| In recent years,hypersonic vehicle(HSV)has attracted more and more attention from various countries in the world due to its advantages such as fast flight speed,large combat radius,short reaction time,strong penetration ability and good concealment performance.The hypersonic platform synthetic aperture radar(SAR)ground moving target indication(GMTI)system has the advantages that traditional airborne and spaceborne SAR GMTI systems do not have,and can achieve clutter suppression and ground moving target detection.Therfore,it lays the foundation for further moving target localization and recognition,thus completing fast remote battlefield detection,precision strike and fire control guide and other tasks.This dissertation focuses on the key issues and technical difficulties of clutter suppression and ground moving target imaging in hypersonic platform multi-channel SAR GMTI system.Based on the issues and difficulties,this dissertation proposes some novel signal processing methods for clutter suppression and moving target imaging in MIMO systems,clutter suppression and high precision moving target imaging in multi-channel systems,clutter suppression and moving target imaging in descending segment mode,and clutter suppression and moving target imaging in squint mode.The main content of this dissertation can be summarized as the following five parts:1.The geometric relationship of hypersonic platform radar multi-channel SAR system is described and discussed,and the corresponding moving target and clutter echo models are established.According to the characteristics of hypersonic platform,the differences between the clutter characteristics of hypersonic platform radar and ordinary airborne and spaceborne radars are discussed.Firstly,the Doppler ambiguity characteristics of the hypersonic platform are studied,and the effect of the Doppler ambiguity on the clutter spectrm is analyzed.Secondly,the clutter walk characteristics of hypersonic platform radar are studied.The reasons for clutter range walk of hypersonic platform are given,and the clutter range walk of hypersonic platform is quantitative calculated and discussed.Finally,two typical multi-channel SAR GMTI methods are introduced,and the advantages and disadvantages of these two methods are compared and discussed.2.In the existing multi-channel clutter suppression methods,the steering vector of the moving target cannot be obtained accurately because the velocity of the moving target is unknown,which results in Signal-to-Clutter-Plus-Noise Ratio(SCNR)loss in clutter suppression.In addition,when focusing the moving target,it usually considers only the effect of range walking,but not the influence of high order phase terms,which can cause performance loss during moving target focusing.In order to reduce SCNR loss in clutter suppression,and to accurately focus the moving target,a method of two-step hypersonic platform multichannel radar clutter suppression and high-precision moving target imaging is proposed.By coarsely setting the radial velocity of the moving target and using the coarse radial velocity to suppress the clutter in the range compression-azimuth CFT domain,the energy of the moving target is effectively preserved so that the SCNR loss is reduced.At the same time,a coarse estimation of the moving target's radial velocity is obtained.Afterwards,the range walking and range curvature of the moving target are corrected,and the Doppler modulation rate is estimated.At the same time,the compensation of the third order phase item is performed,and the moving target can be focused precisely.The proposed algorithm can reduce the SCNR loss during clutter suppression in traditional algorithms,estimate parameters accurately,and focus the moving targets.Simulation results show the effectiveness of the proposed method.3.In hypersonic platform SAR GMTI systems,the echo signals have severe Doppler ambiguities due to the high speed motion of hypersonic vehicle.At the same time,the size of the radar antenna and the number of channel are limited by the spacecraft space,which can result in an insufficient number of channel for clutter suppression,especially in case of the SAR system has a wide imaging swath.In order to solve the contradiction between echo Doppler ambiguity and imaging swath,and the lack of radar antenna size and number of channels in hypersonic platform to accomplish clutter suppression,a hypersonic platform based on MIMO antenna radar clutter suppression and moving target imaging algorithm is proposed.The algorithm can obtain the echo signals of each sub-band respectively by establishing a stripmap narrow-beam transmission and wide-beam reception(NTWR)MIMO SAR system,and transmitting the mutually orthogonal chirp signals using multiple transmit channels.Then,it uses the deramp STAP method to suppression clutter and image moving targets in each sub-band.The proposed method with MIMO SAR system can obtain more channel degrees of freedom than the traditional multi-channel SAR system,and can get larger imaging swath with the same pulse repetition frequency(PRF).4.Hypersonic vehicle has the characteristics of high speed and high maneuverbility,and usually has a skipping flight trajectory.However,the existing SAR GMTI algorithms mostly use the horizontal flight mode,so they cannot be directly applied to the descending flight algorithm.In order to solve the problem caused by the descending flight of the hypersonic vehicle in the skipping flight trajectory,a hypersonic platform multichannel radar clutter suppression and moving target imaging algorithm is proposed for hypersonic vehicle with the skipping flight trajectory.This method can effectively suppress the stationary clutter in the the descending flight,and focus the moving target.Finally,the simulation experiments verify the effectiveness of the proposed method.5.The radar platform in squint mode can use the squint angle to detect the forward target and achieve early warning function.Thus,it can increase the security of the radar platform.In addition,it can scan the area of interest to increase the flexibility of the radar.Therefore,the squint mode has more extensive application value than the side-looking mode,and it is more suitable for hypersonic vehicle to realize battlefield monitoring,accurate strike and precision guidance.In order to solve the problem of pulse compression mismatch and Doppler center migration in the squint mode of hypersonic vehicle multichannel radar,a method of clutter suppression and moving target imaging for hypersonic platforms in the squint mode is proposed. |
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