Galileo Satellites Yield Five-Fold Improvement in Measuring Gravitational Time Dilation

Europe's Galileo satellite navigation system has unexpectedly served as a platform for fundamental physics, facilitating the most precise measurement ever recorded for gravitational time dilation, also known as gravitational redshift. This phenomenon, a core prediction of Albert Einstein's Theory of General Relativity, describes how variations in a gravitational field directly influence the passage of time. The significant experimental advance, first reported in late 2018, was achieved by analyzing data from specific Galileo satellites, namely Galileo 5 and FM06, which were situated in eccentric orbits following an earlier launch anomaly.

This measurement achieved an improvement in accuracy of approximately five-fold when compared to the previous benchmark, the 1976 Gravity Probe-A experiment, which measured the effect to within 0.007 percent. The exceptional precision required for this test was made possible by the highly stable Passive Hydrogen Maser (PHM) atomic clocks aboard the satellites. These PHM clocks are superior to the system's Rubidium clocks; the PHM is capable of losing only one second over three million years, contrasting with the Rubidium clock’s loss of three seconds per million years. The two research consortiums, led by SYRTE Observatoire de Paris in France and Germany's ZARM Center of Applied Space Technology and Microgravity, independently conducted the analysis under the coordination of the European Space Agency's (ESA) Galileo Navigation Science Office.

The eccentric orbits of satellites E14 and E18 provided the necessary variation for this thorough test of General Relativity across different gravitational potentials, causing a time shift modulation of approximately 370 nanoseconds due to changing gravity levels. The utility of this space infrastructure thus extends beyond practical navigation, demonstrating a dual purpose for scientific advancement. The PHM clocks, which utilize the properties of hydrogen atoms to maintain a precision frequency reference, were essential for achieving this level of accuracy. The PHM clock functions as the master clock on each Galileo satellite and is designed for a 20-year operational lifetime, weighing 18 kilograms and occupying 28 liters of volume.

While the Galileo data provided a direct, highly accurate measurement of gravitational redshift on atomic clock frequency, the equivalence principle has also been rigorously tested by other missions. For instance, the French MICROSCOPE satellite confirmed this principle—that all objects fall at the same rate regardless of mass or composition—to a precision of two-trillionths of a percent by comparing the fall rates of different test masses. The MICROSCOPE mission launched in April 2016 and concluded its work in 2017, with final results published subsequently.

The scientific relevance of this gravitational redshift measurement remains pertinent as of February 2026, coinciding with the ongoing enhancement of the Galileo constellation. The system is transitioning toward the deployment of the Galileo Second Generation (G2G) satellites, with the first launch anticipated in 2026 utilizing an Ariane-6 launcher, following the qualification flight in July 2024. As of February 1, 2026, a total of 34 Galileo satellites have been launched, with 26 currently operational, contributing to a system where 10% of the European Union's yearly Gross Domestic Product relies on its precise positioning and timing information. This continuous evolution ensures that the platform for fundamental physics testing, unexpectedly born from orbital anomalies, will continue to advance high-precision scientific inquiry alongside its primary global navigation mandate.