Pointing angle and timing verification of the geoscience laser altimeter using a ground-based detection system
Abstract
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.
Description
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