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The Geoscience Laser Altimeter System (GLAS) is an integral part of the NASA Earth Science Enterprise (ESE). 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. GLAS is carried on the Ice, Cloud and land Elevation Satellite (ICESat), which launched 13 January 2003 00:45 UTC from Vandenberg Air Force Base in California.
The laser altimeter measures the time required for a laser pulse of 5 nanosecond duration to complete the round trip from the instrument to the Earth's surface and back to the instrument. This time interval can be converted into a distance by multiplying with the speed of light, and the one-way distance can be obtained as half the round trip distance. With the position of the instrument in space determined from a high accuracy Global Positioning System (GPS) receiver and from star camera and gyroscopes carried on the instrument/ spacecraft, the laser direction in space will be determined. From the GPS-determined position, the altimeter measurement and the laser pointing direction, the location on the surface of Earth illuminated by the laser pulse can be determined. The series of such laser spot, or footprint, locations provides a profile of the surface. Analysis of the sequence of laser spots over time enables the determination of temporal change in topography.
A diode pumped Q-switched Nd:YAG laser operating in the near infrared (1064 nanometers) is used for the measurement of surface topgraphy. Backscattered light in the green (532 nanometers) is used for measurement of aerosols and other atmospheric characteristics. The return photons will be collected in a 1 meter diameter telescope and the laser will transmit 40 pulses per second to the surface. The spots produced on the Earth's surface will have a 70 meter diameter and the spacing between spots will be 175 meters, caused by the orbital motion of the spacecraft.
Illustrations courtesy of Ball Aerospace
Last Update: January 2003
Last Modified: Fri Jun 22, 2007
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