Advances and Prospects in Gravity Potential Determination Based on the GNSS Frequency Shift Approach

The precise determination of the gravity potential (geopotential) field is one of the foundational tasks in geodesy. The method of using high-precision time and frequency signals to determine geopotential has attracted widespread attention in the fields of geodesy and geophysics. This method, based on the general relativity theory, utilizes high-precision atomic clocks and highprecision time-frequency transfer techniques to directly determine the geopotential difference as well as the orthometric height difference between two arbitrary stations on the ground. Notably, the GNSS time-frequency transfer technique, boasts advantages such as high precision, all-weather capability, flexible networking, and economic feasibility, gradually forming a novel research field of geopotential determination by a GNSS frequency shift approach. First, the principles of geopotential measurement via time and frequency comparison are introduced, and the mathematical model conducive to centimeter-level geopotential measurement is analyzed. Next, the research progress of geopotential measurement utilizing the GNSS frequency shift approach is summarized. Two typical GNSS time and frequency transfer techniques, the GNSS common view (CV) technique and the GNSS precise point positioning (PPP) technique, are investigated. The relevant experiments for geopotential measurement based on the two techniques are reviewed and discussed. Afterward, the challenges that urgently need to be overcome in this field are analyzed, mainly including the modeling of GNSS receiver clock offset series, the error process in GNSS time-frequency transfer, and the data process for the receiver clock offset series. Considering the significant role of highperformance clocks in GNSS frequency shift approach, the development of atomic clocks is described. Furthermore, some discussions on potential applications of the GNSS frequency shift approach are prospected, such as achieving high-precision geopotential measurement, establishing a unified world height datum with high precision, and enabling simultaneous measurement of threedimensional geometric position and corresponding geopotential values. This paper provides references for further research on geopotential determination via the GNSS frequency shift approach and related studies.