Laser technology tackles space debris

As part of the COVER Project, the Institute of Geodesy at TU Graz has seen the combination of gravity field measurements using satellites with the measurement method of satellite laser ranging (SLR) improve both gravity field calculations and the observation of space debris objects and their orbit predictions. The results have been integrated into the gravity recovery object oriented programming system (GROOPS) software, provided by the Institute of Geodesy free of charge.

“The satellite missions Grace, Grace Follow-on and previously GOCE have provided really valuable data for calculating the Earth’s gravity field. However, the long-wavelength of the gravity field, which covers masses of continental size, can not be resolved very well by using these missions,” explained Sandro Krauss from the Institute of Geodesy at TU Graz. 

SLR measurements, however, can resolve this. The process involves a network of SLR stations which point a laser at a satellite with retro-reflectors that reflect the emitted laser light. By measuring the travel time, the position of the satellites can be determined to within centimetres and, through multiple measurements, variations in the orbit resulting from changes in mass on the Earth’s surface can also be detected. 

“If you combine SLR with the other satellite measurement methods, the gravity field can be calculated much more accurately, as you can precisely resolve all wavelengths of the gravity field,” added Kauss. “This allows us to determine the water masses present on Earth in greater detail. At the same time, we can use the data obtained from the measurements to predict the position of satellites and space debris much better, locate them, map them with SLR and predict their future orbits very precisely, which contributes to more safety in orbit.”

There are approximately 40,000 pieces of space debris objects with a size of more than 10 centimetres orbiting the Earth, with one million pieces one centimetre or larger. They travel at around 30,000km/h and not in the same direction. A collision could spell out a significant impact, destroy satellites and endanger human lives in space stations or manned spacecraft. Locating the orbits of these objects and predicting future trajectories is all the more important.

Radar measurements are used to monitor space debris objects, but accuracy is limited. Existing orbit forecasts were faced with accuracy within a few kilometres, which made it harder to locate space debris. 

Using the Satellite Laser Ranging Station of the Austrian Academy of Sciences’ Space Research Institute at the Lustbühel Observatory, progress has been made.

The Institute of Geodesy used its force models to determine the position of a satellite or debris to an accuracy of around 100 metres; a significant step up. This makes tracking and recording space debris objects easier using the surveying laser.

“For orbit prediction, we have to model all the forces on the satellites,” said Torsten Mayer-Gürr from the Institute of Geodesy at TU Graz. “This also includes the Earth’s gravitational force, which is influenced by the presence of masses such as water. The combination of our orbit modelling with SLR measurements now allows much more accurate calculations in our GROOPS software, which is freely accessible to everyone. As far as we know, we are the only ones to offer such a comprehensive package for gravity field determination, orbit determination and SLR processing free of charge. This open source access has the advantage for us that we get feedback very quickly if something needs to be improved.”