An inverse polar format algorithm for turntable spotlight ISAR imaging systems using stepped frequency waveforms

Basic turntable spotlight inverse synthetic aperture radar (ISAR) systems that employ stepped frequency waveforms implement image formation algorithms based on the premise that data collection over uniform frequency and angle steps results in a rectangular sampling of the image spatial frequency (k-space) domain. As a result, a simple image formation algorithm implementing only a computationally efficient inverse 2D discrete Fourier transform (DFT) may be realized. However, this approach imposes limitations on resolution and/or scene size since the conventional data collection procedure actually results in a polar sampling of k-space, invalidating the rectangular grid assumption. This paper introduces a new data collection scheme using stepped frequency waveforms that are nonuniformly stepped in time and frequency so as to collect sample points according to the desired k-space shape. This procedure allows the use of a single inverse 2D DFT as the image formation algorithm, thereby reducing traditional constraints on resolution and scene size while maintaining good image focus and reducing computational complexity.