The mission GOCE
The mission GOCE is an international project for the measurement of the gravitational field and the determination of the geoid. GOCE (Gravity field and steady-state Ocean Circulation Explorer) is a mission dedicated to measuring the Earth’s gravitational field and the determination of the geoid. Selected by ESA in 1999 – as part of the ESA “Living Planet Program” – the mission is the first of the “Earth Exploration CORE missions” and was launched on 17 March 2009.
The objectives of the GOCE are: to determine the anomalies of gravity field with an accuracy of 1 mGal, to determine the geoid with an accuracy of 1-2 cm and to achieve the above objectives with a spatial resolution better than 100 km.
The determination of the overall pattern of the gravity field and the geoid is essential for the understanding of the terrestrial dynamics to large scales as well as for the definition of a single reference system of units worldwide; temporal variations of this field are precise indicators of geodynamic phenomena, such as the postglacial rebound, determinants in the understanding of the dynamics plaque. In addition, the measurement of the gravitational field, if done on orbit low enough, as in the case of GOCE, allows the analysis of geophysical phenomena on a regional scale. The determination of the geoid by using data in GOCE is essential both for cartographic purposes in areas not reached by the measures both on the ground, in conjunction with the altimeter measurements, for the study of ocean currents and their properties.
The main instrument of GOCE is a gradiometer consists of six accelerometers, distributed two on each of the three axes in closed container at a constant temperature. The instrument was designed specifically for the determination of the gravity field stationary. The measured signal is the difference in gravitational acceleration of some proof masses located within the satellite caused by abnormalities of gravity of the terrestrial field. On board is also a GPS receiver with 12 channels and a laser retroreflector for tracing from the ground. The satellite has a very small cross section (CIRA 0.9 m2) and is totally symmetrical to minimize the influence of non-gravitational forces. The orbit is sunsincrona, almost circular, with an altitude of about 250 km. The nominal duration of the mission is of 20 months, including three months for phase commisioning.
Italy, contributed to the construction of the satellite, Thales Alenia Turin being the prime contractor of the project for ESA. Moreover, the Politecnico di Milano, who for decades worked in the international arena for the creation of a European mission to measure the gravity field, is a fundamental part of the Consortium called HPF (High Level Processing Facility) which has the task of transforming the data of satellite in Earth’s field estimation. The ASI is funding the project GOCE-ITALY, which is carried out by the research groups of the Politecnico di Milano, Milan University, University of Padua, Trieste University, Institute of Oceanography and Experimental Geophysics (OGS) of Trieste, Altec and Galileian Plus , and provides for the use by Italian scientists of the products of the mission for the achievement of the following objectives:
a) The determination of a global model of the Earth’s gravity field and geoid with high spatial resolution and high accuracy by integrating the spatial data with the terrestrial;
b) The determination of local models of the gravity field and geoid based on the integration of gravity measurements in situ with data from GOCE;
c) The determination of the geoid in the Mediterranean;
d) The determination of low harmonics of the gravitational field, using the precise orbit determination (POD) by analysis of GPS data, and the improvement of the model of the ocean tides;
e) The development of geodynamic applications, in particular the development of models of postglacial rebound (PGR) to study the effect on the gravitational field at both global and regional scales;
f) The development of oceanographic applications, in particular the combined use of the geoid derived from GOCE, possibly enhanced with gravity data in situ, and measurements from satellite altimetry, in order to measure the currents in the Mediterranean Sea;
g) The development of geological applications, in particular the determination of an advanced model of the crust in the Italian territory and the study of sedimentary basins in a large scale in the lower crust or upper mantle.
The responsibility for the coordination of the research team was assigned to the Politecnico di Milano. Some project activities will be carried out directly by the Centre ASI Space Geodesy “Giuseppe Colombo” including the calculation of time series of geopotential, using data SLR (Satellite Laser Ranging) ILRS network on different geodetic satellites (ie LAGEOS – I and – II, Starlette, Stella, Ajisai etc.), the validation of the orbit and the comparison between the geoid computed using data from GOCE with that derived from the information SLR.
On 21 October 2013, the mission ended with the complete exhaustion of its propellant.