Satellite Laser Ranging

Satellite laser ranging (SLR) began as a concept in 1962 when Plotkin [1] of the Goddard Space Flight Center (GSFC) in Maryland, USA first proposed the development of accurate laser ranging to retro-reflectors on orbiting spacecraft in order to improve geodetic information. At that time optical and radar tracking of satellites was being used to yield tracking station coordinates at a level of accuracy of only about 100 m. The interesting geophysical processes that deform the solid Earth, such as Earth tides and plate tectonic motions, were understood at that time to be affecting station coordinates at the level of only a few centimetres over timescales varying from sub-daily to several years. It was clear that in order to challenge the theoretical work the measurement techniques would have to reach a level of accuracy of a few centimetres and the observations would need to be carried out over many years. The first SLR observations, of the Beacon Explorer-B spacecraft, in 1964 achieved metre-level precision in range, and showed that the technique was viable and potentially capable of reaching the required centimetric precision. These experiments prompted NASA to place retro-reflectors on the GEOS I and II spacecraft and on the Moon, and to begin the development of more SLR stations. In 1975 France launched the first geodetic satellite STARLETTE into a relatively low 950 km orbit, and this was followed in 1976 when NASA launched its laser geodynamic satellite LAGEOS into a near circular, near polar orbit at a height of 6000 km. Both these satellites are inert, dense spheres, uniformly encrusted with retro-reflecting corner-cubes specifically designed to reflect laser light back to the emitting tracking station. STARLETTE has a diameter of 24 cm, LAGEOS a diameter of 60 cm. In 1979 NASA created the Crustal Dynamics Project (CDP) with the aim of ‘developing laser ranging and very long baseline interferometry (VLBI) systems to obtain relative positions with ±2 cm accuracy, to define directions with respect to the inertial reference with a 0.001 arcsecond accuracy to monitor relative rates of motion of different parts of the Earth’s crust well enough to infer irregularities in plate tectonic motions, and to monitor the wobbles and rotational variations to infer their excitations and dampings, as well as to determine accurately the orbits of the distant satellites (higher than 6000 km altitude)’ [2]. This project formalized and consolidated the cooperative efforts between scientists and engineers, which were already underway in several countries, and other independent groups worldwide began to design and build their own SLR systems. During the 1970s the accuracy of the best systems was at the decimetre level, limited mainly by the relatively long laser pulse-lengths that were in routine use at the time. For a typical pulse-length of some 30 cm, a large uncertainty exists in the measurement because it is impossible to relate the detected photons to their position within the pulse. However, advances in both laser and electronic technology continued such