Variations in the Earth’s rotation rate measured with a ring laser interferometer

An exact knowledge of the instantaneous Earth's rotation rate is indispensable for accurate navigation and geolocation. Fluctuations in the length of sidereal day are caused by momentum exchange between the fluids of the Earth (namely, the atmosphere, hydrosphere and cryosphere) and the solid Earth. Since a multitude of different globally distributed and independent mass transport phenomena are involved, the resultant effect on the Earth's rotation is not predictable and needs to be continuously measured. Here we report the observation of minute variations in the rotation rate of the Earth at the level of five parts per billion, namely, with a resolution of a few milliseconds over 120 days of continuous measurements. We employ an inertial self-contained measurement technique based on an optical ring laser interferometer rigidly strapped down to the Earth's crust and operated in the Sagnac configuration. This large-scale gyroscope integrates over three hours for each data point, as opposed to an entire global network of Global Navigation Satellite Systems receivers and Very Long Baseline Interferometry that can only provide a single measurement per day. A self-contained ring laser interferometre measures length-of-day variations due to global mass transport phenomena with a precision of a few milliseconds over several months of measurements.