  The Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) began
millimeter-precision ranging to the Moon in 2006. Until now, a comprehensive
validation of APOLLO system range accuracy has not been possible because of
centimeter-scale deficiencies in computational models of the Earth-Moon range,
and because APOLLO lacked an internal timing calibration system. Here, we
report on the development of a system that enables in-situ calibration of the
timing response of the APOLLO apparatus, simultaneous with lunar range
measurements. The system was installed in August 2016. Preliminary results show
that the APOLLO system can provide lunar range measurements with millimeter
accuracy.

Instrumentation and Methods for Astrophysics;General Relativity and Quantum Cosmology