Unprecedented Daily Measurements Charting Earth’s Rotation

Scientists at the Technical University of Munich (TUM) have made significant progress in measuring the Earth’s rotation with unprecedented precision. The Wettzell Geodetic Observatory’s ring laser can now be used to record data at a level of quality unmatched anywhere in the world. These measurements are crucial for determining the Earth’s position in space, supporting climate research and increasing the reliability of climate models.

Advanced ring laser technology

Would you like to quickly go down to the basement and see how fast the Earth has been spinning in the last few hours? This is now possible at the Wettzell Geodetic Observatory. TUM researchers have improved the ring laser there so that it can provide up-to-date data on a daily basis, which was not possible until now at comparable quality levels.

What exactly does the ring laser measure? During its journey through space, the Earth rotates on its axis at slightly varying speeds. Moreover, the axis on which the planet rotates is not completely static, it wobbles a little. This is because our planet is not completely solid, but consists of different parts, some solid, some liquid. The inside of the Earth itself is therefore constantly in motion. These mass shifts speed up or slow down the planet’s rotation, differences that can be detected using measuring systems such as the TUM ring laser.

The ring laser in Wettzell has been continuously improved since it was put into use. Credit: Astrid Eckert/TUM

“Fluctuations in rotation are not only important for astronomy, we also urgently need them to create accurate climate models and better understand weather phenomena such as El Niño. And the more accurate the data, the more accurate the predictions,” says Prof. Ulrich Schreiber, who led the project at the Observatory for TUM.

Technical refinements and challenges

When overhauling the ring laser system, the team prioritized finding a good balance between size and mechanical stability, as the larger such a device is, the more sensitive the measurements it can make. However, size means compromises in stability and therefore precision.

Another challenge was the symmetry of the two opposing laser beams, the heart of the Wettzell system. Exact measurements are only possible if the waveforms of the two counter-propagating laser beams are virtually identical. However, due to the design of the device, there is always a certain amount of asymmetry present. Over the past four years, geodesists have used a theoretical model for laser oscillations to successfully capture these systematic effects, to the extent that they can be accurately calculated over long periods of time and thus eliminated from the measurements.

Improved precision and applications

The device can use this new corrective algorithm to accurately measure the Earth’s rotation to 9 decimal places, which corresponds to a fraction of a millisecond per day. In terms of the laser beams, this corresponds to an uncertainty that starts at just the 20th decimal place of the light frequency and remains stable for several months. Overall, the observed up and down fluctuations reached values ​​as short as 6 milliseconds over periods of about two weeks.

The improvements in the laser have now also made significantly shorter measurement periods possible. Thanks to the newly developed correction programs, the team can record current data every three hours. Urs Hugentobler, professor of satellite geodesy at TUM, says: “In geosciences, such high time resolution levels are absolutely new for stand-alone ring lasers. Unlike other systems, the laser functions completely independently and no reference points in the room are required. In conventional systems, these reference points are created by observing the stars or using satellite data. But we are independent of those kinds of things and also extremely precise.” Data recorded independently of stellar observations can help identify and compensate for systematic errors in other measurement methods. Using different methods makes the work particularly painstaking, especially when accuracy The requirements are high, as is the case with the ring laser. Further improvements to the system are planned for the future, making even shorter measurement periods possible.

Understanding ring lasers

Ring lasers consist of a closed, square beam path with four mirrors completely enclosed in a Ceran glass-ceramic body, also called the resonator. This ensures that the length of the path does not change due to temperature fluctuations. A helium/neon gas mixture in the resonator allows excitation of the laser beam, one clockwise and one counterclockwise.

Without the motion of the Earth, light would travel the same distance in both directions. But because the device moves with the Earth, the distance for one of the laser beams is shorter because the Earth’s rotation brings the mirrors closer to the beam. In the opposite direction, the light travels a correspondingly greater distance. This effect creates a difference in the frequencies of the two light waves, the superposition of which generates a counting tone that can be measured very accurately. The higher the speed at which the Earth rotates, the greater the difference between the two optical frequencies. At the equator, the Earth rotates 15 degrees eastward every hour. This generates a 348.5 Hz signal in the TUM device. Fluctuations in the length of a day manifest themselves with values ​​of 1 to 3 millionths of Hz (1 – 3 microhertz).

Robust and accurate infrastructure

Each of the sides of the ring laser in the basement of the Wettzell observatory measures four meters. This structure is then anchored to a massive concrete column that rests on the solid bottom of the Earth’s crust at a depth of approximately six meters. This ensures that the Earth’s rotation is the only factor affecting the laser beams and rules out other environmental factors. The structure is protected by a pressure chamber that compensates for changes in air pressure or in the desired temperature of 12 degrees Celsius and automatically compensates for these changes. To minimize such influencing factors, the laboratory is located at a depth of five meters under an artificial hill. Almost 20 years of research work has gone into developing the measurement system.

Reference: “Variations in the Earth’s rotation speed measured with a ring laser interferometer” by K. Ulrich Schreiber, Jan Kodet, Urs Hugentobler, Thomas Klügel and Jon-Paul R. Wells, September 18, 2023, Nature Photonics.
DOI: 10.1038/s41566-023-01286-x