Research On Key Issues For Air- And Missile-borne SAR Imaging

With the increasing requirement on imaging performance of synthetic aperture radar(SAR)imaging technology,the research on SAR is developing toward to multi-platform,high resolution,and various working mode.Regardless of airborne SAR or missile-borne SAR,with the increasing of the imaging scene and the squint angle,the coupling the range and azimuth dimension,the space variance of range and azimuth dimension are all become seriously,leading to the degradation of the imaging performance processed by traditional imaging algorithms.In addition,due to the shaking and the maneuver flight of the carrier platform,carrier maneuver flight,the trajectory in SAR imaging is always not an ideal straight line,but the curve trajectory.In fact,the formation of trajectory is due to the existence of the acceleration.The motion errors caused by acceleration will bring new challenges to the imaging algorithms,and thus how to compensate the influence of acceleration on imaging result has become the focus of attention.Therefore,this dissertation focuses on the removal of serious range and azimuth coupling in large scene and highly squinted angle for the airborne SAR and missile-borne SAR,the spectrum aliasing problem for the highly squinted beam steering synthetic aperture radar(BS-SAR),the compensation of the motion errors caused by acceleration for the missile-borne SAR,the removal of the azimuth space variance for the missile-borne SAR working in spotlight mode in highly squinted angle case with acceleration.The main work and research emphases of this dissertation are summarized as follows:1.In order to solve the serious coupling between the range and azimuth dimension caused by highly squint angel for the airborne SAR and missile-borne SAR,a novel fourth order Nonlinear Chirp Scaling Algorithm(NCSA)and an azimuth NCSA based on range walk removal are proposed,respectively.For the airborne SAR,the key of the novel method is adding a fourth-order filtering step to the traditional NCSA,using the method of series reversion(MSR)to derive an accurate range-Doppler domain point target spectrum,and deriving the nonlinear relationship between the frequency modulation rate and the range,and then adopting the nonlinear chirp scaling theory to image the data acquired in highly squinted case.For the missile-borne SAR,range walk is removed in range frequency and azimuth time domain,and then CS approach is used to correct range curvature.After this,a new high-order of phase filtering factor is introduced into the azimuth time and frequency domain in order to correct the azimuth-dependence.Finally,the signal is well focused in the Doppler domain and azimuth time domain by spectral analysis method.Simulation results illustrate that both these methods satisfy the imaging quality of SAR in high squint mode and large scene swath.2.In order to correct the serious coupling between range and azimuth,to solve the spectrum aliasing problem for the highly squinted beam steering synthetic aperture radar(BS-SAR),a new wavenumber domain mapping method is analysed.Through the geometrical model,the two dimensional(2-D)spectrums of the point targets in wavenumber domain are obtained.After the mapping operation,the 2-D spectrums have a similar form compared with the 2-D spectrums obtained in broadside mode,through this operation,the squint mode can be regarded as an equivalent broadside SAR,and then a novel 2-D two-step approach is introduced to avoid spectral aliasing and skewing caused by the beam steering and the high squint angle.Furthermore,by combining the novel two-step approach and the mapping operation,the baseband azimuth scaling(BAS)algorithm which is only used for broadside cases can be applied for the highly squinted BS-SAR without subaperture.Both the air-borne and missile-borne SAR data can be processed through this approach.3.For missile-borne forward-looking SAR in curve trajectory,time-variant motion errors are ignored,leading to degradations in imaging result.A missile-borne forward-looking SAR algorithm based on motion compensation is proposed to solve the above problem.Through adopting the curve trajectory to make the target and the radar flight path keep a certain angle,thus the forward-looking mode is converted to the squint imaging mode.The acceleration,which leads to the curve trajectory and influences the imaging result,is divided into forward-looking acceleration and cross-track acceleration,the second phase error and cubic phase error caused by these two accelerations are analyzed in detail.For the cross-track acceleration component,it can be divided into the acceleration vertical to the imaging plane and the one in imaging plane further.Via such dividing,the phase errors caused by acceleration are compensated by using vectorial methods.Moreover,for range migration,it is compensated through NCS approach based on the accurate spectrum acquired by the Method of Series Reversion.Therefore,the range-azimuth coupling is removed and the focused image is obtained.The simulated results are given to illustrate the validity of the proposed method.4.In the case of missile-borne SAR working in spotlight mode in highly squinted angle with acceleration,because of the large azimuth space variance and the serious coupling between range and azimuth for the missile-borne SAR,an azimuth frequency nonlinear chirp scaling(AFNCS)algorithm is proposed to save this problem.First of all,the geometrical model and the signal model of the missile-borne SAR are constructed,based on the acceleration model,the accurate 2-D spectrum are performed by adopting the MSR and the space-variant of missile-borne SAR in curved flight path is analyzed,and then an azimuth polynomial phase filter is constructed to make the coefficients of the perturbation function has sufficient flexibility to eliminate the spatial-variant couple terms between range and azimuth.Then,the gradient operation is used to expand the space-variant coefficients of azimuth modulation term,and the perturbation function is applied to eliminate the space-variant in azimuth direction.The proposed focusing method can process the missile-borne SAR data obtained in spotlight mode in highly squinted angle with high efficiency.The simulation results verify the effectiveness of the proposed imaging approach.