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SLR - General description
Short light pulses (<100 picoseconds) generated by a laser are sent through a telescope to a satellite and reflected by on-board corner cube retro-reflectors back to the telescope. The precisely measured time of flight and the well-known speed of light immediately lead to the distance of the satellite from the observing station at the time of observation.
The accuracy of the range measurement is in the order of a few millimeters to about 1.5 centimeter. These observations can be used, among other purposes, to determine the position of the satellites and the participating observing stations in a global geocentric coordinate system.
The International Laser Ranging Service – ILRS
About 40 observing stations regularly peforming range observations to satellites (two of them to reflectors on the Moon, too) are organized in the International Laser Ranging Service ILRS, a service of the International Association of Geodesy IAG – together with data and analysis centers and a Central Bureau A Governing Board determines which satellites to track and it decides about operational rules and procedures. Prof. W. Gurtner was chairman of the Governing Board from 2002 to 2009.
The observed ranges are collected by two ILRS data centers and made available to the analysis centers.
Several ILRS analysis centers determine the orbits of the geodetic satellites, the positions and velocities of the observing stations, the position of the rotation axis and the angular velocity of the Earth. SLR contributes, together with GPS, VLBI and DORIS, to the determination and maintenance of the International Terrestrial Reference Frame ITRF. Scale and origin of the reference frame are mainly determined by SLR (scale together with VLBI).
Long-term range observations to suitable satellites are used for the computation of global models of the earth’s gravity field.