Research on the modified tri-frequency combination model for determining geopotential using China space station...

Determining the gravity potential (GP) is a major task for geodesy. Advances in atomic clocks enable relativistic geodesy. China Space Station (CSS) carries an optical clock with stability ${2\times 10}^{-15}/\sqrt{\tau }$ , but no previous frequency comparison model matches its accuracy. Previous tri-frequency models required fixed frequency ratios and ignored higher-order ionospheric effects. In this study, an improved tri-frequency combination model is proposed that extends to the c-4 terms of the GP and accounts for Doppler shifts, tropospheric and higher-order ionospheric effects, tidal effects, and the Shapiro delay. Crucially, it can be applied to any arbitrary combination of three frequencies. To validate the performance of the modified model, we use CSS tri-frequency microwave links for space-to-ground comparison and simulate GP determination at ground stations. With EGM2008 as the truth, the model achieved 0.7 m²/s² accuracy, supporting centimeter-level unification of global height datums. The model provides theoretical support for future real-data processing.