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Icesat’s Lasers Measure Ice, Clouds and Land Elevations


Sky Image at Bonneville
As part of the calibration of ICESat, scientists and engineers from the Center for Space Research at the University of Texas at Austin photographed ICESat laser pulses in the sky and on the ground at Bonneville Salt Flats, Utah on Tuesday September 30, 2003. The series of near-circular green spots in the sky picture are caused by the green-laser pulses from GLAS illuminating thin clouds or aerosols in the atmosphere. The spots, captured in an 8-second camera exposure, are separated by about 170 meters and by less than 3 hundredths of a second. In a clear sky, the laser light from the satellite would appear as a bright green star. The white streak parallel to the green spots is from lights on an aircraft used to photograph both the infrared and green laser spots on the ground. The small white spots are stars. Credit: NASA/University of Texas

Byrd Glacier, shown in a radar image from RADARSAT -1 (top), is the largest outlet glacier draining ice from the East Antarctic Ice Sheet through the Trans-Antarctic Mountains into the Ross Ice Shelf. The ice sheet is grounded on bedrock and the ice shelf is floating on the ocean. Two ICESat profiles across the glacier show details of the troughs formed on the sides of the glacier as it plows into the ice shelf. The differences between the elevations and widths of the glacier at the inner (red) profile and the outer (black) profile show how the glacier thins and spreads as it merges with the ice shelf. With time, ICESat’s measurements of small changes in the elevations of the ice sheets, outlet glaciers, and ice shelves will provide information on whether the rate of ice discharge into the ocean is increasing or decreasing and thus influencing sea level. Credit: NASA

NASA’s Ice, Cloud and land Elevation Satellite (ICESat) has resumed measurements of the Earth’s polar ice sheets, clouds, mountains and forests with the second of its three lasers. Crisscrossing the globe at nearly 17,000 miles per hour, this new space mission is providing data with unprecedented accuracy on the critical third dimension of the Earth, its vertical characteristics.

"The first set of laser measurements is revealing features of the polar ice sheets with details never seen before, and is detecting dust storms, cloud heights, tree heights and smoke from forest fires in new and exciting ways," said Jay Zwally, ICESat Project Scientist at NASA’s Goddard Space Flight Center, Greenbelt, Md.

The principal mission of ICESat is to measure the surface elevation of the large ice sheets covering Antarctica and Greenland. Measurements of elevation-change over time will show whether the ice sheets are melting or growing as the Earth’s climate undergoes natural and human-induced changes.

The Geoscience Laser Altimeter System (GLAS) instrument on ICESat sends short pulses of green and infrared light though the sky 40 times a second, all over the globe, and collects the reflected laser light in a one-meter telescope. The elevation of the Earth’s surface and the heights of clouds and aerosols in the atmosphere are calculated from both precise measurements of the travel time of the laser pulses, and ancillary measurements of the satellite’s orbit and instrument orientation. This marks the first time any satellite has made vertical measurements of the Earth through the use of an onboard light source.

Operating in a near-polar orbit, ICESat is adding to our understanding of the mass-balance of the Greenland and Antarctic ice sheets. ICESat’s first topographic profiles across Antarctica revealed details of features such as the ice streams of the Siple Coast and the Amery Ice Shelf, as well as the atmospheric phenomena above them.

ICESat is also making unique measurements of cloud heights and global distribution. ICESat detects distributions of aerosols from sources such as dust storms and forest fires. And because its laser pulses continuously, ICESat also measures the Earth’s topography with high accuracy.

"ICESat has already demonstrated the unique capability of lasers to make a variety of Earth Science measurements. When the calibration experiments are completed, we believe the accuracy and sensitivity will exceed previous capabilities by nearly an order of magnitude," stated Bob Schutz, GLAS Science Team Leader, of the University of Texas at Austin.

ICESat was launched January 12, 2003, on a Boeing Delta II rocket from Vandenberg Air Force Base, Calif. On March 29, ICESat’s Laser 1 unexpectedly stopped working after providing 36 days of data. NASA will issue a report shortly on the reason for the anomaly.

"Despite the problem with the first laser, ICESat is providing a new perspective on elements within the Earth System with amazing accuracy. We are especially looking forward to the information this capability will provide on how the polar ice sheets are changing," said Waleed Abdalati, ICESat Program Scientist at NASA Headquarters, Washington.

The ICESat scientists will convene a special session to present the latest results from ICESat at the 2003 Fall Meeting of the American Geophysical Union in San Francisco.

ICESat is the latest in a series of NASA’s Earth observation spacecraft designed to study the environment of our home planet and how it may be changing. NASA’s Earth Science Enterprise is dedicated to understanding the Earth as an integrated system and applying Earth System Science to improve prediction of climate, weather and natural hazards using the unique vantage point of space.

Cynthia M. O’Carroll | GSFC
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