A giant underground motion sensor in Germany tracks Earth’s wobbles


A giant underground motion sensor in Germany has taken its first measurements of Earth’s spin and tilt. Although researchers are still getting the machine’s accuracy up to snuff, their observations could someday keep GPS navigation working reliably on devices like smartphones.

Phenomena like earthquakes and ocean tides continually knock Earth’s rotation off-kilter, requiring constant correction of GPS satellite signals. Typically, corrections use telescope observations, which offer a set of celestial coordinates to determine Earth’s orientation in space. But telescope data can take days to process. The Rotational Motions in Seismology, or ROMY gyroscope array, can monitor Earth’s tiny wobbles continuously, researchers demonstrate in an experiment reported online July 17 in Physical Review Letters.

ROMY is an upside-down pyramid of pipes, about the length of a telephone pole on each side. Its four triangular faces measure motion in different directions. On each side, one laser beam runs clockwise through the triangular piping, while another runs counterclockwise. As the triangles move with Earth’s rotation, laser beams running in the same direction as that motion have to travel farther to loop around the triangle. That stretches out the beams’ wavelength. Meanwhile, the beams going the opposite way have their wavelength compressed by their shorter path. The mismatch between wavelengths reveals the speed and tilt of Earth’s rotation.

Rotational Motions in Seismology lab
About 20 kilometers west of Munich, the Rotational Motions in Seismology, or ROMY, is built into an underground laboratory (shown here from an aerial view). This behemoth motion sensor uses laser light coursing through a pyramid of pipes to reveal tiny deviations in the Earth’s spin and tilt.Fa. Wadle

During a nearly seven-week test run in spring 2019, ROMY was sensitive to changes in Earth’s tilt by less than 0.00014 degrees. That seemingly tiny shift moves the Earth’s poles across the ground by about 15 meters. ROMY could also detect a change in Earth’s spin rate so small that it would add up to only a four-second difference in the length of a day, over a single rotation.

That’s still not as accurate as telescopes, says ROMY team member Heiner Igel, a seismologist at Ludwig Maximilians University of Munich. To be competitive, ROMY must be at least 100 times more accurate. That will require fortifying the machine against temperature shifts that cause its piping to expand and contract, messing with measurements.

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