A Wideband Direction of Arrival Technique for Multibeam, Wide-Swath Imaging of Ice Sheet Basal Morphology

Abstract

Multichannel, ice sounder data can be processed to three-dimensionally map ice sheet bed topography and basal reflectivity using tomographic imaging techniques. When ultra-wideband (UWB) signals are used to interrogate a glaciological target, fine resolution maps can be obtained. These data sets facilitate both process studies of ice sheet dynamics and also continental-scale ice sheet modeling needed to predict future sea level. The socioeconomic importance of these data as well as the cost and logistical challenge of procuring them justifies the need to image ice sheet basal morphology over a wider swath. Imaging wide swaths with UWB signals poses challenges for the array processing methods that have been used to localize scattering in the cross-track dimension. Both MUltiple SIgnal Classification (MUSIC) and the Maximum Likelihood Estimator (MLE) have been applied to the ice sheet tomography problem. These techniques are formulated assuming a narrowband model of the array that breaks down in wideband signal environments when the direction of arrival (DOA) increases further off nadir. The Center for Remote Sensing of Ice Sheets (CReSIS) developed a UWB multichannel SAR with a large cross-track array for sounding and imaging polar ice from a Basler BT-67 aircraft. In 2013, this sensor collected data in a multibeam mode over the West Antarctic Ice Sheet to demonstrate wide swath imaging. To reliably estimate the arrival angles of echoes from the edges of the swath, a parametric space-time direction of arrival estimator was developed that obtains an estimate of the DOA by fitting the observed space-time covariance structure to a model. This thesis focuses on the development and optimization of the algorithm and describes its predicted performance based on simulation. Its measured performance is analyzed with 3D tomographic basal maps of an ice stream in West Antarctica that were generated using the technique.