Geodesy, Geopotential Field Modelling and Oceanography with GOCE & Swarm

The ocean flows generated by lunisolar tides interact with the Earth’s main magnetic field, creating the secondary induced magnetic field observed by low-orbit satellites. We present the results of an analysis of Swarm vector magnetic field measurements that can be used as constraining markers on the spatio-temporal structure of the tidal flows. The Swarm data are processed on a track-by-track basis with careful application of selection criteria, and by applying the corrections for the external field sources. We compare the observations directly with the model predictions obtained by a barotropic numerical model.

GOCE allows the determination of consistent geoid heights with an accuracy of 1-2 cm and a spatial resolution of at least 100 km, quasi globally. This offers new possibilities for the determination of consistent physical heights for a number of applications. These are (1) the global unification of existing national height systems, (2) the determination of consistent physical heights for geometric reference frames like the ITRF and (3) the observation of the sea level and its change in an absolute sense. The paper summarizes results of a project supported by the European Space Agency related to height system unification and identifies to what extent and under what conditions these applications can benefit from the highly precise GOCE geoid.

An update to the global mean dynamic topography model DTU13MDT is presented. For DTU15MDT the newer gravity model EIGEN-6C4 has been combined with the DTU15MSS mean sea surface model to construct this global mean dynamic topography model. The EIGEN-6C4 is derived using the full series of GOCE data and provides a better resolution. The better resolution fixes a few problems related to geoid signals in the former model DTU13MDT. Slicing in the GOCO05S gravity model up to harmonic degree 150 has solved some issues related to striations. Compared to the DTU13MSS, the DTU15MSS has been derived by including re-tracked CRYOSAT-2 altimetry also, hence, increasing its resolution. Also, some issues in the Polar regions have been solved. Finally, the filtering was re-evaluated by adjusting the quasi-gaussian filter width to optimize the fit to drifter velocities. Subsequently, geostrophic surface currents were derived from the DTU15MDT. The results show that geostrophic surface currents associated with the mean circulation have been further improved and that currents having speeds down to below 4 cm/s have been recovered.

A truly global coverage of gravity field measurements by GOCE, supplemented with airborne gravity data in the polar gaps, have now hopefully been realized. A new cooperative airborne geophysical survey, planned to be carried out in the time frame Dec 2015-January 2016, will together with other existing data, provide a near-complete airborne gravity coverage of the GOCE southern polar gap (beyond 83.5°S). Similarly in theArctic, new and older airborne gravity data are currently being re-compiled to provide a corresponding near-complete coverage of the northern polar gap. For future global GOCE spherical harmonic models, the fill-in of polar gap data are expected to provide a globally uniform accuracy, and avoiding the large “spike” in near-zonal current GOCE coefficient error estimates.

In the talk we outline the Antarctic field operations, a truly challenging logistic setup, with Twin-Otter flight operations out of remote field camps, as well as operations out of the South Pole Station, thanks to a special NSF agreement. In addition to the gravity measurements, a whole suite of geophysical data will be collected (magnetics, ice penetrating radar and lidar), and a special dedicated flight around the South Pole are planned with the ASIRAS 13 GHz radar system, to validate CryoSat data in a currently problematic region. The PolarGap project flights are coordinated with matching US C-130 gravity tie flights between South Pole and the Ross Sea, as part of the NSF Rosetta project (Lamont Doherty Earth Observatory, Columbia University), as well as additional NPI flights over the Recovery Lakes region, thus providing a truly international cooperation in filling the GOCE gap.

Gravity in the PolarGap aircraft will be measured by a novel LC&R gravimeter and high-end inertial measurement unit set-up, which has demonstrated the potential of 1 mGal rms accuracy in recent (more benign) surveys. It is therefore believed that the new airborne gravity will be very useful for correcting biases in older airborne survey data. The final goal is to merge all available gravity data – from ESA, IceBridge, and other national sources into a uniform gravity field product, which is then converted to GOCE-like gravity gradient data, and eventually assimilated into the final GOCE models.

This presentation shall first provide a general survey of oceanographic applications using magnetic data from the Swarm satellites. Then, more specifically, the status and highlights of related research currently underway a thet NASA Goddard Space Flight Center (GSFC) will be described. This research includes but is not limited to the numerical forward modeling of ocean flow generated magnetic signals, the extraction of such signals from Swarm data using the NASA GSFC Comprehensive Model, and the interpretation of these signals in terms of oceanic parameters.