GPS/BDS Continuous Network Time Transfer With Estimated Satellite Clocks
The strong coupling between the receiver and satellite clocks implies that traditional precise point positioning (PPP) inevitably inherits the uncertainties of external satellite clock products—such as limited precision and day boundary discontinuities—thereby degrading time transfer (TT) performance. Moreover, most existing network-based solutions are formulated using ionosphere-free (IF) combinations, which not only amplify measurement noise but can also introduce inconsistencies due to misaligned satellite-end products. In this work, we extend the small-scale baseline approach to a large-scale network solution using undifferenced and uncombined observations. Two models are developed: an introduced satellite clock (ISC) model as a control group, and a simultaneously estimated satellite clock (ESC) model, which is particularly robust to day boundary discontinuities in satellite clock products. We evaluate both models using 11 days of GPS and BeiDou Navigation Satellite System (BDS) data from a European network of five stations spanning several thousand kilometers. The results show that, at the 95 % confidence level, the ESC model improves daily frequency stability by 1 %–23 % for GPS and 8 %–25 % for BDS relative to the ISC model when Centre for Orbit Determination in Europe (CODE) products are used. Comparisons across products from different analysis centers (ACs) further indicate that satellite clock products exhibit more pronounced day boundary discontinuities than orbit products. For the longest SPT0-IENG baseline (1455 km), the BDS ISC solution based on GRG products shows more frequent and larger jumps, whereas the ESC model delivers an additional 8.30 % improvement beyond the result obtained with CODE products. In addition, verification using different level products shows that the ESC model can effectively reduce the adverse impact of (near) real-time products. In general, the network-based ESC approach actually mitigates uncertainties in satellite clock products and is well suited for both postprocessed and (near) real-time applications.