Synthetic aperture radar for imaging the basal conditions of the polar ice sheets.

摘要

The far-reaching impacts of future global climate change have driven both the international science community and intergovernmental cooperation to look for predictive capability of both climate-change and its impacts. One area of particular interest is sea level rise and its relationship with the massive polar ice sheets of Greenland and Antarctica. In the last several years, a number of alarming changes have been seen along the outlet glaciers and melt regions of Greenland and the outlet glaciers of the Antarctica Peninsula. The concern is that the steady-state models used in the global climate models to not include important ice dynamics that are now known to occur. Glaciologists in the last few decades have begun grappling with the problem of modeling these dynamics, including basal melting and sliding, till deformation, and ice buttressing. However, much of the work is restricted by the very limited knowledge of the basal conditions.; There are several methods that can be used to observe the basal conditions of an ice sheet. Brute-force methods that involve drilling a borehole to the bottom of the ice sheet are infeasible for wide-area coverage because the resources and time consumed for each borehole are tremendous. Even seismic experiments, which can provide better coverage, require extensive preparation and can only be conducted from the ground. Radar, and specifically side-looking synthetic aperture radar (SAR), on the other hand provides a flexible remote sensing technique that can work with very little field preparation (in comparison to seismology and drilling) and can also be installed on an airborne or space-borne platform. By producing a radar capable of producing SAR images of basal backscattering, we can bring to bear the vast literature base and toolsets developed for traditional SAR applications. To this end, the specification, design, development, fielding, and data processing of a multi-purpose VHF radar for radioglaciological work is described in this work. The purposes of the radar and subsequent data processing are to produce a map of backscatter from the basal interface, map ice thickness, and track internal layers.; The approach taken here starts with the development of an electromagnetic propagation model. This model is used to derive the radar's system specifications such as band of operation, loop sensitivity and dynamic range. It is also used in by the imaging routines for phase-history reconstruction. The system architecture chosen is a ground-based side-looking SAR operating from 120-300 MHz with VV polarization. Ground-based operation is ideal for testing purposes and very few system modifications are needed to convert to an airborne platform. Data conditioning of radar and trajectory data for the imaging algorithms is then detailed. Finally, results from both f-k migration and time-domain correlation imagers are presented and analyzed for a 6.5 km by 25 km area near Summit Camp, Greenland (72.5667° N, 38.4833° W, ∼3200 m). This work represents the first successful application of side-looking SAR to the thick ice found in the ice sheets.