In-orbit Calibration of the Lunar Orbiter Laser Altimeter Via Two-Way Laser Ranging with an Earth Station

Orbiting planetary laser altimeters have provided critical data on such bodies as the Earth, Mars, the Moon, Mercury, and 433 Eros. The measurement accuracy of these instruments depends on accurate knowledge of not only the position and attitude of the spacecraft, but also the pointing of the altimeter with respect to the spacecraft coordinate system. To that end, we have carried out several experiments to measure post-launch instrument characteristics for the Lunar Orbiter Laser Altimeter (LOLA) onboard the Lunar Reconnaissance Orbiter. In these experiments, the spacecraft points away from the Moon and scans the Earth in a raster pattern as the LOLA laser fires (the downlink) while a ground station on Earth fires its own laser to the spacecraft (the uplink). The downlink pulse arrival times and digitized waveforms are recorded at the ground station, the Goddard Geophysical and Astronomical Observatory in Greenbelt, MD, and the uplink arrival times and pulse widths are measured by LOLA. From early in the mission, the experiments have helped to confirm a pointing anomaly when LOLA is facing towards deep space or the cold side of the Moon. Under these conditions, the downlink data indicate a laser bore-sight pointing offset of about -400 and 100 microradians in the cross-track and along-track directions, respectively. These corrections are consistent with an analysis of LOLA ground-track crossovers spread throughout the mission to determine lunar tidal flexure. The downlink data also allow the reconstruction of the laser far-field pattern. From the uplink data, we estimate a correction to the receiver telescope nighttime pointing of ~140 microradians in the cross-track direction. By comparing data from such experiments shortly after launch and nearly 5 years later, we have directly measured the changes in the laser characteristics and obtained critical data to understand the laser behavior and refine the instrument calibration.