Laser Ranging Experiment on Lunar Reconnaissance Orbiter: Timing Determination and Orbit Constraints

Accurate ranges from Earth to the Lunar Reconnaissance Orbiter (LRO) spacecraft Laser Ranging (LR) system supplement the precision orbit determination (POD) of LRO. LRO is tracked by ten LR stations from the International Laser Ranging Service (ILRS), using H-maser, GPS-steered Rb, and Cs oscillators as reference clocks. The LR system routinely makes oneway range measurements via laser time-of-flight from Earth to LRO. Uplink laser pulses are received by a telescope mounted on the high-gain antenna of LRO, transferred through a fiber optic cable to a Lunar Orbiter Laser Altimeter (LOLA) detector, and time-tagged with respect to the spacecraft clock. The range from the LR Earth stations to LRO is derived from paired outgoing and receive times. Accurate ranges can only be obtained after solving for both the spacecraft and ground station clock errors and removing temperature effects. The drift rate and aging rate of the LRO clock are calculated from data provided by the primary LR station, NASA's Next-Generation Satellite Laser Ranging System (NGSLR) in Greenbelt, Maryland. The results confirm the LRO clock oscillator midto longterm stability measured during ground testing. These rates also agree well with those determined through POD. Ten-cm level LR observations are used in the POD procedure to form strong orbit constraints. We have processed the entire LRO mission with the radiometric and LR data, and estimated the impact of the LR data on the orbit reconstruction and accuracy. The orbit residual fits of the LR data over 14 days are less than 10 meters, nominally smaller than the 15-meter residuals of the S-band data. The difference between the orbit results determined with and without LR contribution is up to 10 meters.