Research On Electromagnetic Wave Propagation And Radar Clutter In The Atmospheric Duct

Atmospheric duct is a special propagation mechanism where atmospheric refractionindex down with height abruptly and a anomalous structure obviously different from thestandard atmosphere, which can make the downward bending curvature of electromagneticwave smaller than the earth’s curvature through the atmosphere, and then make a portionof EM wave power trapped in a thin tropospheric layer above the earth. It seems EM wavepropagates in the metal conduit, so it is named atmospheric duct propagation. There,evaporation duct as the highest incidence of ducts arises at tens of meter height close to thesea surface due the interaction between atmosphere and sea make an abrupt down in thevertical humidity and temperature on the sea surface. Atmospheric duct makes EM wavedeviates from the original track, which not only increases the radar’s detecting range, butalso occurs to radar detecting blind area (in this area, EM wave can not arrive), andseriously affects the radar、reconnaissance and communication systems. So analysis on thepropagating character of EM wave and research on the radar clutter in atmospheric ductplay an important role where people improve the operating efficiency of radar andcommunication systems.About atmospheric duct EM wave propagation and the simulation of radar clutter, theessay does the following work. Firstly, this essay describes the definition、classification、genesis and important parameters of atmospheric duct, and analyses the atmosphericrefraction profile along with the refraction profile parameter model, then interprets thefactors0f EM wave forming atmospheric duct propagation and the impact of atmosphericduct on the EM wave propagation,the last this article shows the conception of EM wavepropagation loss and propagating factor in the atmospheric duct, as well as explains theirderivation processes; Secondly, this paper describes three kinds of EM wave propagatingmodel theories: duct model theory、ray tracking technology（geometric optics theory） andparabolic equation, analyzes their advantages and disadvantages, then makes out in detailhow to derivate the parabolic equation, and shows the arithmetic solution of parabolicequation as well. The last and the most important, the essay discusses the sea clutterscattering model and gives the deviation of radar clutter power in atmospheric duct, thenfigures out the EM wave propagation loss in atmospheric duct through the improved discrete mixed Fourier transform (IDMFT), revises the Radar Cross Section (RCS) bymeans of the improved GIT model. In the process, the grazing angle is the key. This paperdescribes the sea roughness by means of Wen ocean wave spectrum which is more suitedto wave spectrum of Chinese offshore, then figures out the changing grazing angle withpropagating range by using modern spectral estimation technique, in the last, the essaysimulates numerically the impact on radar clutter in atmospheric duct from wind、duct andradar parameters.This essay, by researching on EM wave propagating characteristics in atmosphericduct, masters the propagating model of atmospheric duct, analyzes numerically radarclutter echoes in duct from different angles. The simulation result proves these judgments:in different winds, winds almost do not make an impact on EM wave propagation loss, buthave an effect on radar clutter obviously, it proves that winds reacts upon radar clutterechoes through RCS; in different duct heights, atmospheric duct reacts on radar clutterechoes through EM wave propagation loss; in different radar frequencies, the situation iscomplicated and not obvious, it needs the further research. Therefore, people must take theimpact from winds and duct into account when analyze the radar echoes data inatmospheric duct. In a word, the research output plays a very important role where humanmaster and apply atmospheric duct propagation, and then provides the algorithm basis tofurther develop detection equipments which is very important to many radars.