Contribution of laser ranging to Earth's sciences

Abstract Satellite and Lunar Laser Ranging (SLR and LLR, respectively) are based on a direct measurement of a distance by exactly measuring the time transit of a laser beam between a station and a space target. These techniques have proven to be very efficient methods for contributing to the tracking of both artificial satellites and the Moon, and for determining accurately their orbit and the associated geodynamical parameters, although hampered by the non-worldwide coverage and the meteorological conditions. Since more than 40 years, the French community (today ‘Observatoire de la Cote d'Azur’, CNES, ‘Observatoire de Paris’, and IGN) is largely involved in the technological developments as well as in the scientific achievements. The role of the laser technique has greatly evolved thanks to the success of GPS and DORIS; the laser technique teams have learnt to focus their effort in fields where this technique is totally specific and irreplaceable. The role of SLR data in the determination of terrestrial reference systems and in the modelling of the first terms of the gravity field (including the terrestrial constant GM that defines the scale of orbits) has to be emphasized, which is of primary importance in orbitography, whatever the tracking technique used. In addition, the role of LLR data (with two main stations, at Mac Donald (United States) and Grasse (France), since 30 years) has been of particular importance for improving solar system ephemeris and contributing to some features of fundamental physics (equivalence principle). Today, the role of the SLR technique is ( i ) to determine and to maintain the scale factor of the global terrestrial reference frame, ( ii ) to strengthen the vertical component (including velocity) of the positioning, which is crucial for altimetry missions and tectonic motions, ( iii ) to locate the geocenter with respect to the Earth's crust, ( iv ) to avoid any secular and undesirable drift of geodetic systems thanks to a very good accuracy. Now, the future of this technique is to enlarge the technical capability of laser ranging stations for long distances, that is the tracking of space targets orbiting through the Solar System. In addition, the laser technique should participate into time transfer experiments and improve, with mobile systems like the FTLRS and the new SLR2000 concept, the coverage of the international laser network (ILRS). To cite this article: P. Exertier et al., C. R. Geoscience 338 (2006).