Satellite Laser Ranging (SLR) Overview

Introduction

Geoscience Australia operates laser ranging stations at Mount Stromlo in the Australian Capital Territory, and MOBLAS 5, Western Australia. Geoscience Australia coordinates and liaises with the International Laser Ranging Service (ILRS), and the NASA Solid Earth and Natural Hazards Projects (SENH), to define observing programs and quality requirements.

The Geoscience Australia Lunar and Satellite Laser Ranging program began in March 1973 with the signing of a NASA-Division of National Mapping agreement under the USA-Australia Hornig Treaty for cooperation in Science. The Lunar Laser Ranging (LLR) facility at Orroral in the Australian Capital Territory was finally constructed and occupied in March 1975. During the period 1978-1980 LLR was the primary activity at Orroral. Following a grant from NASA, the Orroral observatory was upgraded to Satellite Laser Ranging (SLR) in 1981. In 1984, the first observations to the Lageos satellite were taken.

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The SLR Technique

Diagram showing how SLR works

The primary function of Satellite Laser Ranging (SLR) is the measurement of precise distances between laser telescope and reflectors on passing satellites. This is done by firing extremely short pulses of laser light at the reflectors, and measuring the time it takes for the light to return to Earth. Because the speed of light is known, the distance travelled by the light in that time can be calculated. Diagram image showing the workings of SLR. Select for larger gif_13k image.

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Why SLR

SLR has developed into the most important geodetic instrument for the establishment of an accurate global geodetic infrastructure and earth monitoring science. SLR contributes to:

• The definition of the International Terrestrial Reference Frame (ITRF) by being the only space geodetic technique which defines the earth's centre of mass. In addition, provides scale and the core network for the ITRF.
• Monitoring Earth Rotation and Polar Motion to provide the relationship with the Celestial Reference Frame (CRF).
• Modelling the temporal and spatial variation of the Earth's gravity filed.
• Determination of ocean and earth tides.
• Monitoring tectonic plates and horizontal and vertical crustal deformation.
• Orbit determination for spaceborne altimeters and radar measurements for studies in global ocean circulation and changes in ice masses.

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Lunar Laser Ranging

Lunar Laser Ranging (LLR) in Australia ceased in mid-1985 following five successful sessions to the reflectors placed on the moon by the Apollo-15 mission. Currently, only the stations at Grasse (France) and the McDonald Observatory in Texas, USA operate as LLR stations

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