Research On Synthetic Aperture Radar Imaging Technology For Hypersonic Platforms

Hypersonic vehicles,as one of the three emerging technologies in the global competition,will play an important role in the future military and economy.The flight speed of hypersonic platforms exceeds Mach 5,and the flight altitude is 20-100 kilometers in atmospheric space,which is located between low-orbit satellites and general airplanes,and is the near-space region that has yet to be developed.At the same time,in order to take the high penetration,strong concealment,long-range strike,and rapid response capability into account,hypersonic platforms must have the motion characteristics of high speed,large maneuverability,and long-range multi-angle observation.This paper focuses on the trajectory characteristics of hypersonic platforms and their effects on imaging algorithms.The specific research content contains the following aspects:1.The imaging problem of the curve-trajectory hypersonic spotlight synthetic aperture radar(SAR)is studied.On the basis of the existing polar format algorithm(PFA),an integrated SAR imaging algorithm,the parametric polar format algorithm(PPFA),is proposed,which is capable of simultaneously accomplishing imaging and precise motion compensation.In view of the drastic coupling and spatial-variant problems caused by the large flight span,strong maneuverability,and flexible orbit of hypersonic platforms,the core of PPFA is based on the correspondence between the radar trajectory with its interpolation kernel,which reflect the azimuthal spatial sampling of the echo data,so there is no need to split the radar trajectory,and the conversion from data space to image space can be accomplished by directly resampling the echo data under the real trajectory.In this way,the radar motion errors in the traditional sense are embedded as parameters in the interpolation kernel of the ideal-trajectory PFA,so it is called the(motion error)parametric polar format algorithm.Unlike the coupling twodimensional resampling operation with ”plane wave” assumption in conventional PFA,PPFA adopts a process of ”first decoupling and then one-dimensional azimuthal interpolation”.Then the azimuthal processing in the range domain realizes the fast compensation of the range-variant phase error,which greatly expands the range focus depth.This algorithm,which integrates motion compensation and imaging formation,reduces the approximation caused by functional decomposition in conventional strategy and thus is able to solve the spotlight SAR imaging problem of hypersonic platforms with complex curve trajectory.2.The problems of slant range model mismatch and spatial variance with the diving acceleration trajectory of hypersonic platforms are studied.On the basis of PPFA,a generalized parametric polar format algorithm(GPPFA)is proposed to realize fast imaging of diving and accelerating hypersonic platforms.The hypersonic vehicles inevitably experience the accelerated dive motion state during the process of flying in the atmosphere and finally striking the target.Based on the analysis of the similar problem in accelerated dive imaging with existing missileborne SAR,the core of GPPFA is to extend the application scope of PPFA.To match the equivalent slant range model for diving trajectories,based on the generalized definition of the spatial wavenumber and its inclusiveness to the radar trajectory form,GPPFA employs an ingenious coordinate system rotation operation to transform the altitude change of the radar trajectory into targets’ elevation coordinates,and derives wavenumber expression for the target with three-dimensional coordinates.Based on the principle that wavenumber resampling can complete the transformation of sampling space,efficient imaging can be achieved with the help of fast Fourier transform(FFT).Under the definition of generalized wavenumber,it is proved that the azimuthal resolution of the proposed algorithm is inversely proportional to the effective synthesized angle with this diving track,which lays the foundation for further expanding of the wavenumber domain algorithm.3.The fast imaging problem of the long integral aperture hypersonic SAR is investigated,and a fast factorized parametric polar format algorithm(FFPPFA)is proposed on the basis of the existing fast factorized back-projection algorithm(FFBPA).Due to the high-speed nature of hypersonic platforms,the effective synthetic aperture length is greatly expanded for the same synthetic aperture time compared to low-speed platforms,which makes it easier to obtain high-resolution SAR images.In order to solve the problem of azimuth-variant error with large maneuverable and long integral aperture,a combination of PPFA and fast factorized subaperture technology is proposed to improve the focusing depth of PPFA and the processing efficiency of FFBPA.Firstly,the PPFA is used to replace the back-projection algorithm(BPA)in FFBPA to achieve efficiency improvement,and then a cascaded coordinate transfer relationship between subaperture is proposed for curved trajectories,which makes fast iteration and coherent summation for curved-track subimages feasible.Considering that the target in the real scene has elevation coordinates,FFPPFA updates the interpolation kernel function in the coarse focusing process.On this basis,the effect of radar observation configuration on the focusing quality is analyzed when the accuracy of the digital elevation map is considered.For the hypersonic radar platform with long-range observation,the low precision DEM-assisted imaging still keeps well-focusing effect,which provides guidance for image identification of the low-load hypersonic platforms.4.The high-resolution fast imaging problem of hypersonic SAR with complex bistatic configuration and curved trajectory is studied.Based on the existing fast back-projection algorithm(FBPA),a fast bistatic parametric polar format algorithm(FBPPFA)is proposed.The bistatic SAR system of hypersonic vehicles combined with satellites,low-speed airborne platforms,and ground fixed stations can improve its concealment and increase the observable range.For heterogeneous complex bistatic configuration SAR systems,the core of FBPPFA is the construction of the bistatic imaging coordinate space.A high-precision bistatic slant range model is established in the bistatic range-ellipsoidal parameter angle virtual coordinate space,according to which the wavenumber interpolation kernel of the proposed algorithm is improved.In order to further improve the azimuth resolution,coarse-focusing subimages are fused in the image domain according to the coordinate transformation between the unified Cartesian coordinates and the local imaging virtual polar coordinates.During the process of subimage fusion,in order to ensure accurate projection and non-aliasing of the spectrum,FBPPFA also considers the geometric deformation of the coarse focusing image.Based on the Nyquist sampling requirements,the wavenumber support region and imaging resolution are also analyzed,which provides a basis for the setting of the imaging grid in the unified Cartesian coordinate system.Finally,FBPPFA is able to continuously improve the resolution of the fused image along with the data acquisition.