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.
Satellite Laser Ranging Systems typically operate on the second harmonic wavelength of a pulsed Nd:YAG laser at a wavelength of 532 nm. The absence of sufficiently sensitive photo-detectors with a reasonably large active area made it beneficial to trade the conversion loss of frequency doubling against the higher quantum efficiency of the detectors. Solid state silicon detectors in the near infra-red regime at λ = 1.064 µm also suffered from high thermal noise and slow signal rise times, which increased the scatter of the measurements by more than a factor of 3 over the operation at λ = 532 nm. With the availability of InGaAs/InP compound - Single Photon Avalanche Diodes the situation has changed considerably. Their quantum efficiency has reached 70% and the compound material of these diodes provides a response bandwidth, which is commensurate with high high speed detectors in the regime of 532 nm. We have investigated the properties of such a diode type Princeton Lightwave PGA-200-1064 for its suitability for SLR at the Nd:YAG fundamental wavelength with respect to the quantum efficiency and their timing properties. The results are presented in this paper. Furthermore, we provide remarks to on the performance of the diode compared to state of the art detectors, that operate at the Nd:YAG second harmonic wavelength. Finally, we give an estimate of the photoelectron statistics in satellite laser ranging for different operational parameters of the Wettzell Laser Ranging System.