Performance of the ICESat-2 Precision Pointing Determination

The Advanced Topographic Laser Altimeter System (ATLAS) was launched onboard the ICESat-2 in September 2018. The laser altimeter provides elevation profiles over a variety of surfaces, derived from a geolocated footprint and pulse time of flight (range). In order to determine the precise geolocation of the spot where the laser hit, highly accurate knowledge of the laser pointing direction should be determined by the Precision Pointing Determination (PPD). A small fraction of the transmitted laser is routed into and observed by the Laser Reference System (LRS) in the ATLAS. The crucial part of the PPD is Precision Attitude Determination (PAD) that estimates the orientation of the LRS in an International Celestial Reference Frame (CRF) using the star tracker and gyro data. By using the attitude determined through PAD, the laser centroids in the laser detector are converted to the pointing vectors in the CRF, which are the products of the PPD. After the successful ICESat-2 launch, the star tracker, developed for the PPD, revealed serious performance issues. The PPD algorithm is modified to use measurements of the other star tracker that is primarily intended for the real-time attitude determination and control. After the change to the contingency plan, the current PPD successfully meets the mission accuracy requirement based on various assessments. We will present the ICESat-2 PPD approach and performance evaluation that have been conducted at the University of Texas at Austin.