Last night at 22:17 UTC (24: 14 MESZ) the Satellite Laser Ranging Station on Telegrafenberg in Potsdam was for the first time able to locate the exact positions of the two satellites of the Grace-FO (‚Gravity Recovery And Climate Experiment Follow On‘) mission. The German-American satellite mission that was started successfully during the night of last Thursday to Wednesday consists of two satellites that circle the globe one after another. Like this, they will survey Earth’s gravity field.

The Satellite Laser Ranging Station Potsdam operates continuously since 2003. With a high-precision laser it locates satellites and space debris within the orbit of the Earth. The measurements serve for example for the determination of changes in parameters like the Earth’s rotation. They also support the GRACE-FO mission in calculating changes in the Earth’s gravity field. These changes are based on changes in mass distributions that are caused by changes in ice masses, wind systems, groundwater or sea level as well as tectonic processes. With these data model calculations are supported to investigate climate change, for example.

The GRACE-FO satellites circle the Earth in a polar orbit and overfly the station in Potsdam approximately every twelve hours. A first try to measure the satellites immediately after the start of the mission was, however, unsuccessful, since the orbits of recently launched satellites are not know very well at that point in time. However, 24 hours after the launch, during the third overfly, it already worked. Sven Bauer, head of the Laser station: “We are especially happy that we were able to measure both GRACE-FO satellites during the same overflight, which is not that easy. Therewith we were the first station worldwide.”

Frank Flechtner, leading scientist of the mission: „These are further brilliant news. So far the whole mission runs like we imagined it only in our boldest dreams. Just great.” The GRACE-FO satellites are equipped with GPS receptors which makes the calculation of the orbits much better. Bauer: “We only had a spatial offset of the positions compared to the calculated orbit of 2.3 meters for the first and 3.8 meters for the second satellite. Only 24 hours after the start these are very small deviations.“ Within the global network of Satellite Laser Ranging stations Potsdam was the first to measure both satellites within the same overflight and the first to measure GRACE-FO 1. GRACE-FO 2, however, was for the first time sighted by a station in Simeiz in the Ukraine, already at 20:42 UTC. (ak)

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