Short Period Ocean Tidal variations in Earth Rotation (SPOT)

Short period variations in Earth rotation, quantified as Earth rotation parameters (ERP), are induced predominantly by diurnal and subdiurnal ocean tides. Secondary causes of such variations are thermal tides of the atmosphere, driven by the diurnal solar heating cycle, and the effect of the lunisolar torque on the triaxial figure of the Earth, called libration. Detailed descriptions of diurnal and subdiurnal ERP variations based on a profound geophysical background are essential for various parameter estimation problems in space geodesy, like processing of special VLBI (Very Long Baseline Interferometry) sessions or GNSS (Global Navigation Satellite Systems) orbit determination. Several studies on high-frequency ERP, measured by GNSS or VLBI, reveal significant discrepancies between the observational evidence and the present conventional model for ocean tidal Earth rotation variations. The core intention of project SPOT is the development of an utmost precise model for the impact of short period ocean tides on Earth rotation meeting the requirements of modern geodetic research and applications. The model will be built on the basis of the up-to-date global ocean tide model EOT11a which is computed at DGFI (Deutsches Geodätisches Forschungsinstitut) by means of empirical analysis of multi-mission satellite altimetry data from nearly two decades. As EOT11a does not provide the tidal current velocities which are fundamental contributors to Earth rotation excitation, the calculation of current velocities from the tidal elevations is one of three main areas of research in project SPOT. The second key aspect is the conversion from ocean tidal angular momentum to corresponding ERP variations using state-of-the-art transfer functions. A peculiar innovation at this step will be to consider the Earth`s response to ocean tidal loading based on a realistic Earth model, including an anelastic mantle. The third part of the project deals with the introduction of the influence of minor tides. Ocean tide models usually only provide eight major semidiurnal and diurnal tidal terms and minor tides have to be inferred through admittance assumptions. Within the proposed project, selected minor tidal terms and the corresponding ERP variations shall be derived directly from satellite altimetry data. One of the principal developers of EOT11a will be actively involved in project SPOT. This will enable a very close collaboration of Earth rotation and ocean tide modelers and offer a unique possibility for mutual adjustment and iterative refinement of the ocean tidal ERP model. At IGG Vienna a strong VLBI group exists which is acting as Analysis Center within the IVS (International VLBI Service for Geodesy and Astrometry) and has access to all VLBI data. Thus a validation of the model with measurements through determination of Earth rotation variations from VLBI data can be conducted at any stage of the project and will be an important asset of the project. Besides its relevance for the processing of space geodetic data, a new high-quality ocean tidal ERP model will support the study of secondary effects, such as libration and thermal tides of the atmosphere, and thus, support Earth system research. Since the model is part of the transformation between terrestrial and celestial reference systems, which is important for all precise navigation tasks in space, the results of the project are of significant importance even beyond the scientific domain.

Recent improvements in the development of VLBI (Very Long Baseline Interferometry) and other space geodetic techniques like GNSS (Global Navigation Satellite Systems) require for different tasks a priori information of short period (daily and sub-daily) Earth rotation variations. One significant contribution to Earth rotation variation on this time scale is caused by diurnal and semidiurnal ocean tides. The IERS (International Earth Rotation and Reference Systems Service) conventional model for the short period ocean tidal variations of Earth rotation was evaluated by different groups and they highlight the need for an updated model.In a cooperation between TU Vienna, TU Berlin, GFZ Potsdam and DGFI at the TU Munich, this project aims to develop and evaluate an alternative model for short period ocean tidal Earth rotation variation. This improved model is based on an ocean tide model (EOT11a), considering multi-mission satellite altimeter observations since 1992. By use of a solely observational model of ocean tides, this approach becomes independent from the assimilation of altimeter observations into hydro-dynamic ocean models. On the other hand, it is necessary to determine tidal velocities from the heights of ocean tides by a simplified inversion approach.In the past, modeling of high-frequency ocean tidal Earth rotation variation was an issue, where the ocean tide modeling was independent from Earth rotation modeling. Within project SPOT we developed an alternative model, in which the ocean tidal angular momentum model and the Earth rotation variation was set up jointly. The close collaboration of experts in ocean tide models, on satellite altimetry, Earth rotation modeling and VLBI analysis has provided the basis for this joint approach. Besides the model of the short-period variation of the Earths rotation parameter (ERP), based on the ocean tide model EOT11a, we developed also ERP models, which consider alternatively the ocean tide models FES2012 and HAMTIDE. Furthermore, we assessed the effect of uncertainties in the elastic Earth model on the resulting ERP models. Our proposed alternative ERP model to the IERS conventional model considers the elastic model PREM and 260 partial tides.For the time interval January 2010 to December 2013 VLBI analysis considering the different a- priori models of short-period ocean tidal Earth rotation variation with and without estimation of the ERP was conducted to evaluate the a-priori models. The SPOT model performs in terms of WRMS (weighted root-mean squares) similar to the IERS conventional model, and represents a valid alternative.