Study On Low-Altitude Target Detection And Imaging With Moving Platform Radar Under Complex Environment

Clutter suppression plays the essential role in the signal processing with moving platform radar system.Space-time Adaptive Processing(STAP)technique has been developed for the battlefield reconnaissance application of Airborne Early Warning(AEW)radar system,which can be regarded as the essential manner of Ground Moving Target Indication(GMTI).However,the conventional STAP is a statistical adaptive match filtering methodology,which requires sufficient Independent and Identically Distributed(I.I.D)training samples to meet the convergence demand of clutter covariance matrix(CCM)estimation,and the inter-channel clutter statistical correlation information is subsequently utilized by the Sample Matrix Inversion(SMI)algorithm for the adaptive clutter suppression.Meanwhile,the spatial-temporal steering vector constraint is simultaneously applied for the desired stationary target signal distributed in a single cell under test(CUT),maximizing the output signal-to-clutter-plus-noise ratio(SCNR)of match filtering.However,on one hand,it is rather challenge to obtain sufficient I.I.D trainning samples in the complex nonhomogeneous clutter environment,resulting in the CCM estimation error and the clutter suppression performance degradation inevitably.On the other hand,the fast maneuvering target signal is defocused in the range-Doppler domain,while the utral-low-altitude target also involves the multipath signal with strong coherence.Moreover,in the wide-band based radar system,it is also important to derive the high-resolution radar image of target based on the extended target detection.With the moving platform radar,including the active electronically scanned array(AESA)based and the sum-and-difference channels based radar systems,this dissertion is focused on the robust adaptive clutter suppression and the joint moving taget detection and imaging for different application purposes,in specific for the research of low-altitude target detection and imaging application.1.Aiming at the range-dependent nonhomogeneous clutter suppression problem,a robust adaptive clutter suppression approach is proposed based on the single-snapshot STAP with fast IAA.In this respect,a weighted spectral filtering based IAA is developed for target pollution mitigation with a priori knowledge,while a novel block-wise formation is extended to be referred to as the block-wise Toeplitz temporal covariance matrix reconstruction,which has improved the clutter spectral estimation efficiency with different multi-channels based radar systems,and enhanced the nonhomogeneous clutter suppression performance correspondingly.2.Aiming at the ultra-low-altitude moving target detection problem under the multipath effect backgroud,a robust sum-and-difference beamforming approach is proposed with ΣΔ-STAP from the perspective of anti-target-pollution robustness enhancement,in which the mainlobe of STAP is well maintained with the joint magnitude and phase constraints as well as the joint derivative and zero points constraints.Moreover,a multi-resolution STAP approach is proposed from the perspective of matched filtering performance improvement for the multipath-target coupled signal.Based on the mechanism anlysis of multipath-target coupling effect,the output SCNR is improved with the contribution of the power of multipath signal,by which the ultra-low-altitude moving target detection performance has been improved effectively.3.In view of the maritime moving target joint detection and imaging with airborne radar system,the ISAR imaging approach is developed with the STAP based clutter suppression.At first,the sliding window STAP filter bank is designed from the perspective of temporal dimension,which imposes linear constraints on the azimuth Doppler frequencies of extended target after adaptive clutter suppression,and guarantees the mechanism foundation of high-resolution ISAR imaging.Furthermore,an efficient ISAR imaging approach is proposed based on the multi-resolution STAP from the perspective of frequency dimension.In this regard,a matched filter is designed with respect to the Doppler frequency dimension for the joint adaptive clutter suppression and target motion compensation,by combining the target range-Doppler location coarse estimation of robust sub-band STAP and the post-Doppler dimension-reduced technique.The proposed method achieves significant reduction of computational complexity for the ISAR imaging.4.To solve the long-time coherent accumulation problem of fast maneuvering target under the strong clutter background,a systemic solution is proposed for the moving platform radar in handling the clutter suppression and small maneuvering target detection issues,which is referred to as the sub-CPI STAP based refocusing.With the consistent covariance matrix estimation and the semi-searching mechanism,the proposed method improves the robustness of clutter suppression and alleviates high computational complexity induced by the searching operation of undetected target.Furthermore,the target Doppler history is constrained among different sub-CPIs with the linear phase response,which gives facilities for further SCNR improvement and parameter estimation.