SLR - Satellite Laser Ranging
In Satellite Laser Ranging (SLR), a global network of ground stations measures the instantaneous round-trip time of flight (ToF) of ultrashort laser pulses from the stations to satellites equipped with special retro-reflectors. This provides instantaneous range measurements of millimeter level precision which can be accumulated to provide centimeter accurate orbits and important science products. Currently, the global SLR network tracks over 80 such satellites.
SLR systems are equipped with short-pulse laser transmitters, transmitting and receiving telescopes systems and optic detectors that can range to orbiting satellites (from 300 to 42000 km). Lunar Laser Ranging (LLR) systems can range to retro-reflectors located on the moon.
The International Laser Ranging Service (ILRS) was formed to provide a service to support, through Satellite and Lunar Laser Ranging data and related products, geodetic and geophysical research activities as well as IERS products important to the maintenance of an accurate International Terrestrial Reference Frame (ITRF). The service also develops the necessary standards/specifications and encourages international adherence to its conventions. The ILRS collects, merges, archives and distributes SLR and LLR observation datasets of sufficient accuracy to satisfy the objectives of a wide range of scientific, engineering, and operational applications and experimentation.
Some of the scientific results derived from SLR include:
- Detection and monitoring of tectonic plate motion, crustal deformation, Earth rotation, and polar motion.
- Modeling of the spatial and temporal variations of the Earth's gravitational field.
- Determination of basin-scale ocean tides.
- Monitoring of millimeter-level variations in the location of the center of mass of the total Earth system (solid Earth-atmosphere-oceans).
- Establishment and maintenance of the International Terrestrial Reference System (ITRS).
- Detection and monitoring of post-glacial rebound and subsidence.
In addition, SLR provides precise orbit determination for spaceborne radar altimeter missions mapping the ocean surface (which are used to model global ocean circulation), for mapping volumetric changes in continental ice masses, and for land topography. It provides a means for subnanosecond global time transfer, and a basis for special tests of the Theory of General Relativity.