Experiments of Determining Geopotential Difference Between Two Remote Sites Using Two Hydrogen Clocks

Based on the principle of geopotential frequency shift in general relativity, according to the geopotential frequency shift equation, the geopotential difference between two locations can be determined by measuring the frequency shift of signals exchanged between them. In this study, two hydrogen masers with frequency stability of 5 × 10⁻¹⁵/day were employed, in combination with the GNSS Precise Point Positioning (PPP) technique, to conduct remote clock comparison experiments from March to July 2019. The experiments aimed to determine the geopotential difference between Jiugong Mountain and Wuhan. Through an innovative approach involving alternating zero-baseline and long-baseline observational schemes, systematic clock errors were effectively eliminated. Four experimental sessions were conducted, alternating between zero-baseline and long-baseline setups to mitigate systematic errors of the clocks. To address the complex noise characteristics in PPP observation data, Ensemble Empirical Mode Decomposition (EEMD) method was applied to extract effective signals related to geopotential frequency shifts. The results indicate that the measured orthometric height difference between Jiugong Mountain and Wuhan deviates by -7.8 m from the result derived from both the EGM2008 and EIGEN-6C4 models, with a measurement uncertainty of ±32.4 m, which is consistent with the frequency stability of the hydrogen maser used. This study provides a validation of the feasibility of determining geopotential differences using GNSS PPP-based remote clock frequency comparison in geodetic applications, offering an innovative solution for remote height transfer without relying on fiber-optic links.