Satellite Laser Ranging (SLR) is a method to measure distances to satellites using very short laser pulses. At the SLR station, a very short laser pulse is generated, transmitted to the satellite, and reflected back to the station, where it is detected. A high precision event timer - using time and standard frequency derived from a special dedicated GPS receiver - measures the time of flight of the laser pulse with an accuracy of about 3 ps. Using the known velocity of light, one can thus determine the distance to the satellites - from a few hundred kilometers up to more than 20,000 km - with an accuracy of a few millimeters.
These distances are measured by more than 40 SLR stations around the world. The results are used to calculate precise orbits. Analyzing these orbits allows the determination of the complex rotation parameters of the Earth, its gravitational field, continental drifts etc.