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Lunar laser ranging utilizing a highly efficient solid-state detector in the near-IR

Ranging to the moon in the optical domain sets high demands on the tracking station. Apart from precise pointing and tracking with error tolerances of less than 1 second of arc, it also requires the capability for single photon event timing in the presence of significant background light levels. As a consequence of this technological challenge, there are only very few laser ranging stations like the McDonald Laser Ranging Station (MLRS) and the MeO station near Grasse in France that have successfully tracked the moon over the last almost 50 years. The Geodetic Observatory Wettzell is a fundamental station for Global Geodetic Observing System (GGOS) and therefore combines all the major measurement techniques of space geodesy collocated in one place. While the Wettzell Laser Ranging System (WLRS) has obtained a few observations of the Apollo 15 target in the past, the data volume was too sparse to make a significant contribution. Recent progress in the development of highly sensitive IR photon counting detectors has provided a new generation of diodes that offer high quantum efficiency at the fundamental wavelength of Nd:YAG at 1.064 μm and very short signal rise times at the same time. Furthermore they exhibit a very low intrinsic detector noise level. Together with a 75 mJ pulse energy and a laser pulse width as small as 10 ps, the WLRS has now repeatedly observed the Lunakhod 1 and the Apollo 15 target with a good signal to noise ratio, so that the remaining measurement error is limited to the effective reflector depth of the respective lunar targets in the presence of the libration of the moon. This talk outlines the station characteristics and discusses the detector performance for this high demanding application.