Research On Ship Target Imaging Method Of Squint Sliding Spotlight SAR Systems

The squint sliding spotlight mode is a new Synthetic Aperture Radar(SAR)operating mode.This mode can realize continuous target imaging on rear side,side-looking and front side by changing the antenna squint angle constantly.Thus lots of the same target's highresolution microwave images at different squint angles can be obtained in a short time.It is extremely beneficial for target classification and recognition.At present,the mode is mainly used to imaging in land scene.In this paper,the mode is applied to maritime ship imaging.SAR systems are specifically designed for static scene imaging.The images of moving ships typically appear defocused when they are directly obtained by SAR algorithms.Inverse Synthetic Aperture Radar(ISAR)techniques can deal with moving targets in those cases where SAR imaging fails.Hybrid SAR/ISAR processing is a common method for ship imaging.Therefore,the imaging procedure of ship targets in this paper is organized as follows.Firstly,marine scene images are obtained by squint sliding spotlight SAR systems.Then the sub-images containing ships are cropped and the equivalent ISAR echo is inverted by twodimensional inverse Fourier transform.Finally,refocused ship images are obtained by ISAR imaging algorithms.Due to the low efficiency of existing imaging algorithms of squint sliding spotlight SAR systems,an extended two-step processing algorithm based on the Chirp-Z transform is put forward in this paper.The algorithm does not need Stolt interpolation that is time consuming,and thus it significantly improves the image processing efficiency.Imaging results from simulated experiments prove the correctness of the proposed algorithm,and the obvious improvement in the computation efficiency compared with existing approaches.In view of the low efficiency of existing ISAR imaging methods in choosing time window,a novel time selection algorithm based on the time-frequency analysis is proposed in this paper.The algorithm firstly selects an appropriate range bin to perform frequency analysis,and then fits the curve of the amplitude spectra to sinusoid.Finally,the length and the center moment of the time window are calculated by the derived equations.Compared with the commonly used time window selection algorithm based on the optimal Image Contrast based autofocusing(ICBA),the proposed algorithm does not need multiple alternating iterative search,and thus it improves the computation efficiency significantly.Imaging results from simulated and real data prove the correctness of the proposed algorithm.The significant improvement in the computation efficiency compared with existing approaches is also validated by the performed experiments.The proposed two algorithms are combined,including the extended imaging algorithm of squint sliding spotlight SAR and the novel ISAR imaging method.The echo simulation of ship targets and imaging processing experiments under the squint sliding spotlight SAR mode are conducted.Clear ship images at various squint angles are obtained from simulated experiments.It proves the unique advantage of applying squint sliding spotlight SAR mode to ship imaging.