| As an important branch of array signal processing,Direction of Arrival(DOA)estimation plays an irreplaceable role in military early warning,mobile communication,passive positioning,sonar detection,earthquake monitoring and many other military and civilian fields.After 30 years of development,the basic theory and method of DOA estimation have become mature and have realized the transformation from theory to practical application.However,with the continuous improvement of manufacturing process and hardware level,new technologies and new equipment keep emerging,the electromagnetic environment is becoming more and more complex,and the problems faced by using basic methods to estimate the DOA of targets gradually appear,such as the problem of poor observation and estimation accuracy of high-speed stealth targets and low altitude slow targets,and higher requirements for real-time performance of the algorithm are put forward in practical applications.Therefore,how to improve the accuracy and stability of DOA estimation algorithm in complex environment and reduce the computational complexity of the algorithm has been the goal of the majority of researchers.In this paper,the difficulties in DOA estimation of low-elevation targets and missile-borne radars under complex position conditions are studied deeply,and a series of effective methods are proposed.The main research content of this paper is summarized as follows:1.The multipath signal model for DOA estimation of low-elevation targets in VHF radar under complex position conditions is studied.Firstly,the difficulties in estimating the DOA of low elevation targets for VHF radar are analyzed,and four multipath signal models suitable for different terrain conditions are given.The received signals of different models are derived in detail according to the multipath propagation mechanism of different models,and the applicability of different models is summarized according to the characteristics of different models.Secondly,the previous research results are summarized,and three common types of super-resolution DOA estimation methods are introduced: feature subspace,maximum likelihood,and sparse reconstruction,and the representative methods of these algorithms are introduced in detail.Finally,the advantages and disadvantages of these algorithms and their applicable conditions are explained through theoretical analysis and computer simulation experiments.2.A fast DOA estimation method for low elevation targets based on ADMM is studied.Firstly,aiming at the problem of poor estimation accuracy and low computational efficiency under complex position conditions by using classical multipath model class method,the signal processing flow of two-dimensional array radar was derived in detail,and the target angle was estimated roughly by digital beam synthesis technology.Secondly,the reasons for the performance degradation of traditional methods under complex terrain conditions are analyzed,and a fast DOA estimation method based on ADMM is proposed by combining alternate projection technique and ADMM fast algorithm idea.Finally,the performance of the proposed algorithm is compared with that of common algorithms through computer simulation experiments to verify the effectiveness of the proposed algorithm and its applicability to complex terrain.3.The DOA estimation method of low elevation targets based on multiple reflection centers is studied.Firstly,by analyzing the characteristics of multipath reflection in complex scenes,a multipath signal model based on multi-scattering centers is established.By using the principle of vector addition,an equivalent multipath signal model is proposed.The effectiveness of the proposed model is verified by analysis and simulation experiments.Secondly,on the basis of equivalent multipath model,a DOA estimation method based on particle swarm optimization for low elevation targets is proposed,which solves the problem of high magnification superresolution in the beam range which is difficult to be realized by traditional algorithms.Finally,the proposed model and the proposed algorithm were verified by the results of simulation experiments and measured data.The experimental results show that the proposed multipath equivalent model fully considers terrain factors and is more in line with the actual situation.The proposed method can achieve accurate estimation without any prior information and can be applied to different terrain environments.4.Fast DOA estimation method based on two-dimensional angle optimization is studied.In order to solve the problem of mismatch between the received signal and the signal model in complex environment and high computational complexity of existing algorithms,an equivalent multipath model was introduced and a fast DOA estimation method based on twodimensional angle optimization was proposed.Firstly,according to the equivalent multipath signal model,the angle constraints of target and multipath are obtained,and then the objective optimization function is constructed.Secondly,the target angle value and reflection coefficient are optimized by the combination of alternating projection technique and Newton method,and the superresolution problem is transformed into a parameter optimization problem.Finally,the effectiveness of the proposed algorithm is verified by simulation experiments and measured data results,and compared with the existing mainstream algorithms,it is verified that the proposed method has more advantages under the condition of low signal-to-noise ratio,and the calculation time is shorter.5.The high-resolution DOA estimation method based on spatial synthetic aperture is studied.The problem of poor DOA estimation performance of finite array aperture radar is studied.Firstly,a uniform velocity space synthetic aperture model is introduced to improve the accuracy of DOA estimation.Secondly,a high-precision DOA estimation algorithm based on spatial sampling is designed under this model to meet the needs of accuracy and real-time estimation.Finally,the effects of target state,heading Angle,spatial sampling point,target distance and other factors on the accuracy of the algorithm are fully considered in the simulation experiment,and the effectiveness of the proposed algorithm is verified by the simulation data. |
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