Determining geocenter motion using combined ground and spaceborne GPS observations with ambiguity resolution

Owing to the dense global tracking network and the abundant satellites providing continuous observations, the Global Navigation Satellite System (GNSS) has the potential to measure geocenter motion. However, the high altitude of GNSS satellites and the estimation of phase ambiguities compromise the quality of the estimated geocenter coordinates (GCC). This study conducted a combined zero-difference processing of ground observations from 98 global stations and spaceborne observations from four low Earth orbiters (LEOs), investigating the impact of integer ambiguity resolution and LEO configuration on GPS-derived GCC estimates. The ambiguity resolution in the double-difference mode between ground stations and one LEO can significantly improve the observability of GCC estimates, reducing the formal errors by 69.4 %, 69.4 % and 55.0 % for the X, Y and Z components, respectively. The faster relative motion between LEO and GPS satellites contributes to a faster change of tracking geometry. Integrating four LEOs into ground network, the formal errors of GCC parameters are reduced by 77.8 %, 78.2 % and 50.5 % for the X, Y and Z components, respectively. For the most concerned GCC Z component, its correlation with the B_1C empirical parameter is reduced from 0.69 to 0.41, demonstrating that the GCC Z component is more separable from the orbital parameters. Besides, the 5th and 7th draconitic harmonics of the Z component are mitigated by 67.0 % and 73.5 %, respectively. The derived annual signal shows a good consistency with the external SLR-based solution, and the amplitude differences are only 0.9 mm, 1.6 mm and 1.2 mm for the X, Y and Z components, respectively.