Study On High Accuracy Angle Estimation For Distributed Subarrays Antennas VHF Radar

VHF radar has the prominent advantages over the microwave radar of anti-stealth,anti-ARM and long detection range, while some distinct disadvantages such asnarrower bandwidth, wider beamwidth, lower angular resolution and lower localizationprecision are also severe. How to enhance the angular resolution and localizationprecision is indispensable for the development of VHF radar. An increase in the antennaaperture is an effective measurement for the problems mentioned above, but the stealthrequirement of radar or radar platforms, transportability, and battlefield survivingcapacity are adversely affected. As a solution to these problems, an array system withdistributed subarrays for VHF radar is proposed, consisting of a main transmit subarrayand distributed receive subarrays. Coherently Combination of the signals received bythe individual subarray can achieve high angular resolution of equivalent large antenna.This dissertation addresses some issues of distributed subarrays array VHF radarwith the research task of given VHF radar. The research concentrates on the robustadaptive beamforming, high accuracy DOA estimation, manifold ambiguity resolutionand low elevation angle estimation.The main content of this dissertation is summarized as follows.1. The techniques of robust adaptive beamforming for distributed subarrays arrayVHF radar are studied. Firstly, In terms of the configuration of distributed subarrays,centralized and distributed structures for digital beamforming are proposed. Secondly,An analysis of effects of mismatches caused by look direction/pointing error, sensorsgains and phase errors on the performance of robust adaptive beamformers is made, ascould be used to provide theoretical guidance for selecting appropriate beamformeralgorithem.Lastly, A structured uncertainty model of steering vector is proposed fordistributed array composed of partly calibrated subarrays in which cases the phasecenters of the subarrays are unknown or disturbed. Then, the robust adaptivebeamforming is converted into a non-convex Pseudo-SDP problem, and a Semi-DefiniteRelaxation(SDR)-based beamformer is proposed.2. High accuracy2-Dimensional DOA estimation methods for threeconfiguarations of distributed array are presented. Firstly, direction finding for aninterfermetric-like L shaped array and the effect of the baseline on accuracy of DOA areinvestigated. According to the array structure and the ambiguity caused by grating lobes, one-dimensional unitary ESPRIT algorithm is utilized to achieve lower variance andnon-ambiguous DOA estimates. Secondly,2-Dimensional unitary ESPRIT algorithm isused to estimate DOA for distributed coherent array. Threshold effect in DOAestimation is analyzed by Method of Interval Error(MIE), and the methods toapproximately compute threshold baseline and threshold SNR are presented. Lastly,aperture coupling between azimuthal and elevational aperture of slanting array leads tofine direction cosine estimate standing at a family of straight lines. Then, the directioncosine ambiguity can be resolved by the rule that the vectical distance between a dotand a line is the shortest. Simultaneously, aperture coupling causes the presence ofazimuthal and elevational resolution gain regions for slanting array.3.Manifold ambiguity and matrix completion-based ambiguity resolution algorithmof an interferometric array are studied. Manifold ambiguity does not necessarily implynonidentifiability. Association and model-fitting(Deconvolution) method for ambiguityresolution are derived from pattern recognition theory. Spatial under-sampling is theradical reason to manifold ambiguity. From Caratheodory theorem and Pisarenkoharmonic retrival theory, the manifold ambiguity resolution is automaticallyincorporated if a positive-definite and Toeplitz covariance matrix of a virtual uniformlinear array with the same aperture as the sparse array could be attained. Then manifoldambiguity resolution is translated into a pure math problem. The condition under whichthe various matrix completion method is applicable and the performance are analyzedand compared. For the particularity of an interferometric array configuration andinstability of the existing matrix completion methods, a novel ambiguity resolutionmethod with the combination of auxiliary elements and Toeplitz matrix completion isproposed， which improves stability.Simulation and real data demonstrate theeffectiveness and validity of the proposed method.4. Low elevation angle with high accuracy is studied for an interferometric-likearray VHF radar. Limited by aperture, VHF radar generally has a wider beam. When thetarget is at low grazing angle or lies below about one-third of the beamwidth above thehorizon, multipath signal reflected from ground(sea) surface come into the mainlobe aswell as the direct echo, in which case the performance of low elevation estimation ofVHF radar is highly affected. The interferometric-like array extends the verticalaperture with small pieces of hardware. The interferometric Forward/Backward spatialsmoothing(IFBSS) technique is proposed from the conventional spatial smoothingalgorithm for uniform linear array, being used to decorrelate the multipath and directsignal. Then high accurate elevation angle estimation with dual-size unitary ESPRIT algorithm is achieved. As the surface roughness increases or the target approaches thehorizon, there is a huge mismatch between two-target model and actual multipath. Acomplete theoretical model of multipath is nearly impossible at present. From theperspective of wavenumber spectrum estimation, the nonparametric Multi-taper Method(MTM) is utilized for the feature of adaptive wavenumber of multipath. It isdemonstrated that the diffuse component is characteristic of colored-noise by real dataresult.