Study Of Propagation Characteristics Of Very Low Frequency Electromagetic Waves In Unmagnetized Earth-ionosphere Waveguide

Very Low Frequency(VLF)radio waves have the characteristics of low loss,stable amplitude and phase,and strong permeability,and are widely used in ultra-long-range navigation,timing and communications and other fields.However,affected by the complex space-time variation of the medium(especially the ionosphere)in the earth-ionosphere waveguide,VLF radio waves exhibit complex propagation characteristics.Mastering and predicting the propagation characteristics of VLF radio waves in the earth-ionosphere waveguide is of great significance for improving the accuracy of the VLF navigation/time system and effectiveness of communication.This paper focuses on the influence of the ionosphere in the earth-ionosphere waveguide on the propagation characteristics of VLF radio waves.The waveguide method and Finite Difference Time Domain(FDTD)method are used to study the propagation characteristics of VLF radio waves,and a VLF radio waves propagation prediction software is designed.The details are as follows:(1)The propagation mechanism of VLF radio waves in the earth-ionospheric waveguide is analyzed from two aspects:the calculation method of VLF radio waves propagation and the characteristics of the propagation channel medium.(2)Aiming at the problem of fast qualitative prediction of the propagation characteristics of VLF radio waves in the earth-ionospheric waveguide,the calculation principle of the VLF waveguide method is deduced,and the key modules involved are sorted out.Then,firstly,the implementation process of W.K.B.approximation method and Airy function approximation method is compared and analyzed,and simulation shows:W.K.B.approximation method and Airy function approximation method are respectively applicable to the low frequency band and middle/high frequency band of VLF radio waves.Secondly,a solution to the roots of the modal equation based on Euler-Newton's method is proposed,and analysis shows that the method has the characteristics of high accuracy,high reliability and high efficiency.(3)Aiming at the problem of high-precision prediction of VLF radio waves propagation under the influence of complex ionosphere,an ionospheric correction scheme is proposed,i.e.,modified IRINRL-exponential model(IRINRL:Combination of IRI model and NRLMSISE-00 model),and a VLF radio waves propagation prediction model based on the parallel two-dimensional spherical coordinate FDTD method combined with the modified IRINRL-exponential model is constructed.The prediction model can consider the actual radio waves propagation environment,especially the ionospheric complicated space-time changes.By comparing with actual measured data,it is shown that this prediction model can reflect the influence of actual complex ionospheric changes on the propagation characteristics of VLF radio waves more accurately,and effectively improve the prediction accuracy of VLF radio waves propagation.(4)According to actual engineering needs,a VLF radio waves propagation prediction software with good human-computer interaction is designed.Based on the exponential model and the modified IRINRL-exponential model,the software uses the VLF waveguide method and the parallel spherical coordinate FDTD method to realize the rapid qualitative prediction of VLF radio waves propagation and the prediction under complex ionospheric changes.The research results of this paper can provide certain theoretical and technical support for the accuracy improvement of the VLF ultra-long-range navigation/timing system.