Focus Improvement for Squint FMCW-SAR Data Using Modified Inverse Chirp-Z Transform Based on Spatial-Variant Linear Range Cell Migration Correction and Series Inversion

Two key unsolved problems faced by frequency-modulated continuous-wave (FMCW) synthetic aperture radar (SAR), namely, spatial-variant linear range cell migration (RCM) and range frequency aliasing in a traditional frequency scaling (FS) algorithm, are mitigated in this paper. For FMCW-SAR platform, it usually operates at low altitude space for which the squint angle is range dependent. This range-dependent squint angle in return causes serious spatial-variant linear RCM that deteriorates image quality if it is not properly handled. To focus squint FMCW-SAR, FS is normally utilized. This operation, however, creates an issue, range frequency aliasing that needs to be addressed as well. To overcome these two problems, in this paper, based on the modified inverse Chirp-Z transform (ICZT) and series inversion, a novel imaging algorithm performing the spatial-variant linear RCM correction and eliminating the range frequency aliasing for focusing squint FMCW-SAR is proposed. First, the influences of the range-dependent squint angle on imaging quality are analyzed. Based on the obtained analysis result, a new scaling factor containing the linear RCM spatial variance correction is designed, and then, an efficient modified ICZT is developed to remove the range-dependent RCM and frequency aliasing. As a result, the well-focused SAR image is produced. The proposed method can be efficiently implemented, since it only requires fast Fourier transform and no interpolation is demanded. Finally, the experimental results with simulated data demonstrate the effectiveness of the proposed algorithm.