Pointing angle and timing verification of the geoscience laser altimeter using a ground-based detection system
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The Ice, Cloud, and land Elevation Satellite (ICESat) will begin science operations in 2002 with an emphasis on determination of the ice sheet temporal variations in the Arctic and Antarctic regions. The ICESat bus will serve as the transport for an instrument called the Geoscience Laser Altimeter System (GLAS). GLAS will provide altimetry and lidar measurements with a high level of accuracy. To meet the scientific goals of the mission, specific accuracy requirements for the GLAS data products have been established. For example, the laser pointing angle must be known to within 1.5 arcseconds while the time tag must have an accuracy of 0.1 msec. Both of these data products contribute to the determination of the measured altitude vector from the spacecraft to the ice surface. A unique calibration technique has been developed for verification of the pointing direction and the time tag of the GLAS measurement. This calibration technique is a ground-based system comprised of electro-optical detectors distributed in a grid along the ground track of the satellite. The detectors will trigger "on" when illuminated by the 1064 nm wavelength of the laser footprint. Based on the GPS coordinates of the illuminated detectors and the time tag recorded on the arrival of the pulse, the centroid of the laser footprint can be determined to within 4.5 m, corresponding to 1.5 arcsecond pointing accuracy, and the time tag is determined to within 0.1 msec. This in situ measurement of the footprint location and time tag from the ground array will be compared to the corresponding data products provided by GLAS. The comparison will verify accuracy or will indicate the existence of any errors in the GLAS pointing knowledge or timing determinations. The detectors have been designed and tested in the laboratory. Using a laser pulse similar to what is expected from GLAS, the detectors were analyzed for energy level detectability, system stability, temperature response, and overall performance. In addition, simulations were created to determine possible error sources during the calibration implementation as well as the array sizing and the grid spacing.