Regional Gravity Fields from GOCE and GRACE

The Earth's gravity field is of importance to oceanography, polar science, solid Earth science and to height systems on the Earth. If we consider the oceans as a virtual sea, that is completely at rest with no tides or currents and with the ability to flow at will in a narrow channel under the land masses, then the surface of that imaginary water line is mean sea-level, which scientists call the geoid. The geoid is thus a fictitious surface that approximates mean sea-level and changes with location over the Earth. The change from place to place can be expressed as a sum of components that change with distance giving rise to terms that change slowly (long wavelength terms) and those that change rapidly (short wavelengths). Scientists have only limited knowledge of the geoid over the oceans with the situation generally worse over land. The launch of dedicated gravity field missions, GRAvity and Climate Experiment (GRACE) and the Gravity field and steady state Ocean Circulation Experiment (GOCE) is beginning to change that. The GRACE satellite mission is a pair of tandem satellites some 220 km apart orbiting about 450km above the Earth's surface. Launched in 2001, precise positioning is provided by GPS with the distance between the satellites measured by a micro-wave device. If we consider the satellites joined by an elastic string the length will increase and decrease as the satellite orbits are affected by the Earth's gravitational pull. By utilising this variability it is possible to determine that gravitational effect. GOCE is a European mission scheduled to be launched in 2009. This mission is of a similar concept to GRACE but with the tandem satellites reduced to point masses about 1m apart. The differences in gravity between this three dimensional array of point masses is continuously monitored by accelerometers which provide the rate of change of gravity in a process known as gradiometry. A problem with gradiometry is the large perturbations due to air-drag with the consequence that the accelerometers have to be constructed to avoid saturation. This restricts the wavelengths of the gravity field observable with the long wavelengths not measurable by the gradiometer. These wavelengths are instead to be determined from the GPS tracking but at a lower accuracy than that of GRACE These two missions are thus complementary supplying the gravity field to high precision at long and shorter wavelengths. GRACE yields the gravity field components to high accuracy at the longer wavelengths and GOCE at the shorter wavelengths. In this study we intend to merge the two data sets to produce solutions superior to that obtainable by either missions by itself. We will also obtain gravity fields over regions such as the Arctic and Antarctica as that will enable us to make use of the full gravity field signals from GRACE which can be lost in seeking a single global solution. Of course regional fields can be combined to give a global solution. A highly accurate geoid as determined by this study will enable oceanographers to use the decade and more of satellite altimetric heights to measure absolute ocean currents and eddies etc. The oceans are the most important source of heat transport around the world and knowledge of these transport processes and the heat fluxes are fundamental to our understanding of global climate change. These transportation processes can then be modelled to give us further insight into the effects of climate change due to green house gasses and other anthropogenic effects.