Laser measurements to track space debris and observe water masses

What do the Earth's gravitational field and the trajectories of satellites and space debris have in common? The Earth's gravitational field influences the orbits of our companions in space, while the changes in the orbits in turn allow conclusions to be drawn about changes in the gravitational field and thus existing water masses. In the COVER project, the Institute of Geodesy at TU Graz has now combined gravity field measurements using satellites with the measurement method of satellite laser ranging (SLR), thereby sustainably improving both gravity field calculations and the observation of objects in space and their orbit predictions. The results have been incorporated into the gravity recovery object oriented programming system (GROOPS) software, which the Institute of Geodesy provides free of charge via GitHub.

Precise resolution of the Earth's long-wave gravity field

"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," says Sandro Krauss from the Institute of Geodesy at TU Graz. Measurements with SLR, on the other hand, can resolve this long-wavelength part very precisely. To do this, a network of SLR stations points 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. 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 currently around 40,000 pieces of space debris objects with a size of more than ten centimetres orbiting the Earth; there are around 1 million pieces one centimetre or larger. They are travelling at around 30,000 km/h and are not all flying in the same direction. A collision would therefore have quite a large impact and would destroy satellites and endanger human lives in space stations or other manned spacecraft. This makes it all the more important to locate the orbits of all objects and predict their future trajectories as accurately as possible.

Centimetres instead of kilometres

Radar measurements are currently used to monitor all space debris objects, but their accuracy is limited. And the existing orbit forecasts also suffered from the fact that they were only accurate to within a few kilometres. This subsequently made it more difficult to locate them. Together with the Satellite Laser Ranging Station of the Austrian Academy of Sciences' Space Research Institute at the Lustbühel Observatory, decisive progress has been made here. The Institute of Geodesy used its own force models, which can be used to determine the position of a satellite or debris to an accuracy of around 100 metres. This made it easier to track and record them precisely with the surveying laser. Further measurements during subsequent flybys provided an even more accurate picture of how the orbit behaves, which enabled the researchers to improve the predictions.

"For orbit prediction, we have to model all the forces on the satellites," says 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."