Tuning an orchestra without a conductor: the algorithm decides on its own which instruments should play and how loud, all to reproduce Earth's gravitational symphony.
Two satellites fly in tandem at an altitude of 500 kilometers, constantly measuring the distance between them. When a mountain or an underground lake drifts beneath, gravity shifts slightly, and the satellites either draw closer or drift apart. Over a month, they relayed more than half a million such measurements, roughly one for every 20 kilometers of their path. That's like measuring gravity with a step the length of Manhattan.
Previously, the algorithm was tested only on small synthetic examples, leaving doubts about its performance on real data. Now, for the first time, it processed all measurements from actual orbits. The error came to about 17.5%, concentrated in mountainous regions where the field's details are too subtle to resolve from such a height. But now the method can be used to track glacier melt and underground water movement. The next step is to work with the rawest satellite signals, skipping any preprocessing.
Each GRACE-FO satellite completes approximately 496 orbits around Earth in December 2022, crossing the equator twice per orbit. That gives 1984 points at the equator, corresponding to a resolution of about 20 km: as if you were measuring gravity at intervals the length of Manhattan.
In Arthur C. Clarke's novel 'The Fountains of Paradise', engineers build a space elevator that requires precise knowledge of Earth's gravitational field. The method described here for recovering the potential from satellite data could be part of such technology.