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[1] To provide validation of the ICESat laser altimeter time of measurement and geolocation, a ground-based technique was implemented at White Sands Space Harbor (WSSH), during the Laser 2a and 3a operational periods. The activities used an electro-optical detection system and a passive array of corner cube retro reflectors (CCR). The detectors and the CCRs were designed to provide an independent assessment of the laser footprint location, while the detectors also provide timing verification. This groundbased system unambiguously validated the elevation product time tag to 3 msec ± 1 msec. In addition, the ground equipment provided in situ geolocations of the laser pulse. Comparing the in situ results to the ICESat GLA14 data product the positions differ by 10.6 m ± 4.5 m for Laser 2a (Release 21) operations and 7.5 m ± 6.6 m for Laser 3a (Release 23). These comparisons correlate to pointing validations at this site, for the specific overflight configurations. Citation: Magruder, L., E. Silverberg, C. Webb, and B. Schutz (2005), In situ timing and pointing verification of the ICESat altimeter using a ground-based system, Geophys. Res. Lett., 32, L21S04,

The determination of accurate elevation data using laser altimetry relies on accurate knowledge of the instrument pointing angle. The Geoscience Laser Altimeter System (GLAS) on the Ice, Cloud and land Elevation Satellite (ICESat) uses a novel system for determination of the laser pointing vector. In this paper we describe this system, as well as the method used to process its data, which are required to meet ICESat's science objectives. We discuss the necessary modifications to processing techniques, implemented to optimize accuracy for the various operating conditions. Results to date are compared to calibration/validation data to assess their accuracy. We show that the stated requirements have been met for near nominal operating conditions.

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