The Lunar Orbiter Laser Altimeter (LOLA) aboard the Lunar Reconnaissance Orbiter (LRO) has collected nearly 7 billion measurements of surface height on the Moon with an absolute accuracy of approximately 1 m and a precision of approximately 10 cm. Converting time-of-flight laser altimeter measurements to topographic elevations requires accurate knowledge of the laser pointing with respect to the spacecraft body-fixed coordinate system. To that end, we have utilized altimetric crossovers from LOLA, as well as bidirectional observations of the LOLA laser and receiver boresight via an Earth-based laser tracking ground station. Based on a sample of approximately 780, 000 globally-distributed crossovers from the circular-orbit phase of LRO's mission (approximately 27 months), we derive corrections to the LOLA laser boresight. These corrections improve the cross-track and along-track agreement of the crossovers yielding RMS residuals of approximately 10 m in both directions. From early in the LRO mission, the bidirectional laser tracking experiments have confirmed a pointing anomaly when the LOLA instrument is facing towards deep space or the night side of the Moon, and have allowed the reconstruction of the laser far-field pattern and receiver telescope pointing. By conducting such experiments shortly after launch and nearly 8 years later, we have directly measured changes in the laser characteristics and obtained critical data to understand the laser behavior and refine the instrument pointing model. The methods and results presented here are also relevant to the design, fabrication, and operation of future planetary laser altimeters, and, in particular, their long-term behavior in the space environment.
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