| Recently,NS-HSV has become a hot spot in both civil and military fields,as it enjoys the advantages of both fast response of air-borne platforms and persistent detection of spaceborne platforms.Compared with air-borne platforms,HSV enjoys the characteristics of fast speed,high mobility,large detection area and high survivability.Compared with space-borne platforms,HSV has many advantages such as flexible detection area,low cost,and good concealment performance.Although the HSV radar system cannot replace the air-borne and space-borne radar systems,it can fill the gap between the traditional air-borne and spaceborne platforms due to its operation in the near space.Therefore,NS-HSV provides new opportunities for expanding multichannel SAR/GMTI(MC-SAR/GMTI)systems.In this paper,we study the HSV-MC-SAR/GMTI methods in multiple modes,such as side-looking and highly squint-looking,highly squint-looking with dive trajectory,and forword-looking.For the key technologies of parameter recovery,clutter cancellation and GMT imaging plus localization under ambiguity clutter,we propose the HSV-SAR/GMTI methods in multiple modes,including the following aspects:1.For the HSV-MC-SAR/GMTI system,the slant range model of ground detection is firstly analyzed,and the corresponding signal models of stationary clutter and GMT are constructed.Then,the frequency spectrum distributions of stationary clutter and GMT are discussed,and three key problems in the ground detection of HSV are specifically analyzed:Doppler ambiguity,distance and azimuth coupling,and strong ground clutter interference.Finally,the existing clutter cancellation methods in the case of Doppler ambiguity are discussed,including ISTAP,CFT-center,and CFT two-step methods.Moreover,the performances of the above methods are analyzed based on several key issues of the ground detection of HSV.2.For the HSV-MC-SAR/GMTI system with side-looking,existing Doppler ambiguity clutter cancellation methods will suffer unnecessary energy loss during the clutter cancellation stage,due to the limited channel degrees of freedom or the steering vector mismatch.On the other hand,the existing GMT radial velocity estimation methods rarely consider the Doppler ambiguity.So far,how to pre-estimate the radial velocity in the Doppler ambiguity echo of the IISV-MC-SAR system to achieve accurate extraction,focusing and location of GMT is a key problem to be solved urgently.For the above problems,an HSVMC-SAR/GMTI scheme with cigenspace-based Adaptive Sum and Difference Beams(EASDB)is proposed.First,a new radial velocity estimation method based on EASDB is derived,which is beneficial to simplify and improve the subsequent processing of GMT extraction,imaging and localization.It has the ability to alleviate difference beam distortion and overcome pointing error sensitivity,so it can obtain accurate GMT radial velocity and target real area in the coarsely focused image in range-compressed plus azimuth CFT domain.Then,an accurate GMT steering vector is constructed by using the estimated radial velocity,which effectively suppresses all clutter(clutter and its ambiguity components)and preserves the GMT energy.Compared with the CFT-center and the CFT two-step methods,the proposed method better preserves the envelope of moving objects and achieves higher SCNR.Finally,an improved GMT focusing and positioning method is proposed,which realizes the accurate estimation and phase compensation of the GMT’s two-dimensional velocity,and has high GMT positioning accuracy.Also,it modifies the process of range cell walk correction,and has lower algorithm complexity.3.For the HSV-MC-SAR/GMTI system with highly squint-looking,we analyze and discuss several specific issues caused by characteristics of HSV in highly squint-looking mode,such as higher-order phase errors,motion parameter coupling,Doppler ambiguity,and antenna size limitations.However,the existing SAR/GMTI research is mainly aimed at air-borne or space-borne radar systems,and most of the research on ground detection of HSV-SAR/GMTI systems is aimed at side-looking or single-channel plus squint-looking modes.So far,a HSV-MC-SAR/GMTI scheme with highly squint-looking is proposed.Firstly,the influence of range cell migration is analyzed,and a coarse focusing method based on third-order CFT is proposed.The coarse-focusing microwave images are obtained through range compression,coarse range cell movement correction,and third-order CFT.This method alleviates the influence of GMT Doppler ambiguity and range cell migration,and lays a foundation for subsequent GMT processer.Then,a robust clutter cancellation method based on join-pixel model is provided,which can inhibit the effects of stationary clutter and its ambiguity components,and reduce unnecessary energy loss in the true direction of moving tagets.Finally,for the coarse-focusing microwave images of GMT,the precise GMT focusing and localization method in the highly squint-looking mode is derived.Fro the HSV with highly squint-looking,the influence of the GMT perementers is inhibited,so that the precisely focused microwave image of the GMT can be recovered.4.For the HSV-MC-SAR/GMTI system with highly squint-looking and dive trajectory,the existing problems are analyzed and discusse,such as the slant range model,parameter recovery,clutter cancellation and GMT imaging plus localization.However,most of the existing research on the ground detection of the HSV-SAR/GMTI system is aimed at the horizontal trajectory plus side-squinting or dive-section plus side-looking mode.To cope the problems,a high-squint multi-channel HSV-SAR/GMTI scheme is proposed with dive trajectory.First,an improved equivalent range model(IERM)for stationary clutter and GMT is explored for efficient and high-resolution imaging.Then,combined with the IERM,the clutter cancellation and GMT imaging plus localization methods of the high-squint HSVMC-SAR/GMTI system with dive mode and Doppler ambiguity are derived.Finally,the geometric shift of the GMT is corrected by backprojection.Therefore,the proposed scheme has good performance in parameter recovery,clutter cancellation and GMT imaging plus localization.5.For the HSV-MC-SAR/GMTI system with forword-looking,the signal model and the left and right ambiguity problems are analyzed and discussed.Existing forward-looking SAR research mainly focuses on forward-looking ground-based microwave imaging,however,the research on clutter cancellation and GMT extraction for HSV platform SAR has not been covered.In order to suppress the left and right ambiguity clutter to extract the GMT,a space-time cascade clutter cancellation method for the HSV-MC-SAR with forwardlooking is proposed.First,the time-domain ambiguity microwave images are obtained from the echo signals of the HSV-MC-SAR with forward-looking.Then,the influence of stationary clutter and its left and right ambiguity components is eliminated by constructing a spatial beamformer.Therefore,the proposed method can better suppress the left and right ambiguity clutter and preserve the GMT energy. |
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