Study On The Propagation Of Electromagnetic Waves In The Disturbed Tropospheric Atmosphere

The tropospheric atmosphere is an important transmission medium in modern complex electromagnetic environments,and studying the impact of the disturbed tropospheric atmosphere on electromagnetic wave transmission has important strategic significance for seizing information superiority.In this dissertation,the prediction of important tropospheric electromagnetic parameters,the influence of coherent acoustic waves on electromagnetic wave propagation,and the electromagnetic scattering characteristics of the tropospheric atmosphere disturbed by a high-speed aircraft have been systematically researched under the practical application background of enhancing tropospheric scattering,jamming enemy links,anti-stealth,and so on.Main works and achievements are as follows:According to the theory of atmospheric turbulence and the measurement data of the atmospheric refractive index structure constant profile,the height-time correlation of atmospheric refractive index structure constant has been verified.Then,a high-precision forecast model for atmospheric refractive index structure constant profile is proposed using sliding window technology and deep convolutional neural network,which have excellent capabilities of adaptive feature extraction and nonlinear representation.The proposed forecast model for the atmospheric refractive index structure constant profile has been trained and tested using the measured data,which were measured by the TWP3 wind profile radar in Mianzhu City,Sichuan Province,to verify the accuracy.In addition,experiments have been designed to explore the accuracy differences between one-step-ahead and multistep-ahead forecasts,as well as the forecast accuracy with height.Based on the physical mechanism of acoustic waves changing atmospheric pressure,temperature,and water vapor pressure,theoretical models of coherent acoustic waveinduced atmospheric refractive index fluctuations have been established in the radio and optical waveband,respectively.In the radio band,the wavelengths of electromagnetic waves and audible sound wave can easily meet the Bragg condition.According to the coherent scattering theory of a stably layered structure,the scattering and transmission loss of the electromagnetic wave in the tropospheric atmosphere before and after the disturbance of coherent acoustic waves are calculated,and show the same patterns as the existing measured results.This fully demonstrate the fact that the scattered radio waves will be significantly enhanced when the coherent condition is met,which provides a theoretical foundation for using acoustic waves to disturb the tropospheric atmosphere to enhance troposcatter.In the optical waveband,the impact of the acoustic wave-induced atmospheric disturbance on the phase of optical waves has been studied,and the theoretical model for the additional phase has been established.The influence of the acoustic-induced disturbance on laser propagation has been verified by jamming the Michelson interferometer and the laser heterodyne system.In addition,the directivity factors of different acoustic arrays have been studied,which may guide the design and optimization of acoustic arrays for different practical purposes.Aiming at realizing the indirect detection of stealth aircraft by detecting the aircraft-induced atmospheric disturbances,the region with inhomogeneous atmospheres,which is formed by the collision,compression,and friction effects from the high-speed aircraft,is considered as a "weak scatterer",of which the electromagnetic scattering characteristics have been studied by numerical methods in computational electromagnetics.Firstly,the intense fluctuations of the atmospheric pressure caused by the aircraft at different speeds in the troposphere are obtained by using Ansys Fluent,and the spatial distribution of the dielectric constant fluctuations is based on the relationship between atmospheric pressure and atmospheric dielectric constant.Then,the geometric shape of the region with inhomogeneous atmospheres is extracted based on the previously-obtained point cloud data and a corresponding inhomogeneous "weak scatterer" model is established.The method of moments for solving the volume integral equation is adopted to calculate the electromagnetic scattering characteristics of the "weak scatterer" induced by a high-speed aircraft,thus analyzing its radar cross section(RCS)varying with the scattering angle under different incident angles and frequencies.Finally,the "weak scatterer" formed by the aircraftdisturbed atmosphere demonstrates the radar detectability by the comparison between the RCS of the "weak scatterer" and that of the calm atmosphere,which may provide some certain reference for subsequent research on anti-stealth methods.A novel machine learning method for extrapolating the target’s radar cross section(RCS)versus frequency is presented.The target’s frequency-RCS curve is decomposed into two parts: the global trend and local fluctuations,the non-linear least squares method based on a log-linear function and the Gaussian process regression employing the spectral mixture(SM)covariance function are adopted to capture them,respectively.Experiments based on conducting and dielectric targets are carried out,and the results show that this method can not only accurately extrapolate the conducting target’s frequency-domain RCS responses with simulated and measured data,but also those of the "weak scatterer" caused by a highspeed aircraft.Therefore,the proposed method can be used to alleviate the time-and memory-intensive issues of using numerical methods to explore the target’s RCS characteristics,and has great potential for solving some practical problems,such as obtaining the target’s RCS responses in a wider frequency band under the limited measurement conditions,and extrapolating the RCS responses at other frequencies from the limited observation data of non-cooperative targets.