LASER RANGING -SLR
ILRS Central Bureau NASA's Goddard Space Flight Center (GSFC)
MD 20771 USA
International Laser Ranging Service
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
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
unmodeled drifts of 0.5 ms/month rms and the UT1 estimates have useful accuracy
only for periods shorter than about 60 days.