SLR (Satellite Laser Ranging)

Satellite Laser Ranging measures the round-trip flight time of ultra-short laser pulses from a ground station to retroreflectors on an orbiting satellite, giving instantaneous range to millimeter precision. Coordinated by the International Laser Ranging Service, SLR to the dense passive spheres LAGEOS-1 (tracked since 1976) and LAGEOS-2 uniquely realizes the origin (geocenter) and the scale of the International Terrestrial Reference Frame. That frame is the foundation under which GNSS positions, tide-gauge benchmarks, and altimeter orbits are expressed, so a stable ITRF origin is what lets sea-level trends be separated from reference-frame drift. SLR also recovers Earth orientation and the gravity field.