Research On Some Issues Of Low-angle Target Height Measurement For VHF Array Radar

The very high frequency(VHF)band radar has been paid significant attention because of its potential capability of countering stealth plan and anti-radiation missile(ARM).It is well-known that one of the critical missions corresponding to the modern three-dimensional radar is measuring the altitude of a target.However,when tracking a low-angle target,the localization performance of VHF radar may be affected by the multipath phenomenon caused by ground surface reflection,especially in the case of complex terrain where exact information of reflection paths is unknown.Conventional radar systems may suffer from this problem,thus how to effectively improve the low-angle target tracking accuracy is one of the current key problems to develop the great potential of VHF radar in the modern air defense early warning system.On the basis of the achievements obtained by now,this dissertation starts in the related issues of low-angle target altitude measurement in traditional array VHF radar and VHF Multiple Input Multiple Output(MIMO)radar.The main research efforts can be summarized as the following four aspects:1.The multipath signal model for low-angle target localization in VHF radar and the multipath propagation mechanism are analyzed.Firstly,the primary issue that influence the low-angle target localization accuracy and the technical problem of solving the multipath propagation are introdued.Subsequently,two multipath signal model that under an ideal smooth refelcting surface and based on curved Earth geometry are presented,respectively.Meanwhile,the multipath propagation mechanism is detailed studied from the aspect of the Earth curvature's influence to the multipath propagation,the effective Fresnel reflection region and the specular reflection,respectively.Finally,three commonly altitude measurement techniques of low angle target are also studied,which are spatial smoothing super-resolution method,maximum likelihood based method and method based on compressive sensing,and the advantages and disadvantages of the three methods are discussed through theoretical analysis and simulation experiments.2.The curved Earth-based low-angle target altitude estomation under complex terrain is studied.The second part focuses on the problem that the existing ideal symmetric multipath model based algorithms may become invalid over a complex terrain.To deal with this problem,a practical multipath signal model for VHF radar in complex terrain is presented,where both curved earth geometry and actual ground surface reflection coefficient are considered.Then,the reason why the classical MUSIC-based DOA estimation technique performs poorly for low-angle target tracking is analyzed and a novel altitude estimation technique based on alternating projection is proposed.Simulation results and real data analysis show that the target tracking accuracy can be efficiently improved by using the proposed algorithm under both simple and complex multipath scenarios.3.The low-angle target localization problem for VHF radar is studied in the third part,where the influences caused by both multipath under complex terrain and environment with limited available snapshots are taken into consideration.To deal with the problem that the existing subspace-based algorithms and ideal symmetric multipath model based algorithms may become invalid over a complex terrain with limited available snapshots,a practical multipath signal model for VHF radar in complex terrain is first presented,and a preprocessing step based on the low rank decomposition technique is developed to reduce the data dimension and to formulate the target localization problem with multiple snapshots in the signal subspace domain so that the proposed approach can be available for real-time application.A novel sparse representation based DOA estimation algorithm,combined with alternatingly iterative and dictionary refinement techniques,is then proposed.Meanwhile,the Cramér-Rao bounds(CRB)for the target DOA and multipath attenuation coefficient estimations of multipath model are derived in closed forms.Experimental results based on both simulated data and measured data indicate that the target localization accuracy can be effectively enhanced by utilizing the proposed algorithm under complex terrain and/or limited snapshot scenarios.4.The forth part focuses on the problem of VHF MIMO radar low-angle target altitude estomation under the condition of the reflecting surface height is unknown and under the complex multipath environment,respectively.a.Due to the fact that a priori information of the reflecting surface height may contributes to good performance of target altitude estimation in low-angle tracking environment for VHF MIMO radar,but it is difficult to acquire in practice.Under the condition of the reflecting surface height is unknown,an algorithm for the target elevation estimation is proposed.The algorithm is based on the criterion of maximizing the correlation coefficient of target echo and the atoms of a given dictionary,where the dictionary grids can be iteratively refined to estimate the parameters more precisely.The simulation results indicate the effectiveness of the algorithm.b.For a VHF MIMO radar,multiple reflection paths,i.e.,multipaths,of target echoes may cause severe errors in elevation estimation of low-angle targets,especially in the case of complex terrain where exact information of reflection paths is unknown.To deal with this problem,a practical multipath signal model for MIMO radar in complex terrain is first presented,where both transmitted multipaths and received multipaths are considered.A novel altitude estimation technique based on rank-1 constraint and sparse representation for VHF MIMO radar is then proposed.Simulation results show that the target localization accuracy can be efficiently improved by using our proposed technique under both simple and complex multipath scenarios.