Characteristics of Differential Lunar Laser Ranging

With more than 50 years of distance measurements for tracking the Moon from Earth by using laser pulses, Lunar Laser Ranging (LLR) plays an important role in many research fields, e.g., relativity tests and lunar interior modelling. However, due to the limited LLR accuracy, mainly caused by the Earth’s atmosphere, some Earth-Moon parameters can only be determined with poor quality and certain details of the lunar interior cannot be assessed. A new laser station of JPL will enable a new technique of lunar tracking: Differential Lunar Laser Ranging (DLLR). The DLLR observation is the difference of any two consecutive ranges obtained by fast switching of a station between two or more reflectors. Because of the large reduction of the Earth’s atmospheric error, a big improvement of the observation accuracy of about 30 µm can potentially be obtained. Therefore, DLLR will provide an excellent chance to estimate various parameters with higher accuracies and to achieve a better understanding of the lunar interior. It is also expected to be beneficial for relativity tests, e.g., related to the equivalence principle (EP). For the comparison of DLLR and LLR with respect to the parameter sensitivity, correlation and accuracy, simulated DLLR data has been generated having the same distribution, time span and number of observations as LLR. DLLR and LLR keep the same sensitivity for one group of parameters which include, e.g., the lunar rotation parameters. However, owing to the cancelling effect of DLLR on the station side, DLLR is less sensitive for a second group of parameters, e.g., for the station coordinates. But this can be compensated by its high measurement accuracy. The parameter accuracy of the second group estimated using DLLR remains at the same level as that obtained by LLR, while the parameter accuracy of the first group is significantly enhanced. The DLLR concept increases the correlation of reflectors and stations. Fortunately, some decorrelation can be reached by selecting a larger switching interval from one reflector to the next (e.g., 15 min instead of 1.5 min). Besides the Newtonian parameters, DLLR can also improve the estimation of the relativity parameters. In this presentation, we illustrate the basic principles of DLLR, its typical characteristics and quantify the potential improvement for the determination of various parameters of the Earth-Moon system.

Acknowledgement. This research was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy–EXC-2123 QuantumFrontiers–390837967.