A GNSS-Based Single-Clock Method for Geopotential Difference Determination and Its Outlook with Optical Clocks

Here we propose a single-clock measurement scheme for determining the geopotential difference between two stations using a single clock combined with GNSS Precise Point Positioning (PPP) time-frequency transfer. An experiment was conducted on two remote stations, with a distance 129 km and a height difference 1245 m, using a single hydrogen maser. Utilizing the International GNSS Service (IGS) time as reference, the geopotential frequency shift between the two stations was extracted by comparing the frequency differences between the hydrogen maser and the IGS time before and after clock transportation, and the measured geopotential difference between the two stations is 12075.9±118.5 m²/s², which shows consistency with the value derived from the EIGEN-6C4 global gravity field model, with a deviation of (-79±119) m²/s². Compared with traditional dual-clock methods, this approach obviates the need for inter-clock calibration, reduces operational complexity and equipment investment costs, and improves data utilization efficiency. In the future, the integration of optical clocks into the GNSS observation system is anticipated achieving methodological breakthroughs. Hence, we expect the geopotential difference measurement by integrating optical-clock technology into the GNSS-based single-clock scheme. The prospects include the GNSS receivers connected with optical clock signals, the high-stability optical-to-radio frequency conversion, and the establishment of an integrated space–ground optical frequency network comprising satellite-borne optical clocks and fiber-connected ground stations. This approach is envisioned to enable high-precision geodetic applications, such as equi-frequency geoid definition, centimeter-level orthometric height transfer, global height datum unification, while providing a novel platform for fundamental physics investigations (such as gravitational waves and dark matter detections). These capabilities underscore the transformative potential of optical-clock-enhanced GNSS technology across both geodetic science and fundamental physic. This study is supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 42388102, 42030105, and 42274011), and Gravitation Center Project (NGL-2025-004).