Wavefront Curvature Correction for Radar Imaging in a Degraded Visual Environment

One of the primary challenges in helicopter flight is landing in a degraded visual environment (DVE). In a DVE, the situational awareness of the pilot is inhibited by natural phenomena such as rain, snow, fog, etc., or aircraft induced phenomena such as brownout or whiteout. Typically, the pilot is assisted by feedback from a lidar system, but if the particles are dense enough, the lidar is unable to provide useful information about the terrain and other obstacles. Nonetheless, radar can be used to create useful imagery of the surrounding area, albeit at a reduced resolution, as it has the ability to penetrate precipitation, dust, and other obscurants. In this paper, a forward-looking synthetic aperture radar (FLSAR) concept, which can form three-dimensional (3-D) imagery from a 1-D array, is proposed. A frequency domain imaging algorithm, the polar format algorithm (PFA), is investigated for its applicability to the FLSAR geometry. We show that a wavefront curvature correction (WCC) procedure is required to compensate for the far-field approximation made in the PFA which is not valid at the frequencies and operational ranges under consideration. A filter transfer function for WCC for the FLSAR geometry is derived. Finally, the effectiveness of the derived filter transfer function is demonstrated in simulated 2-D imagery.