The Apache Point Observatory Lunar Laser-ranging Operation (apollo)

Lunar Laser Ranging (LLR) is the only means available for testing Einstein’s Strong Equivalence Principle, on which general relativity rests. LLR also provides the strongest limits to date on variability of the gravitational constant, the best measurement of the de Sitter precession rate, and is relied upon to generate accurate astronomical ephemerides. LLR is poised to take a dramatic step forward, enabled both by detector technology and access to a large-aperture astronomical telescope. Using the 3.5 m telescope at the Apache Point Observatory, we will push LLR into a new regime of multiple photon returns with each pulse, enabling millimeter range precision to be achieved. In order to reap the benefits of this “strong” return, we will incorporate a technologically novel integrated array of avalanche photodiodes—capable of generating a temporal range profile while preserving two-dimensional spatial information. We will also employ a high precision gravimeter at the ranging site to measure local displacements of the earth’s crust to sub-millimeter precision. This approach of obtaining directly relevant measurements relating to the earth surface deformation is to be contrasted with the approach to date that relies strictly on models for this information.