Changes in the area and volume of the two polar ice sheets in Antarctica (fig. 1) and Greenland are intricately linked to changes in global climate and could result in sea-level changes that would severely affect the densely populated coastal regions on Earth. Melting of the West Antarctica part of the Antarctic ice sheet alone could cause a worldwide sea-level rise of approximately 6 m. The potential sea-level rise after melting of the entire Antarctic ice sheet is estimated to be 73 m. In spite of its importance, the mass balance (the net volumetric gain or loss) of the Antarctic ice sheet is poorly known; it is not known whether the ice sheet is growing or shrinking. As a result, measurement of changes in the Antarctic ice sheet has been given a very high priority in recommendations by the Polar Research Board of the National Research Council, by the Scientific Committee on Antarctic Research (SCAR), and by the National Science Foundation's Office of Polar Programs.