Stepped-Frequency Synthetic Aperture Radar Imaging via Polar Format Algorithm

Employing the stepped-frequency waveforms is an efficient way to improve the range resolution of synthetic aperture radar (SAR) retaining the benefits of low instantaneous bandwidth. A conventional stepped-frequency waveform used in high-resolution SAR is stepped-frequency chirp signal (SFCS). Range sidelobe suppression of SFCS can be improved if minor frequency overlap between pulses is introduced. Therefore, the need for SAR image formation algorithm applicable to processing overlapped SFCS arises. In this paper, polar format algorithm (PFA) implementation incorporating SFCS processing is presented. It is demonstrated that image formation procedure comprises 2-dimensional interpolation of a function sampled on a composite nonuniform grid, consisting of several overlapping subgrids. Algorithm imposes no restrictions on SAR trajectory and treats spatial-variant range migration between pulses on different carrier frequencies. Detailed analytical description is provided, as well as simulation results, verifying the effect of the algorithm.