I. INTRODUCTION

SAR Interferometry and Surface Change Detection

It is now possible to detect subtle changes in the Earth's land and ice surfaces over periods of days to years with a scale (global), accuracy (millimeters), and reliability (day or night, all weather) that are unprecedented. The technique involves interferometric phase comparison of successive synthetic aperture radar (SAR) images. Recent examples illustrate how SAR interferometry can be applied to the study of glaciers, earthquakes, and volcanoes. Goldstein et al. (1993) measured ice-stream velocity in Antarctica using Earth Resource Satellite (ERS-1) SAR images taken six days apart. It was the first time ice-stream velocity had been measured directly from space without ground control points. Massonnet et al. (1993) measured coseismic displacement associated with the Landers earthquake based on ERS-1 images taken several months apart. More recent results suggest that the changes are detectable in images taken up to 14 months apart (Massonnet et al., 1994) even without preexisting topographic data (Zebker et al., 1994a). These were the first complete pictures of earthquake-related deformation of the Earth's surface. Massonnet et al. (1995) measured surface deforma-tion at Mt. Etna, an active volcano, with SAR interferometry. Since surface deformation may give clues to future eruptions at active volcanoes, the ability to make these measurements from space without ground control is a fundamental advance in our monitoring capability. These pioneering studies have generated enormous interest in the Earth science community because they point to an entirely new way to study the surface of the Earth.

SAR interferometry can also generate very-high-resolution topographic maps. It is the key technique in a space mission currently under study by NASA to generate global high-resolution digital topographic data, with a 30-m horizontal resolution and height accuracy of several meters. Over longer time scales of several years or more, high-resolution topographic data can also be used for large-scale change detection by comparing elevations at different times. This technique allows changes in glaciers and ice margins to be assessed over long periods and any catastrophic topographic change to be measured (e.g., volcanic eruptions, landslides, and major floods).

Many scientists are unfamiliar with SAR interferometry and its potential new applications, and its technical limitations need to be more fully explored. Thirty-nine scientists representing a broad range of Earth science disciplines met in Boulder, Colorado, on February 3-4, 1994 to consider existing and new applications of SAR interferometry and recommend ways to enhance the collection and distribution of data. After some general sessions, which included a tutorial on the technique and a description of existing and planned radar missions, attendees divided into six discipline-specific groups (volcanoes; earthquakes and faulting; mountain building and erosion; ice sheets and glaciers; hydrology, ecosystem studies, and environmental monitoring; and technology) to discuss applications and make recommendations. Each group drafted an initial set of recommendations at the workshop for general discussion, then worked on more detailed plans for the next 12 months.

This report summarizes the discussion and conclusions of each group and resulting recommendations. It includes a discussion of the theory and practice of SAR interferometry in the general areas of topographic mapping and change detection, major scientific applications, errors and limitations of the technique, and recommendations on collection and distribution of SAR interferometry data using existing data systems. Last but not least, it includes a recommendation and scientific rationale for a dedicated SAR mission, optimized to detect changes on the Earth's land and ice surfaces, to be launched by the turn of the century.