by Bob Schutz
Fifteen thousand years ago, huge ice sheets covered much of North America and parts of Eurasia. As climate warmed during the end of the ice age, sea level rose. We do not know, however, whether the amount of water returned to the oceans in icebergs and runoff balances the snow accumulation. If the ice sheets are shrinking they would be causing the sea level to rise.
The Ice, Cloud, and Land Elevation Satellite (ICESat) will measure the elevation of Earth's land surface with unprecedented accuracy and global coverage. Changes in land elevation, which can be determined during the life of ICESat, will be especially valuable.
Carried aboard the ICESat satellite will be the Geoscience Laser Altimeter System (GLAS), an integral part of the Earth Science Enterprise. GLAS is a facility instrument designed to measure ice-sheet topography and associated temporal changes, as well as cloud and atmospheric properties. In addition, operation of GLAS over land and water will provide along-track topography.
- should detect changes in mass balance
- will take accurate measurements which are essential for making reliable assessments of whether future changes in ice volume will add to the sea level rise, which is already occurring due to the warming and thermal expansion of oceans and worldwide melting of small glaciers, or whether the ice sheets might grow and absorb a significant part of the predicted sea level rise
- is designed to observe changes in surface elevation that are caused by variations in precipitation (snowfall) and surface melting. These data will be used in energy-balance models and to test the results of atmospheric circulation models used to predict climate-induced changes.
- is a laser altimeter designed to measure ice-sheet topography and associated temporal changes, as well as cloud and atmospheric properties
- will operate over land and water to provide along-track topography
- will conduct atmospheric observations which are important for understanding Earth's climate
ICESat/GLAS will measure changes in the volume of polar ice, the height and thickness of the clouds, the amount of pollutants in the atmosphere, and land surface topography, including vegetation canopy heights.
ICESat data collected over land surfaces will therefore enhance data collected by ground-based and airborne surveys, with the added valuable benefit of providing long-term time series of topographic changes, which are especially difficult to obtain by other techniques.
Maps of Earth's topography are needed for a variety of scientific studies in geology, hydrology, volcanology, biology, and meteorology. High accuracy topography of much of the Earth's surface is hardly known. Accurate maps are necessary for a variety of commercial, resource-development, land-use, navigation, and other applications. These measurements are important for understanding the Earth's climate.
GLAS is the first laser-ranging (lidar) instrument for continuous global observations of Earth, which will make unique atmospheric observations as an important component of the ESE climate change program.
GLAS includes an eye safe laser system to measure distance, a Global Positioning System (GPS) receiver, an array to enable laser ranging from the ground, and a startracker attitude determination system.
The laser altimeter will measure the height from the spacecraft to the ice sheet, with an intrinsic precision of better than 10 cm with a 70-m surface spot size. The height measurement, coupled with knowledge of the radial orbit position, will provide the determination of topography. Characteristics of the return pulse will be used to determine surface roughness.
Changes in ice-sheet thickness at a level of a few tens of cm (anticipated to occur on a subdecadal time scale) will provide information about ice-sheet balance and will support prediction analyses of cryospheric response to future climatic changes. The ice-sheet mass balance and contribution to sea-level change will also be determined. The accuracy of height determinations over land will be assessed using ground slope and roughness.
How does it work?
In the 120 days following launch, the ground track will repeat in 8 days to provide multiple overflights of ground verification/validation sites. The main mission will use 183 day repeat track. The near circular, near polar orbit has an altitude of approximately 600 km. Two primary orbits will be used:
- Verification/validation orbit: this orbit has a ground track repeat cycle of 8 days to enable overflights of specific locations on the Earth which will be instrumented to support measurement verification or data production validation
- Mission orbit: this orbit has a ground track repeat cycle of 183 days to enable uniform sampling of the surface with high resolution. At the equator, the separation between ascending tracks will be about 15 km after 183
Global sea level is believed to be rising slowly. This can be attributed to thermal expansion as the ocean warms, to small glaciers melting around the world, or possibly human activities such as burning trees, filling reservoirs, pumping ground water, and draining wetlands.
Atmospheric observations are important for understanding Earth's climate. Clouds cool the atmosphere and Earth's surface by reflecting solar radiation back to space and they also heat the atmosphere by absorbing infrared energy that is emitted from the surface and escapes to space in the absence of clouds. Knowledge of the height, coverage, and thickness of cloud layers is essential for both modeling the complicated feedback process between clouds, radiation, climate, and the understanding of climate change.
We do not know which will be stronger: the effects of increased melting at ice sheet margins, or increased snowfall over the entire ice sheet. The lower elevations of the ice sheets melt more in summer. However, a warmer atmosphere carries more moisture, so more snow will fall on the ice sheets causing large areas of the ice sheets to thicken.
GLAS will provide first estimates of ice thickening/thinning for all major Greenland drainage basins by comparing measurements to data on major ice drainage basins on the Greenland Ice Sheet.
Observations of elevation changes over the entire ice sheets will enable assessments of the ice mass balance and dynamics in individual drainage basins and major outlet glaciers, as well as for the entire ice sheet. The continuous satellite observations will also detect changes in surface mass balance and enable evaluation of whether the changes are caused by recent or longterm changes in climate and/or ice dynamics.
Last Updated: 8 February 2011