A novel atomic clock in space may assist in gauging Earth’s elevations.
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To prevent costly construction errors, in 2015 scientists in the International Association of Geodesy voted to adopt the International Height Reference Frame, or IHRF, a worldwide standard for elevation. It’s the third-dimensional counterpart to latitude and longitude, explains Sanchez, who helps coordinate the standardization effort. Now, a decade after its adoption, geodesists are seeking to update the standard—by utilizing the most precise clock ever to fly in space.
That clock, known as the Atomic Clock Ensemble in Space (ACES), launched into orbit from Florida last month, destined for the International Space Station. ACES, developed by the European Space Agency, consists of two connected atomic clocks: one operating with cesium atoms and the other with hydrogen. These components work together to produce a single set of ticks with greater precision than either clock could achieve alone.
Pendulum clocks are only accurate to about a second per day, as their rate can fluctuate with humidity, temperature, and the accumulation of dust. In contrast, atomic clocks in current GPS satellites will lose or gain a second on average every 3,000 years. ACES, however, “will not lose or gain a second in 300 million years,” notes Luigi Cacciapuoti, an ESA physicist involved in the project. (In 2022, China installed a potentially more stable clock on its space station, though there has been no public disclosure on its performance after launch, according to Cacciapuoti.)
From space, ACES will connect with some of the most accurate clocks on Earth to create a synchronized clock network designed to perform tests of fundamental physics. This network is of particular interest to geodesists as it can facilitate gravitational measurements, establishing a more precise zero point from which to gauge elevation across the globe.
Alignment over this “zero point”—essentially where you position the end of a tape measure to determine elevation—is crucial for international collaboration. It simplifies the monitoring and comparison of sea-level changes worldwide and is especially beneficial for infrastructure projects involving flowing water, such as dams and canals. In 2020, the international height standard resolved a longstanding dispute between China and Nepal regarding Mount Everest’s height. While China reported the mountain at 8,844.43 meters, Nepal measured it at 8,848. The application of the IHRF allowed both countries to agree that the Everest height is 8,848.86 meters.
A worker performs tests on ACES in a cleanroom at the Kennedy Space Center in Florida. ESA-T. PEIGNIER
To establish a standard zero point, geodesists create a model of Earth called a geoid. Every point on this uneven, potato-shaped model experiences uniform gravity; hence, if a canal is dug at the height of the geoid, the water within it will remain level and not flow. The distance from the geoid establishes a global system for altitude.
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