Multi-GNSS clock combination for PPP with an epoch-wise clock alignment and variance component estimation

Abstract Applications of Global Navigation Satellite Systems (GNSS) for positioning, navigation, and timing rely on precise orbit and satellite clock corrections provided by multiple Analysis Centers (ACs) of the International GNSS Service (IGS). To address modeling errors and data gaps inherent to individual ACs, the GNSS community develops and refines procedures for receiving a single, robust combined product. While the IGS has recently extended its combination products with demonstrational multi-GNSS solutions for GPS, Galileo, and GLONASS, these are not yet final operational products and do not include BeiDou. This study introduces an integrated operational framework for multi-GNSS clock combination, covering GPS, Galileo, GLONASS, and BeiDou, and implemented in open-access, open-source software using an epoch-wise alignment strategy and variance component estimation. The quality of the combined clocks is assessed through pairwise AC comparisons, frequency stability analysis, and Precise Point Positioning (PPP) results for IGS stations. The epoch-wise alignment strategy successfully removes the issue arising from the non-linear behavior of the ACs’ clock products and reduces inter-AC dispersion to around 0.05 ns. This approach also yields more stable AC weights over time and achieves a higher agreement with the AC products for the GPS and Galileo blocks by 30–70% than the linear-fit alignment strategy. PPP solutions based on the epoch-wise combination deliver coordinate estimates that match or surpass those obtained from individual ACs. Finally, the combination strategy improves reference frame stability, improving the parameter repeatability by up to 40% compared to individual ACs.