Building an Integrated View of Antarctica
by Kenneth C. Jezek, The Ohio State University
NASA and the Canadian Space Agency began planning in the early 1980s for an imaging campaign using Synthetic Aperture Radar to be carried by RADARSAT-1. The technical goals set forth in the early planning were to obtain two complete mappings of Antarctica. These would result in the first, high-resolution radar maps and provide nearly instantaneous snapshots of the icy continent.
The science driving the technical goal was to obtain benchmarks for detecting changes in the continent by comparison with earlier and subsequent data. Radar interferometry for ice sheet studies was proven with the European Earth Remote Sensing Satellite, but the Antarctic Mapping Mission (AMM) was the first dedicated use of RADARSAT-1 for radar interferometry for large-scale studies.
Preparing for the campaigns, organized under the RADARSAT-1 Antarctic Mapping Project (RAMP), took years of effort. The first AMM commenced in September 1997. During this phase, the primary mapping goal was achieved, capturing an extraordinary view of Antarctica that has been widely distributed to the science community in digital and hard-copy form.
The campaign also acquired 24-day, repeat-pass data suitable for interferometric analysis. Repeat-pass interferometry allows the computation of sub-wavelength scale estimates of surface displacement. Consequently, the surface velocity of the ice sheet can be calculated with such data from the ice divides to the ice margin.
The second campaign to image Antarctica began in September 2000 with a modified set of technical and scientific goals. Based on the quality of the AMM image map, there was great interest in securing a second map that could be used to measure changes in the coastal areas of Antarctica and changes in fast glaciers and ice streams. Because of the proven ability to acquire interferometric data, there was also great interest in using RADARSAT-1 to map as much of the surface velocity field as possible.
Three complete repeat mappings were acquired with each mapping containing complete ascending and descending data, the coverage extending from about 80 degrees south to the coast. The acquisition phase, known as the Modified Antarctic Mapping Mission (MAMM), was completed successfully. While the analysis of this large and complex data set is ongoing, the results are already proving to be of exceptional scientific interest.
The RAMP Antarctic image mosaic shown on this page was created using map attributes of high resolution and continental coverage. The mosaic has been used to: map the coastline of the continent in great detail; study and contrast glaciological regimes about the continent; and examine glaciological processes, such as the evolution of ice shelves, by observing ice shelves at various stages of development about the continent.
A single instrument or data set alone is rarely able to answer complex scientific questions, so it became useful and very interesting to use the RAMP mosaic as a basis for integrating other continental scale observations into a common framework referenced to features observable on the surface. We have assembled several continental data sets, including velocities computed from the MAMM data set, into a Geographic Information System format. In each case, the base map for the image is the AMM mosaic.
We examined surface topography patterns using the Digital Elevation Model developed as part of RAMP. The figure at the top right, on the next page, shows surface elevation expressed as colored tones overlaying the mosaic that governs the brightness. This image clearly shows the relationship between the ice divides which partition the ice sheet into different catchment basins and well defined tonal patterns in the SAR mosaic. The physical link between ice divide and backscatter remains unknown, but the obvious correlation provides useful information on the detailed positions of ice divides.
We next compare the RAMP mosaic with the BEDMAP compiled, basal topography shown in the center of the figure on this page. Good correlations between the basal topography and backscatter strength are notable in the vicinity of the Belgica Highlands (lower right quadrant). These results are exciting, while not completely unexpected, since established theory relates basal topography, surface topography, and accumulation patterns.
The correlations found between basal topography and backscatter strength suggest that inferences about basal topography and properties can be made using the image mosaic in regions where basal topography data are sparse or completely absent.
Surface velocity is a key, kinematic variable for estimating ice sheet mass balance and is a diagnostic indicator of the forces governing icesheet dynamics. A comparison of the RAMP model of surface balance velocities and the mosaic is shown at the bottom on this page. Balance velocities are computed using information on accumulation rate, ice thickness and surface slope, under the assumption that the ice sheet is in mass balance. The velocity patterns capture the extensive network of ice streams draining into the Filchner Ice Shelf (upper left of center quadrant).
These ice streams are revealed in the RAMP mosaic by the intense crevassing that occurs along their margins and which appears bright in radar imagery.
The RAMP balance velocity model becomes more interesting when compared to measured velocities. Statistically significant differences can be attributed to places where the ice sheet is in fact either thickening or thinning. A comparison between the balance velocity model and MAMM interferometrically derived surface velocities reveals that the balance and measured velocities agree generally between the two, but what is especially interesting is the presence of numerous small ice streams or outlet glaciers revealed in the MAMM data and that snake through the area north of the Amery Ice Shelf.
Continental scale, high-resolution data sets of Antarctica are becoming available from a host of satellites carrying on board a suite of sensors that span the electromagnetic spectrum. New sensors are being planned or are already on the way that will greatly improve existing measurements, and some novel instruments are now being designed to acquire key geophysical observations of Antarctica’s physical properties from space for the first time.
Continuing the acquisition and compilation of these data into the future will provide a crucial fourdimensional look at Antarctica, and indeed, our entire home planet. We need such a look to understand how our planet is changing and what role we play in that change.
Acknowledgements: The RADARSAT Antarctica Mapping Project is a collaboration between the Canadian Space Agency and NASA. Processing and analysis of the RAMP data is supported by a grant from NASA’s Pathfinder Program and Polar Oceans and Ice Sheets Program. RAMP participants include the Jet Propulsion Laboratory, the Alaska Satellite Facility, Vexcel Corporation and The Ohio State University.