| As a microwave active imaging system,synthetic aperture radar(SAR)can obtain two-dimensional(2-D)high-precision imagery of targets interested in different areas,which greatly improves the information acquisition and perception capabilities of traditional radars,and has been demonstrated great value in a myriad of application fields.The SAR imaging guidance technology that combines SAR and a maneuvering platform(MP)can enhance the autonomy and anti-jamming capability of a seeker precision guidance in complex electromagnetic environments and has become an important research hotspot.However,the traditional imaging methods are no longer applicable in this scenario because of the flexible and variable motion trajectory of MP,early observation for the squinted region,and the requirements on acquiring different resolution imagery.Therefore,this dissertation studies several key issues in the imaging algorithm for an MP-mounted SAR with a curved trajectory,considering the characteristics of the curved trajectory of the MP and the different requirements on the image resolution and swath in different scenarios.Major progress and accomplishments are summarized in the following:?.A two-step equivalent slant-range model is configured for the MP-mounted SAR imaging with a curved trajectory because the traditional azimuth shift-invariant characteristics are no longer available in the presence of three-dimensional components in both velocity and acceleration.The model is universal and can be applied in arbitrary imaging mode and flight mode.Based on this model,the calculation methods for some pivotal parameters in the MP-mounted SAR imaging procedure with a curved trajectory are discussed,which lays the foundation for obtaining well-focused images.Besides,the data acquisition plane in the MP-mounted SAR imaging is rotated compared to that in a traditional SAR imaging method,and therefore it is necessary to obtain a near distortion less ground image through geometric correction.Because measurement errors existing in the motion parameters will affect the effectiveness of correction,therefore,this dissertation analyzes the effect of errors in different parameters on the distortion of the corrected image,which can be used as a reference in further work to optimize system parameters.?.Traditionally,the pulse repetition frequency(PRF)for an MP-mounted SAR with a curved trajectory is independent of the platform height and is set to a higher value to take care of different platform heights,which leads to an increment in data acquisition amount.This dissertation proposed a universal platform-height dependent PRF design method for the MP-mounted SAR based on the concept of constant multi-distance rings,which is introduced to calculate the bandwidth caused by the offset of center Doppler frequency.Thus,the lower limit of PRF is obtained for a given platform height by taking the system and motion errors into account.Then,dividing the platform height into segments,we can dynamically select PRF for the current height segment to relieve the burden in echo data acquisition with fewer PRF adjustment times.By suitably adjusting some of the parameters,the proposed method can be applied to different scenarios,including different platform motion modes such as horizontal flying or diving,different motion trajectories such as straight line or curved,different beam steering angles such as side-looking or high-squint,and different imaging modes such as spotlight or strip-map.?.2-D space variance appears in both the range envelope and azimuth phase in the high-squint sub-aperture imaging for an MP-mounted SAR with a curved trajectory when there exist severe range-azimuth coupling,range-dependent squint angle,and/or three-dimensional components in its velocity and acceleration.Therefore,a two-step frequency filtering approach is proposed to correct the 2-D space variance in the diving high-squint imaging.Firstly,a preprocessing is applied to reduce the severe 2-D coupling and compensate for the influence of acceleration on the 2-D frequency spectrum.Then,a unified Range Cell Migration(RCM)correction is performed by using a first-stage frequency filtering(FsFF)approach through constructing a space-range envelope correction factor to mitigate the azimuth-dependent RCM.Lastly,a second-stage frequency filtering(SsFF)approach,by constructing an azimuth-dependent phase correction factor to correct the spatial Doppler modulation frequency and the third-order phase coefficient,which can improve the azimuthal focusing depth and complete the MP-mounted high-squint sub-aperture SAR imaging with a curved trajectory.?.In the MP-mounted sliding-spotlight SAR imaging with a curved trajectory,the beam steering will cause the azimuth bandwidth of echo signal to exceed the PRF width,which leads to an aliasing phenomenon in the 2-D spectrum.Moreover,the space variance in the range envelope and azimuth phase will severely influence the image focusing quality in the MP-mounted high-squint sliding-spotlight SAR imaging with a curved trajectory.Therefore,a new hybrid method that combines the modified chirp scaling algorithm(CSA)and time filtering processing(TSF)is proposed for the full-aperture high-squint sliding-spotlight SAR imaging in a MP-mounted SAR with a curved trajectory.The echo characteristics of the sliding spotlight SAR with a curved trajectory is analyzed,and the calculation methods of some pivotal parameters are discussed as well.Then,a preprocessing is applied to the azimuthal signals with spectral ambiguity,which greatly reduces the influence of high-squint,acceleration and beam steering on 2-D spectrum.And then,a modified CSA is proposed to eliminate the azimuth-dependent space-variance of RCM caused by the space-variance of the Doppler parameter,in order to realize the unified RCM compensation.Lastly,a TSF is introduced to correct the spatial variation of the residual acceleration and Doppler coefficients with the azimuth position,and the unified azimuthal focusing is completed based on the spectral analysis. |
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