Research On Localization Technique Of Shortwave Emitter For Multistatic Radar Systems

Direct localization of shortwave emitters technology plays an important role in electronic warfare today.The location information of shortwave emitters plays a pivotal role,in the field of Over-the-Horizon radar(OTHR)detection.In the increasingly complex and changeable electromagnetic environment,military radars have increasingly higher requirements for positioning accuracy.The traditional techniques designed for localization of shortwave emitters are mostly based on two-step methods.Since the relatively low computational complexity,the two-step methods are suitable for practical engineering applications.However,There are also some limitations.The internal constraint of the received data is ignored,and a part of the received signal information will be lost in the process of extracting relevant parameters in the first step.Besides,for the localization of multiple emitters,extra procedure of data association is required.These factors cause the positioning result estimated by the two-step method to be a suboptimal solution,and the positioning performance cannot be guaranteed.In order to overcome the shortcomings of the two-step method,the direct position determination(DPD)methods have received extensive attention.In the complex electromagnetic environment,the DPD methods do not need to estimate the intermediate parameters,but directly exploit the properties of the observations,which achieve better positioning performance than the traditional two-step positioning algorithm.At present,many positioning algorithms are given under ideal signal channel conditions in the emitter radar system.In practical scenarios,considering the over-the-horizon situation of shortwave communication,the signal will pass through the ionosphere multipath reflection to the station.However,virtual height of the reflection in an unknown ionosphere is hard to estimate,the relation between the location of the emitter and the steering vector can not be determined.In such scenario,the traditional DPD method is no longer work.Therefore,the main content of this thesis is the direct localization of the shortwave emitter technology based on the angle information under the ionosphere multipath reflection channel.The main research contents of this thesis are as follows:First,this thesis introduces the propagation theory of shortwave signals,including the structural characteristics of the ionosphere and the propagation form of shortwave signals,which provides a theoretical basis for the research of shortwave emitters positioning algorithms.The classic localization of shortwave emitters methods are analyzed,including the traditional direction-finding cross location method and the traditional direct localization method based on angle-of-arrival(AOA)information.Second,a novel ionosphere multipath reflection model is proposed.Without considering the ionospheric tilt,shortwave signals pass through the ionosphere at multiple reflection points,and make a reflection to the station,and the signals of shortwave emitters incident at the ionosphere reflection point produces specular reflection.Based on this model,this thesis derives the mathematical relation between the emitter position and the azimuth and elevation under this model.Third,due to the number of the ionospheric reflection layer and the virtual height of each reflection layer are unknown,and the reflection signals have the same azimuth but different elevation.In order to solve the multi-dimensional search problem caused by the unknown elevation,a decoupled technique of angle suitable for any station array structure is proposed.Fourth,based on the traditional direct localization model of the shortwave emitter,the ionospheric reflection channel condition is extended in this thesis.By taking adcantages of the decoupled technique,two multi-station direct localization algorithms based on AOA information are proposed.The direct localization methods do not require multi-dimensional search,and only require a two-dimensional search in the latitude and longitude direction to determine the location of emitter.The simulation results show that the localization accuracy of the proposed methods are improved compared with the traditional direction-finding cross localization method.Compared with the traditional DPD method,we can avoid multi-dimensional search and decrease the computational complexity.