GEOLOCATION FOR THE ICESAT LASER ALTIMETER

摘要

The NASA Ice, Cloud and land Elevation Satellite (ICESat) was launched in January 2003 into a near circular, 94° inclination orbit with an altitude of approximately 600 km. The primary instrument carried by ICESat is a. precision laser altimeter, known as the Geo-science Laser Altimeter System (GLAS). GLAS measures the scalar altitude, but these measurements are augmented with instrumentation that enable determination of the spatial position and orientation of the GLAS optical bench. By combining the spatial position, the orientation information and the scalar altitude, the position of the spot on the Earth's surface illuminated by the laser can be inferred. The time sequence of these inferred positions gives a vertical profile of the surface in the along track direction of ICESat. By comparing successive profiles along the same track, changes of surface elevation can be determined. The narrow footprint of the laser enables measurements over sloping surfaces with high accuracy. Over smooth, flat surfaces, the altitude measurement precision is 2-3 cm. The methodologies used to geolocate the illuminated laser spot are summarized, i.e., determine the coordinates of the spot in an appropriate Earth-fixed reference frame. More than 500 million laser shots have been acquired by GLAS since launch and each shot has been geolocated. Using a calibration/validation site at White Sands Missile Range, the geolocation methodology has been validated using several techniques: a) airborne photography of the illuminated laser spot, b) ground array of laser energy detectors, c) visual observations and d) ground array of laser cube corners to introduce unique signatures in the GLAS on-board echo digitizer.