Double-difference and precise point positioning processing strategies to determine OTL effect in GNSS time series

The study investigates the determination of ocean tidal loading (OTL) effects in GNSS time series using two primary processing techniques: double difference (DD) and precise point positioning (PPP). OTL-induced surface displacements, driven by lunar and solar gravitational forces, poses significant challenges in geodetic applications, requiring precise modeling to improve GNSS positioning accuracy. This research analyzes GNSS data from selected stations to assess the strengths and limitations of DD and PPP in estimating OTL displacements. The study eliminates solid earth body tides (EBT) effects and focuses on dominant tidal constituents such as M2, N2, O1, and Q1, comparing observed GNSS-derived displacements with modeled predictions. The results indicate that applying an OTL correction significantly enhances GNSS solution stability, leading to at least a 20% reduction in the RMS of station positions, with the most substantial reduction observed in the up component. The DD technique exhibits lower RMS values and better coherence with ocean tidal models in both vertical and horizontal components, making it more effective in mitigating common-mode errors. In contrast, PPP provides globally consistent positioning but shows greater sensitivity to systematic biases, particularly in solar-related tidal constituents (S2, K2, K1, P1), where discrepancies with models exceed 2 mm in certain cases. Tidal analysis reveals that major constituents such as M2, N2, O1, and Q1 align well with modeled predictions, while K1 and S2 exhibit residual deviations, particularly in the PPP solution. The largest residual appears in the PPP-derived vertical component for K1, suggesting potential limitations in current OTL modeling due to data being affected by satellite orbit/clock mismodeling errors and daily repeating multipath effects at the ground station. These findings underscore the importance of selecting appropriate GNSS processing techniques based on application needs. While DD offers rapid convergence and higher accuracy in relative positioning, PPP remains valuable for global applications despite its longer convergence time and sensitivity to systematic errors. The study highlights the necessity for improved multi-GNSS integration and refined tidal models to enhance OTL displacement estimation, which is essential for advancing geodesy, sea-level monitoring, and Earth deformation research.