Study On Some Issues Of Altitude Measurement Of Low-angle Target For VHF Array Radar

Very High frequency(VHF)Radar attracts extensive attention in recent years because of its advantage and potential in long detection rang,anti-stealth effect and anti-arm capacity.However,since the wavelength of VHF radar is long,limited by array aperture,VHF radar usually has wide beam,which leads to bad angular resolution.For low elevation target,the echo of target is affected by serious multipath phenomenon,and it is difficult to distinguish the multipath echo and direct echo from distance domain,spatial domain and Doppler domain.Especially in complex terrain,the multipath signal often divide into several paths,both the amplitude and phase of the multipath signal is perturbed by irregular reflection.Consequently altitude measurement of low elevation target is traditionally a difficult engineering problem urgently to be solved in the low angle detection and tracking domain.With practical engineering application of the VHF radar as the background,on the basis of the achievements obtained by now,the dissertation focus on some issues of low elevation altitude measurement for VHF radar in complex terrain,the main content of this dissertation is summarized as follows:1.The technology of altitude measurement of low elevation target is studied,include the signal model of altitude measurement for low elevation target and some common used algorithms in practical engineer.Firstly,to deal with the model mismatch for altitude measurement of low elevation target in complex terrain,three different signal models of altitude measurement for low elevation target are established,that is classical multipath model,perturbation multipath model and scatter reflecting multipath model.In the classical,the multipath signal and direct signal are considered as a pair of closely spaced and coherent signal,and the reflecting coefficient is determined by factors such as reflecting medium and the roughness of the reflecting surface etc.As for the perturbation multipath model,by introducing the perturbation parameters,the perturbation multipath model measure the extent that the multipath signal is influenced by irregular reflecting.A scatter reflecting multipath signal propagation model for low-angle target is proposed with considering the influence of the irregular reflection,where the multipath signals consist of a group of coherent signals with the number of these signal unknown.Whereafter,some common used altitude measurement algorithms is studied and the amplitude and phase property of the echo under the classical multipath model is analyzed.Finally,to study the amplitude and phase property of the echo and the performance of some common used algorithms,real data of a VHF radar located at three different types of terrain are processed.2.A novel method based on Orthogonal Matching Pursuit is proposed to deal with altitude measurement of low-elevation target for very high frequency radar in complex terrain.Firstly,a perturbation multipath propagation model is derived where the influence of irregular reflecting in complex terrain is taken as a random perturbation of the multipath signal.Secondly,by imploring the sparseness of the targets in the spatial domain,a sparse recovery question is constructed with the received signal as the sparse matrix.Then a method based on OMP is proposed to solve the problem.The elevation the target and the perturbation parameter can be obtained when the algorithm is convergence.Subsequently,we can get the altitude of the target.Computer simulations results illustrate that the proposed method can estimate the pre-set perturbation parameters with higher accuracy,and the proposed method is of high capability of distinguishing coherent signals.Finally,we validate the performance of the proposed method with real data of a VHF radar collected in complex terrain.3.A novel method based on Sparse Bayesian Learning is proposed to deal with altitude measurement of low-elevation target for very high frequency radar in complex terrain.Firstly,the perturbation parameters that defined in the perturbation multipath model is introduced to the framework of sparse Bayesian learning.Then,the signal model of low elevation target for sparse Bayesian learning is established,by using the EM(Expectation Maximization)algorithm,the perturbation parameters and the elevation of the target can be obtained.Subsequently,we can get the altitude of the target.Computer simulations results illustrate that the proposed method can estimate the pre-set perturbation parameters with higher accuracy,and with the condition of diffident SNR or size of perturbation,the performance of the proposed method is of obvious superiority over the existing methods.Finally,we validate the performance of the proposed method with real data of a VHF radar located at complex terrain,real data of two scheduled flight with different azimuth are processed,the performance of the proposed method is of high precision,which indicates that the proposed method is not sensitive to the complexity of reflecting surface.4.A novel method based on Array Covariance Vectors Sparse Representation is proposed to deal with altitude measurement of low-elevation target for very high frequency radar in complex terrain.Firstly,a scatter reflecting multipath signal propagation model for lowangle target is proposed with considering the influence of the irregular reflection,where the multipath signals consist of a group of coherent signals with the number of these signal unknown.Then,the signal received by the array is transformed to the array covariance vectors,by using the flexibility of the sparse Bayesian learning,a high precision DOA estimation method based on array covariance vectors sparse representation is proposed to solve the problem.Several computer simulations and a real data analysis indicate the method proposed in this paper can get the altitude of low-angle target with the influence of complex terrain with high precision.The scatter reflecting multipath signal propagation model indicate the distribution of the signal of low elevation target in spatial domain,and is much more adapt in dealing with altitude measurement of low-elevation target for very high frequency radar in complex terrain in comparison with the classical multipath model.5.The DOA estimation method of spatially closely spaced and coherent signal is studied.Aim at Direction of Arrival(DOA)estimation for closely spaced and coherent signals in the scenario with multipath effect,a Maximum likelihood DOA estimation method based on spatial filtering is proposed.Firstly,both the advantage and disadvantage of the traditional method of coherent signal such as subspace based super-resolution algorithm and maximum likelihood based method are analyzed.Subsequently,by analyzing the theory of sparse recovery,we find that the potential project function of the sparse recovery is unsuitable for the demand of high precision DOA estimation.The performance of the sparse recovery method degrade greatly when the resolution of the source or the signals is coherent.Finally,the proposed method eliminate the influence of signal coherence by separating the coherent signals with spatial filtering,and have conquered the drawback that multidimensional parameter estimation in traditionally Maximum likelihood methods.The proposed algorithm has better DOA estimation performance than the existing methods.The simulation results indicate that the root mean square error(RMSE)could achieve Cramer-Rao Low Bound in the scenario of sufficiently high Signal to Noise Ratio(SNR).