A Less-Memory and High-Efficiency Autofocus Back Projection Algorithm for SAR Imaging

The back projection (BP) algorithm is an accurate time-domain imaging method for synthetic aperture radar. However, the influences of wind field and turbulence on the platform make the antenna phase centers (APCs) greatly deviate from the designed linear trajectory and degrade the BP performance. Although the inertial measurement unit can be used to measure the trajectory, the measurement error still affects the image quality to some extent. The autofocus BP algorithm under the criterion of maximum image sharpness has been proposed to compensate the motion error effectively. However, this method needs to compute and store the per-pulse back-projected values for all pixels, which results in the heavy burden for memory and time and limits its practical application. This letter demonstrates an improved way to overcome these two drawbacks without loss of focusing performance. In our new method, only minority pixels are selected for autofocus to estimate the phase error, which is used to obtain the APCs by solving a system of nonlinear equations with an optimization method. This procedure is particularly suitable for the wide application of the autofocus BP. The experimental results strongly validate the efficacy and efficiency of the improved autofocus BP algorithm.