| The navigation system is essentially a positioning system,which can measure the navigation parameters such as the position,speed and even course of the person or the vehicle in real time.Provide personnel or vehicles with a forward direction so that they can reach the established position on time without deviating from the running direction.The navigation system has a wide range of applications in the military field,and it is also an indispensable and important presence in the civil field,and plays an important role in people's lives.It makes the navigation system an important source of military information and an indispensable part of the national economic infrastructure.Very low frequency(VLF)radio waves,because of their long wavelengths,stable propagation characteristics,and low transmission attenuation,have been widely used in long-range and ultra-long-range navigation and positioning.They play an irreplaceable role in communication and navigation,especially in military communications and ship navigation.It means that the study of the VLF waves cannot be ignored;in-depth study of the propagation mechanism of the VLF band radio wave and the establishment of an accurate and efficient VLF radio wave propagation prediction model are highly value and necessary in practical and theoretical aspects.Electromagnetic waves in this band mainly propagate in the space formed by the ionosphere and the earth's surface.Due to its wide wavelength range,the ratio of the wavelength to the space's characteristic size also changes greatly,making VLF radio waves have different propagation modes in propagation engineering.The main propagation modes are: diffraction along the ground,propagation along the ground-ionosphere waveguide,"wave hop" propagation between the ground and the ionosphere,and "side wave" propagation along the ground.This thesis will mainly introduce and analyze the problems related to these two propagation modes: ground propagation and earth-ionosphere waveguide.In this thesis,we will use analytical methods to study and analyze the propagation modes of VLF electromagnetic waves propagating along the ground and in the earth-ionosphere waveguide,so that the understanding of the propagation characteristics of VLF waves is more detailed and in-depth.This thesis will summarize and analyze the propagation characteristics of VLF radio waves,and briefly introduce a more feasible correction method for very low frequency navigation radio waves.For ground wave propagation,we first consider the situation when the transceiver is installed at a relatively short distance,in this situation the propagation path can be regarded as a uniform and smooth flat ground.Next,we consider the situation when the transceiver device is installed at a remote location.In this situation,the influence of the curvature of the earth must be considered.In this situation,the simplest idealized model is first considered,that is,a uniform and smooth spherical ground.Under normal circumstances,the ground cannot be regarded as a uniform and smooth sphere,so another situation needs to be considered,a segmented uniform ground.For the waveguide propagation in the earth-ionosphere waveguide,first consider the case of the planar waveguide.At this time,the ground and the ionosphere are regarded as two parallel reflective walls.On this basis,then the situation of the spherical waveguide is considered..Then on the basis of the above situation,the error factors of the VLF navigation model are analyzed.In this thesis,the propagation problem of VLF waves propagating along the ground and in the earth-ionosphere waveguide is studied.While gaining a deeper understanding of its propagation characteristics,the wave propagation characteristics in the two cases are analyzed and summarized to extract factors that may cause errors,analyze these factors,and briefly describe the method of error correction.It has certain help to the establishment of accurate VLF radio wave propagation prediction model. |
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