Tidal Inversion over the Japan Sea from Sentinel-3A Altimetry Using the MHACS Method

As a semi-enclosed marginal sea, the Japan Sea has long been hindered in tidal studies by sparse tide-gauge observations and limited satellite altimetry coverage. Although Sentinel-3A provides high-quality sea-level observations, its 27-day exact-repeat sun-synchronous orbit introduces severe tidal aliasing into conventional harmonic analysis—most critically, the S2 constituent degenerates into a constant and cannot be recovered independently. In this study, we apply the Modified Harmonic Analysis model based on the Credo of Smoothness (MHACS) with ridge regression to eight years (2016–2024) of Sentinel-3A altimetry in the Japan Sea for the first time, estimating tidal constants for the eight major constituents. By exploiting the smooth connections among major astronomical constituents, MHACS mitigates the aliasing problem, with severely aliased constituents such as S2 inferred from the smooth admittance relationship rather than independently resolved. The inverted tidal fields show excellent agreement with the independent FES2014 global tide model, with a spatially averaged vector difference of only 1.39 cm, whereas the traditional tidal response method yields 4.52 cm—3.3 times larger. Independent validation against 22 GESLA-3 tide-gauge records further confirms this ranking (3.012 cm vs. 5.676 cm). As a consistency check, the MHACS-derived P1/K1 and K2/S2 amplitude ratios (0.316 and 0.278) agree closely with equilibrium tidal theory, although this test is not fully independent. These results demonstrate that MHACS is well suited for tidal inversion from 27-day repeat altimetry.