3D image autofocus for millimeter-wave forward-looking SAR

Abstract

The U.S. Army Combat Capabilities Development Command (DEVCOM) Army Research Laboratory (ARL) has been investigating the capability of using 3-D millimeter-wave synthetic aperture radar (SAR) technology to provide navigation for aircraft through terrain and obstacles in a degraded visual environment (DVE). One of the key challenges associated with this technology is the focus quality of the 3-D millimeter-wave SAR image, which requires very-high-accuracy data from the positioning measurement system for the platform motion in the 3-D space. The level of accuracy needed is currently not achievable by state-of-the-art positioning measurement systems. Thus, an effective autofocus algorithm is critical for the success of this program. In this paper, we propose an image metric-based autofocus algorithm to address the SAR image focusing challenge associated with the Ka frequency band, our MIMO radar configuration and forward-looking 3-D geometry. The algorithm models the errors in the measured radar positioning data as the rotation and translation parameters of the MIMO antenna frame in 3-D space. The algorithm solves an optimization problem that maximizes the specified SAR image metric. We evaluate the proposed algorithm and compare its performance with the Phase Gradient Autofocus (PGA) algorithm using both linear side-looking SAR and our proposed forward-looking 3-D SAR geometries.