Squint Spotlight SAR Imaging by Two-Step Scaling Transform-Based Extended PFA and 2-D Autofocus

Abstract

In this article, a novel imaging algorithm by combining two-step scaling transform (TSST) with structure-aided 2-D autofocus is proposed for the squint spotlight synthetic aperture radar (SAR). First, on the basis of planar wavefront assumption, a modified range-frequency linear scaling transform (MRFLST) and an azimuth-time nonlinear scaling transform (ATNST) are proposed to eliminate the coupling between range-frequency and azimuth-time of the received echo. Furthermore, to improve the efficiency, the MRFLST is implemented by using the principle of chirp scaling (PCS), which involves only complex multiplications and fast Fourier transforms (FFTs) without any interpolation, meanwhile, a constant scaling factor (CSF) selecting criteria is defined to avoid range spectrum aliasing. Then, to correct the phase error caused by the range measurement error and atmospheric propagation effects, the prior 2-D phase error structure implied in the TSST is analyzed. Finally, by integrating the derived 2-D phase error structure and range frequency fragmentation technique, a new 2-D autofocus algorithm is presented to improve the image quality. Simulated and real data experiments are carried out to verify the proposed algorithm. Index Terms-2-D autofocus, planar wavefront assumption, principle of chirp scaling (PCS), range frequency fragmentation, two-step scaling transform. I. INTRODUCTIONB ENEFITING from the unique all-weather and all-time detecting ability, synthetic aperture radar (SAR) becomes the popular and extensively adopted means in supervising and imaging interesting areas [1]. The spotlight mode SAR can provide us with high resolution through continuously adjusting the antenna beam of the radar pointing to a preselected district in time of the echo acquisition [2].The coherence of data processing over the whole aperture synthesis period is the assurance for getting a well-focused imaging result and the imaging processes require precise measuring positions of the radar footprints. However, the ubiquitous Manuscript

A Prior 2-D Autofocus Algorithm With Ground Cartesian BP Imaging for Curved Trajectory SAR

“…After the above two step preprocessing, the 2-D phase error can be computed and corrected in the spectrum domain. In this algorithm, according to the known analytical structure of (35), only a simple APE estimation processing is required to achieve a 2-D phase error estimation processing. APE estimation processing can be realized by traditional 1-D autofocus algorithms.…”

“…, and then the 2-D phase error can be obtained according to the analytic mapping relationship in (35) ( )…”

“…A Prior 2-D Autofocus Algorithm with Ground Cartesian BP Imaging for Curved Trajectory SAR Nowadays, an important prerequisite for efficient autofocus algorithms to refocus defocused images is the spectrum alignment of all targets in the image [32]- [35]. [43] proposes an autofocus algorithm based on the combination of the multi-aperture mapdrift method and PGA for ultra-high resolution stripmap SAR.…”