Multi-target Detection For VHF Radar In Multipath Conditions

In recent years,the potential advantages of VHF radar in anti-stealth and anti-ARM have attracted renewed attention in the field of radar.However,the wavelength of VHF radar is long,so its beam is wide with limited aperture,which cause the bad angle resolution.In the detection of low angle target,the received signals include the direct path echo from targets and the multipath echo reflected by the ground(sea)surface.The multipath echo is highly correlated with the direct path echo,which reduces the accuracy of the target localization.Therefore,the resolution and accuracy of VHF radar are too low to meet the demand of target localization.The existing target detection methods usually suppress the multipath to improve the performance of the VHF radar systems.In this thesis,aiming at the problems of multipath in the detection of low angle target,the methods are analyzed to improve the resolution and accuracy of the target detection.The main contributions of this thesis are summarized as the following two aspects:1.For the flat ground,the multipath echo includes specular reflection component and a little diffuse reflection component.The spatial resolution of the traditional array radar is limited by the aperture size of the antenna.The temporal-spatial stochastic radiation fields based microwave correlated imaging can obtain high spatial resolution,which is beyond the restriction of antenna aperture.In this thesis,a multi-target detection method is proposed based on the temporal-spatial stochastic radiation fields.Firstly,multi-target detection model of the VHF radar is established and temporal-spatial stochastic radiation fields in the multipath conditions is constructed.The specular multipath signals is used to enhance the spatial-temporal random property.So the multipath signal is changed from unfavorable factors to favorable factors.Secondly,a robust compressed sensing algorithm is used to suppress the effect of the diffuse multipath signal to reconstruct the targets with superresolution.Finally,the effectiveness of the proposed method is verified by the simulation results.2.For the complex ground,the reflective surface is very rough and the roughness dissatisfies the Reileigh rule.The reflective surface is rougher,the power of diffuse reflection component is stronger.In this case,the multipath signal energy obeys a certain distribution in the space.In this thesis,the multipath signal model of distribution source is studied and the observation matrix in this model is derived.Then a compressed sensing algorithm is applied to achieve the super-resolution and high precision estimation of target elevation with less number of snapshots.Finally,the effectiveness of the proposed method is verified by the simulation results.