Lunar Laser Ranging

Lunar Laser Ranging (LLR) is a unique tool to determine the distance between stations on Earth and retroreflectors on the Moon with highest accuracy.  For this purpose short laser pulses with a length of 70 to 200 ps are sent from a ground-based telescope to the Moon. Lunar reflectors which were deployed between 1969 and 1973 by the Apollo 11, 14 and 15 missions and by two unmanned Soviet rovers (Lunokhod 1 and 2) reflect the laser pulse back. On Earth the laser pulse is then registered by the telescope. The observation is the round trip travel time of the laser pulse.

© Nasa/ D. Scott
A partial view of the Lunar Laser Ranging retroreflector field installed by the Apollo 15 mission

The huge distance and signal loss due to beam divergence in the atmosphere and from reflector diffraction effects makes LLR challenging and only a few observatories on Earth are capable to track the Moon. Today, only three observatories observe the Moon regularly: Grasse (France), APOLLO (USA) und Matera (Italy).  

Since the earliest measurement in 1969 the accuracy has improved from several decimeters to just a few millimeters at the newest LLR-observatories. With LLR a number of parameters can be estimated which provide an insight into the Earth-Moon system. These include the following topics:

  • Reference systems, e.g. station- and reflector coordinates, tie of the Lunar orbit to VLBI-based ICRF, the determination of Earth orientation parameters and their comparison with other independent techniques;
  • Dynamics of the Earth-Moon system, e.g. orbit and libration parameters, mass of the Earth-Moon system and secular tidal acceleration;
  • Physics of the Moon, e.g. the interior structure of the Moon, dissipation effects;
  • Gravitational physics/tests of Einstein’s theory of relativity, e.g. the principle of equivalence or a possible temporal variation of the gravitational constant.
Lunar Analysis Center

The Institute of Geodesy acts together with the Forschungseinrichtung Satellitengeodäsie (FESG) at TU München as a Lunar Analysis Center of the International Laser Ranging Services (ILRS). It also cooperates with the Analysis Working Group of ILRS in order to establish consistent standards and results at the ILRS.