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The primary function of Satellite Laser Ranging (SLR) is the measurement of precise distances between laser telescope and mirrors on passing satellites. This is done by firing extremely short pulses of laser light at the mirrors, and measuring the amount of time it takes for the light to reflect from the mirrors and return to Earth. Because the speed of light is known, the distance travelled by the light in that time can be calculated.
The measurements are made to determine precise distances between points on the Earth's surface. This enables highly accurate global modelling, allowing Australian regional geographic networks to be related to others throughout the world. The satellites provide an external view of the Earth, so that distances can be measured more accurately than terrestrial surveying techniques. For example the measurement to a satellite of more than five thousand kilometres distance can be done with a precision of better than 1 cm. The change in these distances indicates the changing shape of the Earth. Australia operates two SLR stations, one at Mt Stromlo in the Australian Capital Territory and the other at Yarragadee in Western Australia. The distance and orientation of the two stations relative to each other provides a baseline which is relevant to the rest of Australia's geodetic networks.
Lunar Laser Ranging is SLR to the moon and was first achieved in 1969. Lunar Laser Ranging lets us study the complex motion of the moon and has even shown us that the moon is steadily moving away from the earth. Laser Ranging is an invaluable tool for monitoring movement of the tectonic plates and changes in the Earth's rotation. This information contributes to national and international research into understanding tectonic motions and climate change. By monitoring changes in land height SLR contributes to the measurement of sea level rise which is a key factor in the Greenhouse Effect. SLR networks are being developed to improve strategies for dealing with earthquakes in countries such as Japan.
SLR also supports non geodetic satellite missions such as remote sensing and it is hoped that SLR will be able to provide extremely accurate transfer of time across continents and between countries.
