Benefits of tidal admittance functions for refining GNSS-observed solar and lunisolar tidal constituents

Ocean tide loading (OTL) displacements observed by the Global Navigation Satellite System (GNSS) are generally less accurate for solar (S2 and P1) and lunisolar (K2 and K1) tidal constituents than for lunar tidal constituents (M2, N2, O1 and Q1). We propose to construct the tidal admittance function (TAF) in the complex domain for tidal inference, aiming to refine noisier GNSS-observed solar and lunisolar constituents. The vertical OTL displacements are estimated from the single GNSS (GPS and Galileo) and multi-GNSS (GPS + Galileo + GLONASS) solutions, using kinematic precise point positioning under undifferenced ambiguity resolution to process 2.5 years of continuous GNSS observations from 49 global GNSS stations. Results show that TAF inferred P1 and K2 agree better with FES2014b model predictions than GNSS estimates for most GNSS stations. We find that the P1 and K2 inferences are only perturbed by less than 0.3 times the observed K1 and S2 errors, respectively. In contrast, the K1 and S2 inferences suffer from over three times the observed P1 and K2 errors. As a result, the tidal inference can only provide limited refinement of GNSS K1 and S2 estimates for a few stations. By comparing the two different TAF approaches for the tidal inference, we find that direct fitting of phase lags is susceptible to phase addition and subtraction of 360°, without disrupting our approach. In addition, the OTL displacements predicted at most GNSS stations are found to satisfy the smoothness of the tidal admittance, which is fundamental to the robustness of the tidal inference. Our approach is also applicable to improve the horizontal solar and lunisolar constituents estimated by GNSS, but should be used with caution in cases where smooth admittances may be distorted by the tidal resonance.