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Source: Satellite laser ranging

In satellite laser ranging (SLR) a global network of observation stations measures the round trip time of flight of ultrashort pulses of light to satellites equipped with retroreflectors. This provides instantaneous range measurements of millimeter level precision which can be accumulated to provide accurate measurement of orbits and a host of important scientific data. The laser pulse can also be reflected by the surface of a satellite without a retroreflector, which is used for tracking space debris.
Satellite laser ranging is a proven geodetic technique with significant potential for important contributions to scientific studies of the earth/atmosphere/ocean system. It is the most accurate technique currently available to determine the geocentric position of an Earth satellite, allowing for the precise calibration of radar altimeters and separation of long-term instrumentation drift from secular changes in ocean topography.
Its ability to measure the variations over time in Earth's gravity field and to monitor motion of the station network with respect to the geocenter, together with the capability to monitor vertical motion in an absolute system, makes it unique for modeling and evaluating long-term climate change by:

providing a reference system for post-glacial rebound, plate tectonics, sea level and ice volume change
determining the temporal mass redistribution of the solid earth, ocean, and atmosphere system
determining Earth orientation parameters, such as Earth pole coordinates and length-of-day variations
determining of precise satellite orbits for artificial satellites with and without active devices onboard
monitoring the response of the atmosphere to seasonal variations in solar heating.
SLR provides a unique capability for verification of the predictions of the theory of general relativity, such as the frame-dragging effect.
SLR stations form an important part of the international network of space geodetic observatories, which include VLBI, GPS, DORIS and PRARE systems. On several critical missions, SLR has provided failsafe redundancy when other radiometric tracking systems have failed.