Satellite Laser Ranging (SLR)
SLR measures the time intervals required for pulses emitted by a laser transmitter to travel to a satellite and return to the transmitting site. The "range", or distance between the satellite and the observing site, is approximately equal to one half of the two-way travel time multiplied by the speed of light.
Several satellites are equipped with specially designed reflectors to return the incoming laser pulse back to the transmitting site. For IERS applications, the most commonly used satellites are the dense spherical satellites known as Lageos-1 (launch 1976) and Lageos-2 (launch 1992). These two satellites are at an altitude of 5900 km but in different orbital planes. It is now possible to measure the range to the Lageos satellites with a precision of less than 1 cm from a single laser pulse. SLR measurements require a correction for the dry component of the troposphere; thus, the SLR sites measure atmospheric pressure, temperature and relative humidity to calculate this correction.
Since 1976, more than 100 sites have obtained SLR measurements to Lageos-1. Coordinates with an accuracy better than 2 cm have been obtained for most of these sites and site velocities have been obtained for about 50 of the sites.
SLR is sensitive to the location of the earth's centre of mass; the time history of its motion with respect to the origin of the terrestrial reference frame has been obtained since 1987 with an accuracy of a few millimeters.
SLR from Lageos has provided daily polar motion estimates with an accuracy of about 0.3-0.4 milliarcseconds.
Although UT1 and nutation can, in principle, also be obtained, there are problems in separating them from orbital errors. For example, UT1 is correlated with the longitude of the ascending node of the orbit plane so that if the node location were perfectly known, UT1 could be estimated with high accuracy. In practice, the Lageos-1 node exhibits unmodelled drifts of 0.5 ms/month rms and the UT1 estimates have useful accuracy only for periods shorter than about 60 days.
In the IERS, SLR is represented by the International Laser Ranging Service (ILRS).