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 I 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 2-D 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 non-uniformly 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 2-D DFT as the image formation algorithm, thereby reducing traditional constraints on resolution and scene size while maintaining good image focus and reducing computational complexity.