Estimation of ocean tidal loading through hourly GNSS processing and analysis of ocean tides in the Adriatic Sea

Ocean tidal loading (OTL) represents the elastic response of the solid Earth to the varying weight of ocean tides. Although these deformations are typically at the millimeter to centimeter scale, they carry important information on the coupling between ocean dynamics, crustal elasticity, and geodetic observation. Detecting and modeling such small signals remains particularly challenging in regions where tidal amplitudes are weak and ocean models have limited spatial resolution. The Adriatic Sea, with its shallow bathymetry, semi-enclosed geometry, and strong resonance at diurnal and semidiurnal periods, provides a natural laboratory for addressing this problem. The study investigates how reliably OTL can be detected and quantified from GNSS coordinate time series and how the combination of geodetic and oceanographic observations can improve the characterization of tidal forcing and crustal response. The problem was tackled comprehensively, analyzing tidal constituents in GNSS observations, and testing the ocean tidal models against sea surface observations from tide gauge (TG) and GNSS reflectometry (GNSS-R). The first focuses on methodology, assessing the sensitivity and feasibility to detect OTL constituents of different GNSS processing strategies (absolute and differential). The second applies these methods regionally to the northern Adriatic, integrating GNSS with TG and GNSS-R data to analyze both the ocean tides and the associated surface deformation. Hourly GNSS processing, applied to continuous observations, allows the recovery of tidal displacements at millimetric level. Introducing modern OTL corrections consistently reduces coordinate scatter by up to 22% in the vertical and 10-15% in the horizontal components, confirming that current models capture most of the tidal load. However, residual spectra reveal coherent tidal energy, dominated by the diurnal K1 and semidiurnal M2 constituents, pointing to remaining model–data mismatches and orbital aliasing. PPP solutions retrieve a greater number of tidal constituents, whereas DD tends to suppress weaker harmonics due to differencing. The detectability of tidal lines depends critically on record length and noise. Multi-year series enhance spectral resolution and amplitude stability, while higher noise levels can obscure weak tides even in long datasets. In the regional application of the Adriatic sea, TG analyses confirm a northward amplification of semidiurnal energy (M2 and S2) and a coherent phase progression consistent with resonance. The global model FES2014b reproduces the main pattern, though with localized discrepancies attributable to bathymetric and coastal complexity. GNSS-R observations provide an effective complement to TGs, recovering sea level variations with 2–3 cm agreement and correlations exceeding 90%, demonstrating the potential of low-cost, autonomous coastal monitoring. GNSS-derived OTL displacements reveal a coherent semidiurnal elastic response consistent with model predictions, while diurnal discrepancies primarily reflect GNSS orbital effects rather than true ocean model errors. The work documents the first hourly GNSS detection of OTL in the Adriatic Sea. By integrating GNSS, GNSS-R, and TG observations, it establishes a cross-validated framework capable of resolving sub-centimeter crustal deformation in a small and resonant basin. The findings advance methodological understanding of GNSS-based tidal analysis and highlight the benefits of combining geodetic and oceanographic techniques for studying ocean-solid Earth interactions. Beyond scientific implications, the integrated monitoring approach offers practical relevance for coastal geodesy, hydrodynamic model calibration, and risk assessment in regions affected by subsidence and sea level rise.