BOULDER, Colorado -- NASA is a little more than a year away from launching a satellite capable of making regular measurements of even the minutest changes in the thickness of the ice capping the Arctic, Antarctica and Greenland.

The Ice, Cloud and land Elevation Satellite, or ICESat, will carry a single science instrument to accomplish the task, regularly tracking any changes in depth over time.

The instrument, called the Geoscience Laser Altimeter System, or GLAS, will bank on knowledge of two things to do its work: the speed of light and the spacecrafts location in space.

The instrument will bounce a laser beam down onto the ice 40 times a second, timing how long it takes for the pulse to make the roughly 744-mile (1,200-kilometer) roundtrip. 

Since the satellite will have very precise knowledge of its position in space -- thanks to the combined use of Global Positioning System receivers, star-tracking cameras and gyroscopes -- the time a single pulse takes to make its trip down and back can be converted into distance.

And as the satellite passes over Earth, those distances can be built up to create a profile of the surface elevation. With each regular pass of the satellite over the same spot, any changes in elevation in icecap or sheet height point to a thickening or thinning of the ice itself.

"It will know within a centimeter how much its changed month to month," said Zubin Emsley, the ICESat program manager for satellite-builder Ball Aerospace & Technologies Corp.

Scientists will use the data to track ice melt, which spells an attendant rise in global sea levels.

While launch of the mission is slated for July 2001, that is likely to slip to later in the year.

Once in orbit, the NASA Goddard Space Flight Center satellite mission will last anywhere from three to five years.

Since 1991, NASA has used aircraft-based laser altimeters to determine how accurately it could measure ice-surface elevations. It has already established a benchmark data set for all of Greenland.