Study On High-squint Wide Swath SAR Imaging Technology Of Hypervelocity Maneuvering Platforms

Synthetic aperture radar(SAR)imaging can obtain high-resolution microwave remote sensing images similar to optical photography under complex meteorological conditions.And it has the ability of camouflage recognition and covering penetration,which has been widely used in military and civil fields.The maneuvering platform SAR can realize battlefield reconnaissance and surveillance,and target recognition and strike in complex environments.At present,the maneuvering platform SAR shows the development trend of higher platform speed,wider observation swath and more flexible flight trajectory.The new development characteristics have also brought many new problems,making the conventional SAR imaging algorithm no longer applicable.This dissertation focuses on the motion characteristics of maneuvering platform and the application requirements of SAR imaging detection in the future.Based on the real-time processing framework of smallaperture imaging and multi-aperture image synthesis,the key problems in high squint wide swath area SAR imaging of hypersonic maneuvering platform are studied.The main research contents of this dissertation are summarized as follows.Aiming at the difficulties of maneuvering trajectory and high squint beam steering,Chapter2 focuses on the research of system timing sequence design and signal azimuth spatial variation correction,which lays a foundation for the subsequent multi-mode wide swath imaging processing of high squint maneuvering platform SAR.First of all,from the perspective of practical resolution requirements,the accurate analytical parameters of the resolution ellipse of ground plane are derived.According to the criterion of uniform distribution of ground resolution,the synthetic aperture time and beam steering speed under different SAR modes are designed.In order to solve the problem of azimuth spatial variation in high squint maneuvering platform SAR data,an azimuth spatial variation correction method based on Doppler resampling is proposed in this part.To adapt to the coordinate rotation caused by the linear range walk correction(LRWC)operation,an orthogonal coordinate slant range modelling method is proposed.In this model,the azimuth spatial variation characteristics of Doppler parameters are analyzed in detail.Then,the spatial variation components of the azimuth phase coefficients are corrected by Doppler resampling in the azimuth frequency domain.Finally,the unified focusing is completed by SPECAN operation.Combined with the azimuth block operation,this method can effectively solve the signal spatial variation problem of high squint maneuvering platform SAR imaging in wide swath scenes.Due to the limitation of beam width,the azimuth illumination swath of a single beam in a short time is limited.In order to realize azimuth wide swath observation of small aperture data,the direction and speed of beam scanning in TOPS SAR mode of maneuvering platform based on beam steering are designed in Chapter 3.Aiming at the problem of signal spectrum aliasing caused by beam steering,a focusing method based on joint time-Doppler deramp(JTDD)is proposed in this chapter.Firstly,according to the beam steering characteristics of maneuvering platform TOPS SAR,the corresponding concentric circle range model is established.On this basis,the time-frequency characteristics of the echo signal are analyzed,and then a modified linear deramp operation in the azimuth time domain is proposed to obtain the 2-D spectrum without aliasing.Then,the range cell migration(RCM)is corrected in the non-aliasing frequency domain after recovery.The time-frequency conversion in time domain deramp will lead to time domain expansion and signal time domain aliasing.To solve the problem,a nonlinear deramp function in the Doppler domain is proposed to obtain the aligned time-frequency lines without aliasing in time domain.Meanwhile,the frequency domain NCS function is introduced to correct the spatial variation Doppler parameters.Finally,the signal can be focused in Doppler domain by SPECAN technology.This method can realize azimuth wide swath SAR imaging of maneuvering platform with limited single beam illumination swath.With the continuous increasing of maneuvering platform flight speed and signal instantaneous bandwidth,the contradiction between range ambiguity and azimuth ambiguity is irreconcilable.Azimuth multichannel SAR can avoid range ambiguity and achieve wide range swath imaging without range ambiguity.However,the channel array vector of maneuvering platform multichannel SAR is time-variant and inconsistent with the radar velocity vector,which leads to irregular space-time spectrum and brings difficulties to signal reconstruction.For this issue,Chapter 4 proposes a high-squint multichannel SAR imaging algorithm for high speed maneuvering platform.Firstly,a space-time spectrum correction function is proposed to correct the irregular space-time spectrum.Then,the unambiguous Doppler spectrum can be reconstructed by an improved range-dependent spatial domain filter,and the equivalent high PRF sampled signal can be obtained.To solve the spatial variant problem of wide range swath SAR data,an improved Omega-K algorithm is proposed to complete the RCM correction(RCMC).However,the deramp operation before Stolt mapping will cause time domain aliasing of small aperture data.To solve this problem,a time-frequency reconstruction method based on time domain spectrum compression is proposed,which can realize accurate focusing without the zero-padding process.This method can avoid range ambiguity and obtain a well-focused SAR image.Chapter 5 focuses on the study of remotely observation of sea sparse targets by hypersonic single-channel SAR.On the condition of remote detecting sea targets with a small grazing angle,the systematical 2-D ambiguity is an urgent problem due to the minimum antenna area constraint.This chapter presents a sea target focusing and positioning method based on 2-D ambiguity resolving for single-channel SAR.The high system ambiguity degree caused by the minimum antenna area constraint is analyzed at first.For this issue,an optimal-PRF SAR concept is proposed,where the signal ambiguity is evenly distributed to the range and azimuth direction by designing PRF.For the range ambiguity,an ambiguity suppression method based on the orthogonal phase-coded waveform is proposed.In this method,the emission timing sequence of the orthogonal phase-coded waveform is designed to make the target echoes in different ambiguity regions orthogonal.Therefore,the energy in the ambiguity regions can be suppressed by orthogonal matching filtering.Aiming at the azimuth ambiguity problem of small aperture data,a target refocusing and positioning method based on Doppler ambiguity resolution is proposed.The Doppler ambiguity number can be estimated by using the residual envelope migration of the ambiguity target,and then the accurate focusing and positioning of each target can be realized.This method can realize the focusing and positioning of sea sparse targets at the wide swath for the single-channel SAR mounted on hypersonic maneuvering platform.