COTAGA (Combined Ocean Tide Analysis by GRACE and Altimetry Data) is a joint project between the German Geodetic Research Institute (DGFI) in Munich, the Institute of Theoretical and Satellite Geodesy (ITSG) of the Technical University Graz and GFZ within the DFG Special Priority Program 1257 "Mass Transport and Mass Distribution in the Earth System".

Precise models of ocean tides are crucial because (a) the effect of ocean tides has to be removed from CHAMP, GRACE and GOCE observations in order to obtain models describing exclusively the Earth’s gravity field and (b) altimetric sea surface heights should be "tide-free" in order to be comparable with each other and to be assimilated into numerical models. As deficiencies in the currently available ocean tide models, especially over continental shelves and in the polar oceans, are well known, the COTAGA project has the following strategies:

Empirical improvements to the widely used state-of-the-art ocean tide model FES2004 shall be derived by (a) solving residuals for major tidal constituents with a spatial resolution of ~1000 km from GRACE data only (long wavelengths) and (b) applying response and harmonic analysis to multi-mission altimeter data to estimate corrections for the dominant tidal constituents and non-linear effects like M4 in particular for shallow water areas (short wavelengths). The latter corrections derived from altimetry are then used to generate an upgraded FES2004 model first, which is called EOT??a (Empirical Ocean Tide model, where ?? denotes the year of generation e.g. “08”, “10” or “11”) and which is subsequently used for a more precise de-aliasing of the gravity field models and for improved corrections of altimetry data. The optimal combined solution of altimetry information and GRACE information (using variance component estimation) will lead to an ocean tide model EOT??ag. Several methods to validate all models are used.

The current project achievements are:

• A series of new global Empirical Ocean Tide models (EOT08a, EOT10a, EOT11a) has been generated by empirical analysis of (harmonized, upgraded and cross-calibrated) multi-mission satellite altimeter data. Analysis yield shallow-water amplitude corrections up to 15 cm, identified large scale corrections for M2 and K2 with 1-2 cm amplitude with a clear de-correlation of critical constituents (mean & S2, K2 & Ssa). EOT11a has been used for the GRACE RL05/typo3/  reprocessing and will serve as the basis for an altimetry & GRACE combined EOT??ag which should become available at the end of the project.
• Ocean tide residuals relative to the EOT11a model have been estimated using 10 years (2003-2012) of monthly GRACE RL05 normal equations. Results for the M2, K2, K1 and O1 tides are shown in the figure below. It can be observed that GRACE is able to add information in high-latitude areas (> 66 deg. N/S) where some essential altimetry satellites (TOPEX, JASON-1/2) cannot contribute due to their inclination, but also in many mid-latitude regions, mainly along coast lines, some large-scale signals are estimated from GRACE data. The maximum estimated residual amplitudes are in the range of 1-3 cm depending on the particular tidal constituent and thus already within the current accuracy of time-variable GRACE gravity field solutions.