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Lunar laser ranging provides a number of leading experimental tests ofgravitation -- important in our quest to unify General Relativity and theStandard Model of physics. The Apache Point Observatory Lunar Laser-rangingOperation (APOLLO) has for years achieved median range precision at the ~2 mmlevel. Yet residuals in model-measurement comparisons are an order-of-magnitudelarger, raising the question of whether the ranging data are not nearly asaccurate as they are precise, or if the models are incomplete orill-conditioned. This paper describes a new absolute calibration system (ACS)intended both as a tool for exposing and eliminating sources of systematicerror, and also as a means to directly calibrate ranging data in-situ. Thesystem consists of a high-repetition-rate (80 MHz) laser emitting short (< 10ps) pulses that are locked to a cesium clock. In essence, the ACS deliversphotons to the APOLLO detector at exquisitely well-defined time intervals as a"truth" input against which APOLLO's timing performance may be judged andcorrected. Preliminary analysis indicates no inaccuracies in APOLLO data beyondthe ~3 mm level, suggesting that historical APOLLO data are of high quality andmotivating continued work on model capabilities. The ACS provides the means todeliver APOLLO data both accurate and precise below the 2 mm level.

An absolute calibration system for millimeter-accuracy APOLLO measurements