Satellite Radar Unveils Subtle, Slow Wrinkling Of the Planet's Surface
By WILLIAM J. BROAD
Published: September 3, 1996
THE surface of the earth is alive with motions too subtle for direct human observation, the slow wrinkling of the planetary skin, that pose considerable risks to cities and people. Slow shifts of the earth's plates regularly build up strains that cause earthquakes. Volcanoes swell before they explode. Glaciers move and polar ice packs break up as the earth warms, raising seas and threatening to flood coastal areas.
Since the birth of modern science, such motions have been known to exist but have been excruciatingly hard to monitor on a regular basis, which is the kind of surveillance needed for thorough study and hazard warning.
Now, however, a new kind of space radar is beginning to map the earth in stunning topographic detail and to reveal a wide range of previously hidden creepings, swellings and subsidences. With at least four satellites in orbit and more planned, the results of this mapping are exciting scientists around the globe and energizing a number of fields, including geology, climatology, hydrology, ecology and hazard prediction.
''It's a great leap forward,'' said Dr. Howard A. Zebker, a geophysicist and electrical engineer at Stanford University who helped develop the technique. ''It's like suddenly having X-rays to see inside a body.'' He said it was now possible to see things happening on the earth's surface that were previously hidden from scientific view.
The method goes by the tongue-twisting name of synthetic aperture radar interferometry.
To date, it has produced spectacular images of slight but significant changes in places like Landers, Calif., where a 1992 earthquake shattered the high desert east of Los Angeles; Kobe, Japan, where a 1995 earthquake killed thousands of people; Mount Etna, Sicily, where an eruption between 1991 and 1993 unexpectedly shrank the old volcano; the ice fields of Chile, where a glacier surged forward in 1994; and Antarctica, where a huge ice floe that may have have been produced by global warming was monitored in 1992 in great detail.
Snapped by satellites hundreds of miles up, the radar images of the planet's surface include bands that indicate elevation shifts on the ground as small as three centimeters, or a little more than an inch.
Last week, Dr. Zebker and some 200 other experts met in Sioux Falls, S.D., to help plan a new generation of American radar satellites meant to advance the field beyond the craft now in orbit, which were built by Europe, Japan and Canada. One aim is to increase the precision of change measurement to less than a centimeter, or about a third of an inch.
Although the United States pioneered the technology of space radars in the late 1970's, it now has no civilian radar satellites, a void that American scientists are increasingly eager to fill.
The Sioux Falls meeting was sponsored by the United States Geological Survey, which wants to improve its earthquake predictions, and by NASA's Jet Propulsion Laboratory in Pasadena, Calif., which built the first space radars nearly two decades ago.
''The science community is saying, 'Wow, there's all kinds of things we can do with this,' '' Dr. Tony Freeman, head of radar science and engineering at the propulsion lab, said in an interview. ''It's a measurement tool. It makes very precise measurements of topographic change, and we're just beginning to investigate the implications of that.''
Dr. David T. Sandwell of the Scripps Institution of Oceanography in La Jolla, Calif., who discusses the radar method in the current issue of the journal Science, said it had major implications for monitoring not only natural changes but also those caused by human activity.
For instance, pumping underground oil and water to the surface can cause the ground to sink over wide areas. Monitoring the beginnings of such sinking could aid in planning land use and putting controls on pumping. And nuclear explosions deep underground make telltale depressions at the surface, the tracking of which could aid arms control.
''It's revolutionary,'' Dr. Sandwell said of the radar technique in an interview. ''There are all kinds of neat things you can do.''
Since the start of the space age, most satellites that peer over great distances to observe regions on the ground, both for spying and environmental assessment, have done so with cameras similar in principle to those of any tourist. They work in or near the visual part of the electromagnetic spectrum.
In 1978, however, the National Aeronautics and Space Administration launched a satellite of an entirely new kind that made its images not with light but by bouncing radio waves off the ground. Known as Seasat, this craft publicly revealed how radar satellites could portray both land and sea in striking new ways.
A major advantage was that the satellite worked at night or in foul weather. The radio waves with their long wavelengths easily pierced the clouds, a feat impossible for visible light and its short wavelengths.
The name ''synthetic aperture radar'' came from the method used to overcome the setup's main weakness. By nature, the long radio wavelengths of a radar system produce very low resolution, not showing details anywhere near as small as those delineated with light.