Mountain-Top Hydrology in Support of Lunar Laser Ranging

Apache Point Observatory (APO) sits on a promontory at 2880 m in the Sacramento Mountains, New Mexico and houses a 3.5 m optical mirror used for lunar laser ranging (LLR). One of the goals of LLR is a precise measure of Earth-Moon separation in the solar gravity field to test the strong equivalence principle. To that end, local knowledge of the telescope's local displacements would add useful knowledge to the solution of the lunar distance, measured to 1 mm accuracy by LLR. Combining GPS from a neighboring Plate Boundary Station P027 with a time series of gravity data from a superconducting gravimeter at the telescope gives information on vertical displacements. The problem with GPS is the noise levels in the vertical, and the problem with gravity turns out to be the significant response of the gravimeter to local rainfall. In addition the traditional flat-terrain loadings provided by global services such as EOST (Strasbourg) and IMLS (NASA) need to be modified when there is significant topography. We present the best reduced time series for local hydrology modeling and loading (displacements and gravity) and discuss how these time series are used in the determination of the lunar distance.