Laser pointing angle and time of measurement verification of the ICESat laser altimeter using a ground-based electro-optical detection system

 The Ice, Cloud, and land Elevation Satellite (ICESat) will begin science operations in 2003 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. For altimetry, the GLAS data will enable determination of the laser pointing angle to within 1.5 arcsec and the laser pulse time of arrival on the ground to within 100 sec. Both of these data products contribute to the determination of the measured altitude vector from the spacecraft to the ice surface. Verification of both the laser pointing angle and the timing can be achieved by using a unique experimental technique designed to ‘capture’ an altimeter pulse on the surface of the Earth. The capture of the laser pulse is accomplished by covering the illuminated area with devices designed to detect the arrival of energy within the altimeter footprint. This ground-based technique will supply an independent, unambiguous determination of the laser footprint geolocation and the epoch time associated with the arrival of the pulse on the surface. Knowledge of the laser footprint centroid on the ground will infer the laser pointing direction in the geocentric reference frame. This in situ measurement of the footprint geolocation and time of arrival will be compared to the corresponding data products provided by GLAS. The comparison of the GLAS laser pointing and the timing data with an independent measurement will verify the accuracy and/or will indicate the existence of any biases or errors in the generation of the GLAS altimetry data products. The detectors have been designed and tested in the laboratory and analyzed for energy level thresholds, system stability, temperature response and overall performance. Timing hardware has been tested and software has been written to achieve event detection within the desired accuracy.