Satellite Laser Ranging (SLR) is a space geodetic technology which can measure the distance between a ground station and a satellite most precisely in current methods. A very short laser pulse is transmitted from a telescope in a ground station, and it is retro-reflected vise versa by a corner cube reflector on a satellite and goes back to the station. The round trip time is measured, that is equivalent to measure the distance. Not only the satellite orbit but also many geodetic parameters such as station coordinates can be determined by analysis of SLR data. In other words SLR measures the absolute time of flight of photon so that the geometry of satellite and laser station can be determined precisely as long as the system calibration error is controlled in a negligible level.
It is now possible to accurately measure this up to 30 picoseconds or better that is equivalent to the accuracy of 1 centimeter or less. With the range measurements that are taken from SLR stations around the world, we are able to determine rotation of the Earth, plate tectonic motion on a global scale, the orbits of satellites, our own position relative to Earth's center of gravity to name just a few of the many applications of SLR.