Multistatic Synthetic Aperture Radar Autofocus for Back Projection Imaging of a Moving Target

Synthetic Aperture Radar (SAR) plays a vital role in the surveillance of terrestrial and maritime targets, which are commonly in motion. As such, the ability to perform accurate real-time focusing and localisation on moving targets, particularly those moving with complex motion, is desired. Many existing autofocus algorithms struggle to achieve this and rely on sub-aperture processing of SAR data to estimate and compensate for phase errors attributed to unknown target motion. This paper presents a new metric-based autofocus approach, called Localised Threshold Sharpness (LTS), which employs multistatic SAR data to localise and focus a target moving with up to six degrees of freedom motion on a real-time, pulse-by-pulse basis. The algorithm is verified with experimental data, and its performance is compared against the performance of an existing measure of image sharpness suitable for pulse-by-pulse autofocusing, namely the intensity-squared metric, with varying levels of added noise. Normalised cross-correlation results demonstrate a resemblance of at least 80% between Multistatic SAR images focused via LTS autofocus and Multistatic SAR images ideally focused using target motion knowledge for signal-to-noise ratios above 3 dB.