UAV-Based SAR-Imaging of Objects From Arbitrary Trajectories Using Weighted Backprojection

Abstract

Synthetic aperture radars (SARs) based on uncrewed aerial vehicles (UAVs) are advantageous in comparison to existing airborne systems. Apart from cost, their main advantage is the flexibility of their flight path. It can be optimized specifically for every application, which is not possible to this extent with other airborne systems. Both the distance between radar and targets as well as the viewing angle of the radar change significantly during flight. A common imaging algorithm for close-range and nonlinear flightpaths is therefore backprojection (BP), since it respects the nonlinear flightpath. The remaining effects, e.g. caused by changes in elevation or squint angle, are then commonly compensated for with the goal of consistent image brightness as well as low sidelobes. In this work, a weighted BP for UAV-based imaging of objects is proposed. The presented method assumes a horizontal ground surface with approximately constant properties over the imaging area. A full system model is introduced, including system effects as well as geometric effects and properties of the ground surface. Based on this model, the expected signal-to-clutter ratio (SCR) of any point target in a single measurement can be predicted. This allows weighting of the contributions with the goal of maximizing target contrast in the synthetic aperture radar (SAR) image. Using UAV-based SAR measurements, it is shown that significant improvements in imaging quality can be achieved by employing the proposed method.