3D SAR beamforming under a foliage canopy from a single pass

Coherent detection of changes on the ground under a forest canopy by repeat-pass synthetic aperture radar (SAR) imaging is problematic due to the mixture of ground and canopy responses. 3D SAR imaging, by beamforming data from multiple low-frequency across-track acquisitions, offers a way to separate the forest components in height. However, data acquired from multiple passes require precise registration of the flight tracks, which is often not possible, particularly for small airborne platforms. This study analyses the potential for SAR imaging of the ground under a forest canopy using data from a single pass of a multichannel across-track radar system. We focus in particular on the case of a two-channel alternating-transmit interferometer giving three effective input channels. 3D image formation in one pass by phase-preserving adaptive beamforming is shown to provide sufficient attenuation of the interference from a model forest volume to permit a reasonable estimation of the ground coherence across two passes for coherent change detection.