LASER POINTING DETERMINATION SYSTEM FOR THE GEOSCIENCE LASER ALTIMETER ON ICESAT. Initial In-Flight Performance Assessment

摘要

The primary mission of the Geoscience Laser Altimeter System (GLAS), launched on the Ice, Cloud and land Elevation satellite (ICESat) January 12th, 2003, is to measure changes in the topography and mass balance of the polar ice caps over 3 to 5 years. The altimetry measurement includes firing the laser at the surface at 40 Hz, collecting and timing the surface return. The time of flight is calculated to determine the elevation of the surface at each laser spot. The error of the measurement includes the accuracy of the GPS solution for spacecraft position, timing uncertainty in the ranging measurement, and pointing knowledge of the laser with respect to inertial coordinates. The pointing knowledge accuracy requirement is 1.5 arcseconds (1σ) for every laser shot fired at 40 Hz. Therefore, a highly accurate means to determine the attitude with respect to inertial space for the laser beam was developed. This is the first time that a spaceborne laser altimeter system directly measures the pointing uncertainty of the laser beam resulting in a highly accurate altimetry measurement. The paper discusses the Stellar Reference System (SRS), which is comprised of two star trackers, one having a narrow field of view for increased accuracy, a hemispherical resonant gyro (HRG) and angle preserving beam steering optics. Absolute laser pointing determination is achieved by simultaneous laser far field imaging and attitude determination in the system. Recent data from independent in-flight calibration of the system is presented. Compliance with requirements has been preliminarily verified.