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Laser Altimeter Measures Ice Sheets

Photonics Spectra
May 1999
Michael Van Belle

Scientists are gearing up to shine lasers from the sky onto Greenland and the Antarctic to monitor changing conditions in the areas' huge ice sheets that could contribute significantly to the levels of Earth's oceans. Carried aboard NASA's Ice, Cloud and Elevation Satellite, the Geoscience Laser Altimeter System is on track to start its orbit in July 2001.

Topography changes

The altimeter is designed to measure ice-sheet topography and the associated temporal changes, according to Charles Bentley, a glaciologist at the University of Wisconsin and one of the team members involved in this project. Surface topography measurement relies on a diode-pumped Q-switched Nd:YAG laser operating at 1064 nm, and backscattered light in the green (532 nm) that measures aerosols and other atmospheric characteristics.

NASA's Ice, Cloud and Elevation Satellite — on track to go into orbit in July 2001 — will carry a laser altimeter to measure ice sheets in Greenland and the Antarctic.

With the laser transmitting 40 pulses per second to the surface, the return photons will be collected in a 1-m-diameter telescope. The spots produced on the Earth's surface will have a 70-m diameter, and spacing between spots will be 175 m, caused by the orbital motion of the satellite.

The laser altimeter measures how long it takes a light pulse to travel from a satellite to the surface and back, as well as that height distance. It offers a significant advantage over radar altimeters, Bentley said. The radar altimeter, the standard for height measurement from satellites, records the return, he said. However, the surface of ice sheets is rough and sloping, and because the wavefront is very broad, it hits the surface in a number of places. Therefore, "you get returns from all the high spots, but you don't know where those high spots are. You just get an echo time, which you can measure, but you don't know where it's coming from."

The laser altimeter will be used mainly to measure mass that is added to or subtracted from the ice sheets, because that ice must come from -- or be returned to -- the ocean, Bentley said. "We don't know what the contributions of the Antarctic or Greenland to sea-level change are, so we'll be looking for any parts of the ice sheets that may be changing rapidly."

The instrument is being built under the direction of Bob Schutz of the University of Texas at Austin.

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