Implementing a Common Clock Framework in GNSS: Harnessing Fiber-Optic Links and Global Hydrogen Maser Networks for Enhanced Parameter Estimation

The fundamental reliance of GNSS on one-way signal travel time measurements necessitates precise clock synchronization. This introduces high correlations between satellite/receiver clock offsets and nearly all other estimated parameters—such as station coordinates, tropospheric delays, and orbital elements—creating a fundamental bottleneck in modern high-precision geodesy by limiting the independent determinability of these parameters.

Recent breakthroughs in time-frequency technology offer promising pathways to mitigate this issue. Ultra-stable optical clocks and fiber-optic time transfer have emerged as transformative tools. Fiber-optic links can synchronize the clocks of GNSS receivers to a remarkable degree, achieving fractional frequency stability of clock difference at the 10^-18 level—several orders of magnitude beyond GNSS-based synchronization. Consequently, receivers connected via fiber can be treated as sharing a common clock. In parallel, highly stable hydrogen masers, already deployed at many permanent GNSS stations, provide another foundation for common-clock processing. When two receivers are each equipped with a hydrogen maser, the stability of their clock offset difference can approach that achievable via fiber links, effectively constituting a "virtual" common clock even in the absence of a physical connection.

To leverage these advancements, we developed and implemented a novel module that incorporates common-clock constraints into the widely used Bernese GNSS Software. This module enforces that multiple receivers share a single common clock parameter per epoch. Initial processing results demonstrate that applying this constraint significantly reduces noise in key estimated parameters, notably in station height time series and high-frequency (e.g., 10 to 30 minutes) tropospheric delay estimates.

The implementation of a common-clock framework opens several avenues for future enhancement of GNSS. Beyond reducing parameter noise through decorrelation, it raises the prospect of establishing a more stable time reference for GNSS networks—potentially realized as a software-generated composite clock. This work represents a critical step toward integrating next-generation timekeeping infrastructure into global geodetic networks, with the goal of improving the stability of the terrestrial reference frame and the precision of all GNSS-derived geodetic products.