An algorithm for reliable normal point calculation of noisy LLR measurements

Lunar Laser Ranging (LLR) data can be used to determine parameters of the Earth-Moon system (e.g. lunar gravity, tidal parameters) and relativistic effects in the solar system. Moreover, LLR contributes to the realization of the International Terrestrial Reference Frame (ITRF) and provides a set of Earth Orientation Parameters (EOP) like UT0 or nutation coefficients. A prerequisite for a good analysis is the availability of sufficient observations with high accuracy from globally well distributed sites on the Earth to the reflectors on the Moon. However, the measurement of the Earth-Moon distance is difficult and much more challenging for the laser system than satellite laser ranging (SLR). The signal to noise ratio of the received signal is low because a lot of power is lost on the round trip from the Earth to the Moon. Therefore special observation strategies and processing algorithms are required. To improve the visibility of the real lunar returns in the noise, a semi-pulse pattern is incorporated in the analysis of the raw data by applying a correlation procedure. Based upon these results and after subtracting the noise, the final normal point is computed. Here, all relevant processing steps are explained, where the big advantage of the new processing method will be shown by examples.