All-frequency IGS phase clock/bias product combination to improve PPP ambiguity resolution

Satellite product combination has been a major effort for the International GNSS Service Analysis Center Coordinator to improve the robustness of orbits, clocks and biases over original AC-specific contributions. While the orbit and clock combinations have been well documented, combining phase biases is more of a challenge since they have to be aligned with the clocks precisely to preserve the exactitude of integer ambiguities in precise point positioning (PPP). In the case of dual-frequency signals, frequency-specific phase biases are first translated into an ionosphere-free form to agree with the IGS satellite clocks, and they can then be integrated as integer clocks to facilitate a joint combination. However, regarding multi-frequency phase biases, forming their ionosphere-free counterparts would be cumbersome as they are linearly dependent. We therefore propose a concept of “frequency-specific integer clock” where all third-frequency phase biases are integrated individually with satellite clocks to enable an efficient frequency-wise combination. The resultant combined product will ensure all-frequency PPP ambiguity resolution over any frequency choices and observable combinations. Our combination test based on the GPS/Galileo satellite products from four IGS-ACs in 2020 showed that the mean phase clock/bias consistencies among ACs for all third-frequency signals (i.e., GPS L5, Galileo E6 and E5b) were as high as 10 ps, and the ambiguity fixing rates were all around 95%. Both quantities reached the same levels as those for the baseline frequencies (i.e., GPS L1/L2 and Galileo E1/E5a). The combined products outperformed AC-specific products since outlier contributions were excluded in the combination.