The GOCE Gravity Field Space Mission as an Important Step for the Exploration of our Planet

As an integral response to its mass distribution, the structure of the Earth’s gravitational field represents an important source of information about our planet. Its knowledge is therefore important for the Earth as a system and for geodesy, solid Earth physics, and oceanography in particular. For the first time in space geodesy, dedicated gravity field missions are being realized during this decade. The three missions are based on different observation concepts but they have one element in common: GPS-based high-low satellite-to-satellite tracking (SST). A second dedicated on-board sensor makes the three missions focus on different aspects of the Earth’s gravity field: the determination of the static field with utmost precision and resolution by gravity gradiometry (GOCE mission) versus the monitoring of the temporal variable field with reduced spatial resolution by inter-satellite range measurements (GRACE mission). The two sensors of the GOCE mission (GOCE = Gravity field and steady-state Ocean Circulation Explorer) combined will deliver a global gravity field with unprecedented accuracy and resolution. The GOCE derived gravity field, if combined with satellite altimeter data, will allow the precise determination of the absolute ocean circulation on a global scale. Considering the interchange between ocean and atmosphere, the results of the GOCE mission have the potential of substantially contributing to the further improvement of climate research. When combined with seismic data, the GOCE mission will enable a significant advance in the understanding of the dynamics of the Earth’s interior and will contribute to a better understanding of processes such as plate tectonics, volcanism, and earthquake phenomena. Finally, the GOCE mission will provide a global height reference at the centimetre level, which is of paramount importance for positioning and navigation. The paper provides an overview of the GOCE satellite gravity field mission in terms of mission parameters, goals, instruments and products and describes in more details the sensor and processing systems. Special emphasis is given to the interaction between the various sub-systems in space and on ground, which are required in order to successfully operate mission and exploit its data. Introduction to the GOCE Mission Mission Goals. GOCE is the acronym for „Gravity field and steady-state Ocean Circulation Explorer mission“. It is the first core satellite mission of the newly defined ESA “Living Planet” programme (see ESA, 1999a). The objective of GOCE is the determination of the stationary part of the Earth gravity field and geoid with highest possible spatial detail and precision. The gravity and geoid model derived from the GOCE mission will serve science and application in the fields of solid Earth physics, oceanography, geodesy and glaciology, compare for example (ESA, 1999b; Johannessen et al, 2003; Le Grand, 2003). An overview of the fields of application is shown in Figure 1. Figure 1: Application fields of GOCE gravity field mission and products (Courtesy ESA1999b). Two main applications can be distinguished. Firstly, the spatial variations of gravity and geoid are directly related to density anomalies in lithosphere and upper mantle, respectively, and consequently to interior stresses and ultimately to mass motion. In this respect GOCE provides important new information to studies of continental and oceanic lithosphere and upper mantle. Its information is complementary to that of seismic tomography, magnetic field models, geokinematic studies and laboratory results. Secondly, a detailed geoid surface when combined with satellite altimetry yields ocean topography, the quasi-stationary deviation of the actual mean ocean surface from its hypothetical surface of rest. Under the assumption of geostrophic balance the sea surface topography can be directly translated into a global map of ocean surface circulation. Thus, ocean surface circulation becomes directly measurable, globally and uninterruptedly. In conjunction with higher resolution ocean models and in-situ measurements, GOCE is expected to improve significantly estimates of global mass and heat transport in the oceans (see Le Grand, 2003). Furthermore, the global geoid will permit height systems to be connected globally with almost cm-precision. Sea level variations in Australia, or East Asia will become directly comparable to those measured in Europe or America. These and other expected scientific benefits from GOCE gravity and geoid models demonstrate that this mission represents an important element of global observation of mass anomalies, mass transport and mass exchange. The science goals and the requirements on the mission performance are summarized in Table 1. To reach the specified science goals, precondition is that GOCE can determine gravity and geoid with a precision of 10·g (corresponding to 1 mgal) and 1-2 cm, respectively, with a spatial resolution of better than 100 km half wavelength and that these results are achieved free of long wavelength systematic errors. The mission performance depends on the gravity sensor system on-board GOCE. Table 1: Science Goals and Mission Requirements of GOCE Accuracy Application Geoid [cm] Gravity [mgal] Spatial Resolution (half wavelength – D in km) SOLID EARTH Lithosphere and upper mantle density structure 1-2 100 Continental lithosphere: Sedimentary basins 1-2 50-100 Rifts 1-2 20-100 Tectonic motions 1-2 100-500 Seismic hazards 1 100 Ocean lithosphere and interaction with asthenosphere 0,5-1 100-200 OCEANOGRAPHY Short scale 1-2 0.2 100 200 Basin scale ≈0.1 1000 ICE SHEETS Rock basement 1-5 50-100 Ice vertical movements 2 100-1000 GEODESY Levelling by GPS 1 100-1000 Unification of worldwide height systems 1 100-20000 Inertial navigation system ≈1-5 100-1000 Orbits ≈1-3 100-1000 SEA LEVEL CHANGE Many of the above applications, with their specific requirements, are relevant to studies of sea-level change. Mission Concept. The gravimetric concept of GOCE consists of two sensor systems. The first follows the high-low satellite-to-satellite tracking concept (hl-SST). The satellite positions are observed with cm precision by the on-board GPS receiver. These observations between the GPS satellites at high altitudes and the GOCE satellite at very low altitude can be used to determine the long wavelengths of the Earth gravity field by analyzing the orbit variations (the satellite can be regarded in free-fall). The second and completely new element of the GOCE satellite mission is the direct measurement of the anomalous static gravity field by pairs of accelerometers forming a 3-D gravity gradiometer (SGG: Satellite Gravity Gradiometry). By observing differential accelerations in three directions in a drag-free environment and at very low altitude it will become possible to determine the gravity field globally to a much higher spatial resolution than with any satellite mission flown before. The gravity gradiometer, due to its short baseline between the accelerometers and due to its band limitation will not be able to observe the long wavelength part of the Earth gravity field with sufficient accuracy. The two measurement concepts therefore provide complementary information. Figure 2 shows the general mission concept of the gravity field sensor system. Mission Parameters. In order to determine the Earth gravity field from space with high accuracy and high spatial resolution the mission has to fulfil specific conditions. First of all the ground track coverage of the satellite orbit has to be sufficient in order to reach a spatial resolution of at least 100 km globally. This is ensured by the chosen orbit parameters and by the fact that GOCE mission control will keep the satellite during operational phases in an orbit where no short repeat periods on ground will occur. Nevertheless, due to power consumption constraints the satellite has to fly in a sun-synchronous orbit with an inclination of 96.7 degrees. This implies that the polar areas cannot be observed completely and that there is a polar gap of 6.7 degrees in the Northern and Southern hemisphere. The definition of the operational phases is also driven by power consumption constraints. This means that the satellite can only be operated in the chosen altitude during direct sun-light and using electric energy from batteries during short eclipse phases. During long eclipse phases the solar energy is not sufficient to fully operate the satellite and its instruments in the science measurement mode. Therefore, for GOCE we have measurement phases interrupted by a so-called hibernation phases during the long eclipse period. A final driving parameter for definition of the mission altitude is the sensitivity of the instrument compared to the expected signal. The strength of the gravity field decreases by the square of the distance from the attracting body (Earth). This means, in order to enable highly precise gravity field determination by means of satellite observations the altitude has to be low enough and/or the instrument sensitivity has to be high enough, respectively. In other words the optimal configuration will be a compromise of both factors using the best possible instrument at lowest possible satellite altitude. The results of this compromise are the selected mission parameters as summarized in Table 2 for a launch date in August 2008. Depending on the actual launch date the mission profile might have to be adapted. Table 2: Summary of GOCE Mission Profile