An absolute calibration system for millimeter-accuracy APOLLO measurements

Abstract

Lunar laser ranging provides a number of leading experimental tests of gravitation—important in our quest to unify general relativity and the standard model of physics. The apache point observatory lunar laser-ranging operation (APOLLO) has for years achieved median range precision at the ∼2 mm level. Yet residuals in model-measurement comparisons are an order-of-magnitude larger, raising the question of whether the ranging data are not nearly as accurate as they are precise, or if the models are incomplete or ill-conditioned. This paper describes a new absolute calibration system (ACS) intended both as a tool for exposing and eliminating sources of systematic error, and also as a means to directly calibrate ranging data in situ. The system consists of a high-repetition-rate (80 MHz) laser emitting short (10 ps) pulses that are locked to a cesium clock. In essence, the ACS delivers photons to the APOLLO detector at exquisitely well-defined time intervals as a ‘truth’ input against which APOLLO’s timing performance may be judged and corrected. Preliminary analysis indicates no inaccuracies in APOLLO data beyond the ∼3 mm level, suggesting that historical APOLLO data are of high quality and motivating continued work on model capabilities. The ACS provides the means to deliver APOLLO data both accurate and precise below the 2 mm level.

Observational Constraint on the Radius and Oblateness of the Lunar Core‐Mantle Boundary

“…Future extension of the Apollo seismometer network with a better coverage (Mimoun et al, ) would allow a better determination of ρ c and thereby improving current LLR estimations. With advancements in the LLR measurement (Adelberger et al, ; Courde et al, ) continuing to accumulate high‐accuracy data sets and emerging observational techniques (Dehant et al, ), future LLR analysis will allow unprecedented access to the dynamical nature of the lunar interior. Moreover, the methods described here can be applied to study the influence of the liquid core on the rotation of other planets such as Mars (Folkner et al, ).…”

APOLLO clock performance and normal point corrections

“…In February 2016, APOLLO installed a cesium clock (Microsemi 5071A) as a first phase of a new absolute calibration system [6], which became fully operational in September 2016. A Universal Counter (UC; Agilent 53132A) began collecting essentially continuous measurements of the XL-DC frequency with reference to the Cs 10 MHz output on 10 s intervals to 10 −13 resolution.…”

Constructing a High-Accuracy Geometric Model for Moon-Based Earth Observation

“…The JPL ephemeris is in good agreement with lunar laser ranging (LLR), which now achieves millimeter-scale accuracy [28]. In this paper, we use DE430 to calculate the positions and velocities of the helio-center, geocenter, and seleno-center in the ICRS, as well as the amplitude and derivative of the libration.…”