The Apache Point Observatory Lunar Laser-ranging Operation (APOLLO): Two Years of Millimeter-Precision Measurements of the Earth-Moon Range1

ABSTRACT.In 2006 April, the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) began its science campaign to measure the Earth-Moon separation to millimeter precision. Since that time more than 280 “normal-point” measurements have been made of the distance between the Apache Point Observatory (APO) 3.5-m telescope in New Mexico and retro-reflector arrays on the surface of the Moon. If only statistical errors are considered, then the median nightly range measurement uncertainty for all of our data is 1.8 mm of one-way path, and is 1.1 mm for data after 2007 September. We present an analysis of the APOLLO system performance, highlighting the record-breaking photon return rates and the ability to perform high-cadence observations of multiple lunar retro-reflector targets in a short (30–60 minute) time span. We also show that there is no evidence to suggest that the APOLLO apparatus introduces drifts in the lunar-range measurement over timescales of minutes to an hour.