| Airborne radar faces serious clutter when detecting targets in down looking work,so clutter suppression is an important problem to be solved.Space time adaptive processing(STAP)combines space-time information to suppress clutter adaptively.However,its clutter suppression effect is affected by the independent identically distributed(IID)characteristics of sampled data.For the side looking array radar,its clutter meets the IID characteristics,while for the forward looking array radar and the complex situation of range ambiguity clutter,the performance of STAP method will be seriously affected.Firstly,in the case of no range ambiguous clutter,training samples distribution characteristics of different range gates of forward-looking array airborne radar have range coupling,and the clutter suppression performance of STAP will decline;Secondly,in the high repetition frequency pulse system,clutter echoes from different ranges will overlap when the radar enters the steady state.The traditional non-uniform clutter suppression method can not align the two types of clutter from the unambiguous range clutter region and the range ambiguous clutter region at the same time,resulting in the decline of radar target detection performance.How to obtain better STAP clutter suppression performance in the case of non-uniform range ambiguous clutter is an important problem faced by airborne radar.In addition,with the development of radar system from monostatic to bistatic,airborne bistatic radar not only has larger living space and better target detection ability,but also faces more complex clutter background,and its difficulty of non-uniform clutter suppression increases accordingly.With the increase of system freedom,how to reduce the complexity of clutter suppression algorithm is also an urgent problem for bistatic radar clutter suppression.On the other hand,the development of jamming technology brings more complex battlefield environment to radar target detection,which seriously affects the effectiveness and accuracy of radar target detection.The development of digital RF storage technology enables the repeater jammer in the main lobe to accurately copy multiple false target signals,and the range and speed of false targets can be modulated at will.Dealing with main lobe deception jamming is one of the important problems faced by radar target detection in jamming environment.Aiming at the problems faced by target detection under the condition of high-speed movement of airborne FDA-MIMO(Frequency Diversity Array Multiple Input Multiple Output)radar system,such as range ambiguity clutter pollution,range dependent nonuniformity of short-range clutter,store and forward jamming and bistatic high-dimensional data processing,this thesis makes overall use of multiple degrees of freedom information such as radar angle,range and frequency,and adopts heuristic algorithm,compressed sensing and other algorithms.This thesis studies the clutter suppression and anti-jamming of radar under various systems such as monostatic,bistatic,linear array and planar array.By analyzing the echo characteristics of radar jamming and clutter signals different from traditional radar,the purpose of distinguishing and suppressing undesired signals is realized and the performance of radar clutter suppression and anti-jamming is improved.Focus on the FDA-MIMO radar signal model based on nonlinear frequency increment,anti-main lobe deception jamming method,range ambiguity clutter and short-range dependent clutter characteristic analysis and two clutter suppression methods.Algorithms such as range ambiguity clutter suppression,bistatic clutter suppression,3D data dimensionality reduction search,etc.based on pattern main lobe correction.The FDA-MIMO radar signal model based on nonlinear frequency increment,anti main lobe deception jamming method,range ambiguity clutter and short-range dependent clutter characteristic analysis and two kinds of clutter suppression methods are studied.The algorithms of range ambiguity clutter suppression based on pattern main lobe correction,bistatic clutter suppression,threedimensional data dimensionality reduction and search are studied in detail.The main work and innovations are as follows:1.The suppression method of nonlinear frequency increment main lobe range deception jamming is studied.The main lobe range deception jamming forms an active deception signal by storing,modulating and forwarding the radar signal into the radar receiver,which greatly reduces the radar detection efficiency.For this type of jamming,this thesis proposes a main lobe range deception jamming suppression method based on simulated annealing algorithm nonlinear frequency increment FDA MIMO Radar(NL-FDA MIMO).Firstly,the nonlinear FDA frequency increment is calculated by simulated annealing algorithm.Its pattern has the characteristics of single peak decoupling,which is different from the traditional FDA pattern.Secondly,after receiving signal compensation,the nonlinear frequency increment makes the false target echo show discrete distribution in the power spectrum.Finally,the main lobe range deception jamming suppression is realized by adaptive filtering while retaining the desired signal echo.Simulation results show that the proposed method can effectively suppress the main lobe range deception jamming,and the suppression performance is improved compared with MIMO and linear frequency increment FDA radar.2.The clutter suppression method of nonlinear frequency increment is studied.We focusing on the serious range dependence and range ambiguity of ground clutter in forwardlooking array airborne radar system under high repetition frequency pulse system,a range ambiguity clutter suppression method of FDA MIMO Radar based on annealing algorithm is proposed in this thesis.According to the frequency increment design criteria.Firstly,the nonlinear FDA frequency increment is calculated by simulated annealing algorithm.Its pattern has the characteristics of single peak decoupling,which is different from the traditional FDA pattern.Secondly,the nonlinear frequency increment breaks the fixed geometric relationship between spatial angular frequency and Doppler frequency,making the clutter echo show discrete distribution in the power spectrum,Finally,the clutter suppression in the range ambiguous area is realized by compensation while retaining the signal echo from the unambiguous area.The simulation results show that the proposed method can improve the clutter suppression performance compared with STAP and linear frequency increment FDA radar.3.The radar range ambiguous clutter suppression method based on main lobe correction(MLC)is studied.For the serious range dependence and range ambiguity of ground clutter in forward-looking array airborne radar system under high repetition frequency pulse system,a range ambiguity clutter suppression method based on transmitting main lobe correction FDA MIMO radar is proposed.Firstly,the designed pulse transmission weight is used to compensate the initial phase formed by the accumulation of frequency increment between pulses,so as to solve the problem of inconsistent high gain irradiation angle of main lobe between MIMO equivalent transmission pattern pulses synthesized by the receiver in the traditional FDA MIMO system.Secondly,the received echo is restored and compensated.Eliminate the influence of the main lobe correction weight of the transmitter on the signal echo.Finally,using the characteristics of transmitting end correction and compensation to distinguish different range ambiguous clutter,the echo in the clutter area presents discrete distribution in the power spectrum while retaining the signal echo in the observation area,so as to realize the suppression of range ambiguous clutter.The simulation results show that the proposed method can improve the clutter suppression performance compared with MIMO and linear frequency increment FDA radar.4.The bistatic range ambiguity clutter suppression and dimension reduction methods are studied.Under the high repetition frequency pulse system,the ground clutter of the side looking array MIMO(multiple input multiple output)bistatic radar system has serious range dependence and clutter spectrum spreading.At the same time,the bistatic radar has a multidimensional clutter structure,which makes it difficult to obtain enough training samples and the complexity of adaptive weight algorithm.In this thesis,an FDA(frequency diversity array)-MIMO bistatic radar range ambiguity clutter suppression method based on transmit main lobe correction is proposed.In this method,the main lobe of the equivalent emission pattern at the receiving end is corrected to solve the problem of the moving angle of the main lobe;Through the correction and compensation of the main lobe of the equivalent transmission pattern and the anti compensation processing after receiving the signal,the echo has the phase term of range coupling,which realizes the effective distinction of clutter at different ranges,solves the problem of clutter range ambiguity,and completes clutter suppression;The three-dimensional search is divided into the form of "2+1".Firstly,the clutter is suppressed and the target parameters are searched in the transmission angle frequency domain and Doppler domain.Then,the dimension reduction matrix is constructed according to the obtained target transmission angle and Doppler information,and the dimension reduction search is carried out in the reception angle frequency domain,which solves the problems of insufficient clutter samples and large amount of computation.This method finally solves the ambiguity problem of ground range clutter in large scenes of bistatic early warning radar.Simulation experiments verify the effectiveness of the proposed method.5.Range ambiguous clutter is an inevitable issue for airborne forward-looking array radars,especially with the high pulse repetition frequency(PRF).In this paper,a method to suppress the range ambiguity clutter is proposed in a FDA MIMO radar with a forwardlooking planar array.Compressed sensing FDA technology is used to suppress the range ambiguity clutter and the range non-uniformity short-range clutter of forward-looking array radar.In specific,first,the range ambiguous clutter in different regions is separated by the characteristics of the planar array radar elevation dimension and FDA radar range coupling.Meanwhile,aiming at the problem of the FDA radar beampattern main lobe moving between different pulses,a main lobe correction(MLC)algorithm is proposed to solve the problem that the FDA radar cannot be coherently accumulated in the case of non-full angle illumination.Finally,Compressed sensing technology and elevation dimension filtering are utilized to suppress the range ambiguous clutter at the receiver and the approach alleviate the range dependence of clutter in the observation area.A small number of clutter snapshots can obtain approximately ideal clutter covariance matrix through compressed sensing sparse recovery.The method not only reduces the number of training samples,but also overcomes the problem of non-uniformity of clutter in the forward-looking array.Therefore,the clutter suppression problems faced by the high repetition frequency airborne radar forward-looking array structure are solved.At the analysis stage,a comparison among the traditional MIMO and FDA methods is carried on by analyzing the Improved factor(IF)curves.Numerical results verify the effectiveness of the proposed method in range ambiguity clutter suppression. |
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