improved ENN is employed to build the relationship between the electron density and the amplitude/phase of the VLF wave in the Earth-ionosphere waveguide.The genetic algorithm is combined with the trained ENN to retrieve the optimal electron density at different altitudes and solar zenith angles using measured VLF amplitude and phase in the waveguide.Results show a good agreement between measurements and predictions using the inverted electron density.5.The propagation characteristics of ordinary and extraordinary waves in anisotropic ionospheric near-field region are studied.Starting from Maxwell’s equations,dispersion equations for ordinary waves and extraordinary waves are derived,and corresponding expressions for the refractive index are provided.Subsequently,expressions for various field components of ordinary waves and extraordinary waves based on their electromagnetic characteristics in the Earth-ionosphere waveguide modelled by the twodimensional FDFEM are derived.The time-and location-varying electron density of the lower ionosphere is considered as the FDFEM numerical simulation is performed.Finally,the propagation characteristics of ordinary waves and extraordinary waves in the anisotropic ionosphere near-field region are discussed separately. |
