Satellite Laser Ranging

Satellite Laser Ranging (SLR) is a method to measure the distance to satellites or space debris by using very short laser pulses. Since 1982, IWF has operated one of world-leading SLR stations at the Lustbühel Observatory.

The Graz SLR station measures the distance to satellites up to an orbital height of 36000 kilometers. To certain targets - by using statistical analysis - it is possible to reach an accuracy of less than one millimeter. Currently, the station routinely measures the distances to more than 140 targets and uploads the data to various analysis centers within the International Laser Ranging Service (ILRS). The measurements are the starting point for determining e.g. the rotational parameters, gravitation or the center of the Earth. As a result of the reliability and accuracy of its data the Graz SLR station was selected by ILRS as one of the five core stations worldwide.

The satellites measured by the SLR station can be split up in four main groups: (1) passive/geodetic satellites, (2) satellites in low Earth orbit, (3) navigation satellites, and (4) space debris.

Passive/geodetic satellites are of spherical shape and constructed in a way not to be influenced by external forces except gravitation. Typically, a large amount of retro-reflectors leads to response signals for SLR measurements, which can be easily identified. Distances range from 800 to 20000 km and their main area of use is highly precise measurements of the Earth's gravitational field.

Satellites in low Earth orbit can be found at distances between 450 and 1350 km. Their field of use is versatile and ranges from the measurement of Earth's ice mass, ocean currents, sea level rise etc. up to high resolution radar images. All of them need to know their accurate orbit, which is measured and determined by SLR data.

Besides the American and European satellite navigation systems GPS and Galileo several other countries such as China, Russia, and India sent their own navigation satellites to space. Distances vary between 20000 and 36000 km, with total masses of 600 to 1400 kg. Their field of use is the exact positioning and navigation on Earth.

The scientific emphasis of the Graz SLR station currently lies on distance measurements to space debris. Such measurements deliver valuable data to improve orbit predictions to space debris, which could reduce significantly reduce the amount of collision avoidance maneuvers necessary.