GOCE – Surfer of the gravitational field
ILA/Berlin, Friedrichshafen, 10 May 2004
From August 2006, the European research satellite GOCE will start "surfing" the Earth’s gravitational field and investigate the interior of the planet from an altitude of about 250 kilometres. The Gravity-Field and Steady-State Ocean Circulation Explorer (GOCE) will circle the Earth for at least two years and measure the gravitational field as well as the shape of the Earth more precisely than ever before. The data collected can then be used to study the structure of the Earth’s interior and, at the same time facilitate in oceanographic investigations, like the measurement of the possible rise in the level of the ocean, or the analysis of ocean currents. EADS Astrium in Friedrichshafen is responsible for the construction of the satellite platform. Its engineers and scientists benefit from earlier satellite develpments, such as the environmental and climate satellite, CryoSat, currently being assembled in the clean room in Friedrichshafen. From the scientific standpoint, GOCE represents the extension of the CHAMP and GRACE missions, which also were created under the direction of EADS Astrium.
The potato-shaped gravitational field of the Earth
Every star and planet generates a force, or field, of gravity. This force of attraction ensures that the Earth flies around the Sun and the Moon around the Earth, and is, by the same token, responsible for the fact that man and animals stay put on the surface of the Earth. If the Earth were a perfect sphere, the gravitational force field around our planet would be completely symmetrical and would diminish uniformly in all directions away from it. However that is not the case.
On the one hand the rotation of the Earth creates a centrifugal force. It is strongest at the equator and diminishes to nothing at the poles. This centrifugal force pulls the planet apart so that it looks more like a rugby ball or an ellipsoid: The diameter at the equator is about 21 kilometres more than the diameter from pole to pole. This has the effect that a man of normal weight weighs about 350 grams more at the poles than at the equator.
In addition, on a smaller scale, there are variations from the perfect ellipsoid, for instance from high mountains to deep sea trenches. This irregular topography leads to corresponding irregularities in the outer gravitational field. Moreover the interior of the Earth is not uniformly composed. There are zones of very dense and heavy rock where stronger terrestrial attraction prevails. In other locations, the crust material is lighter and the terrestrial magnetic field is less. Such so-called anomalies arise, for instance, where continental plates bump into one another or drift apart.
These irregularities in the structure of the Earth are directly mirrored in the structure of the gravitational field. Representing the field in spatial mapping, the Earth looks like a potato. A gravitational field atlas is as valuable for a geophysicist as a topographical map is for a surveyor. It contains a wealth of information.
Seething magma and streaming oceans
The structure of the Earth’s interior cannot be determined from knowledge of the terrestrial gravitational field alone, because it is impossible to recognize whether a "dent" in the gravitational field has its origin in the interior of the Earth or on the surface. Only in conjunction with other methods, like seismology, can the causes be separated. Geophysicists want to study two aspects above all with GOCE:
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Deep in the Earth’s interior the rock is hot and viscous. Like water in a kettle, magma rises, cools and flows back again. These fluid movements of rock are the cause of continental drift and earthquakes. Researchers want to study this phenomenon with GOCE.
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The Earth’s crust is spread over the entire globe like the pieces of a puzzle. As these continental plates are pushed towards each other they bump together in some areas and descend into the interior of the Earth. This is where earthquakes frequently occur. In other places they drift apart and this is where material from the deep inside the Earth rises to the surface. The researchers are interested in what is hidden beneath these fault zones.
The second main area of application is oceanography.
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Climate researchers are discussing whether the sea-level will rise as a result of global warming. The measurement of sea levels, to the required degree of accuracy of about a centimetre, is very difficult, because, until now, there has been no precise reference surface, to which measured changes can be related. Geological researchers call this surface a geoid. Currently the values for this "Normal Null" vary by up to a metre between continents worldwide. Therefore the sea level in one part of the world cannot be compared with that in another. But that is what is required to prove global changes. GOCE is intended to establish this reference surface worldwide at one centimetre and in certain areas at a few millimeters.
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This global "calibration" is also required for other operations. For instance, ocean currents can be much better studied. These have a decisive influence on the climate, because they transport great quantities of water and energy. If there were no North Atlantic current, (Gulf stream), the air temperatures in the North Atlantic region would fall by five to ten degrees. Important measurements, like the amounts of water and energy transported, can be modelled pretty exactly relative to a clearly defined reference surface.
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Last but not least, the new reference system also aids geodesy and thereby the creation of topographic maps. These are based on the average sea level “Normal Null”.
GOCE surfs in the gravitational field
Satellites offer the only possible way to survey the entire gravitational force field of the Earth uniformly. This works as follows: The satellite circles the Earth in a fixed orbit where the gravitational force directed toward the Earth and the outwardly-directed centrifugal force are exactly in balance. In a perfectly symmetrical gravitational field the satellite would move in an elliptical or circular orbit, but if it passes over a "bump" or a "dent" in the gravitational field, then it experiences something similar to what a surfer faces in the ocean: It rides over a slightly wavy washboard. In the region of stronger gravitational force, it speeds up and climbs, whereas in a region of weaker gravitational force it slows down and drops. Following the path of the satellite exactly, the terrestrial gravitational field can be reconstructed from the orbital deviations.
Researchers have made giant strides in the past couple of years using the CHAMP satellite (launched in July, 2000) and GRACE (launched in March, 2002). Both spacecraft were designed and manufactured under the leadership of EADS Astrium. GOCE will carry on the work of these two successful missions and deliver even more precise data. It should be able to measure details in the terrestrial gravita-tional field down to 70 kilometres, and variations in its strength down to a millionth of the average field.
The precision of these measurements can only be achieved with costly technology. Since the gravitational field weakens with the distance from the Earth, GOCE orbits at an altitude of only 250 kilometres. However at that altitude there is still a partial atmosphere and so to keep the resistance due to friction from the air to a minimum, the satellite was "streamlined". Its cross-sectional surface perpendicular to the direction of flight is only about a square metre. This was achieved by stretching out the body lengthwise and rigidly mounting the solar panels almost parallel to the direction of flight. In addition the satellite is almost symmetrical in shape.
EADS Astrium delivers the satellite platform
Two technical conditions must be met so that the challenging plan can succeed. First, the orbit of the satellite must be followed exactly. This is achieved using a GPS receiver on the upper side of GOCE. The GPS makes it possible to determine the altitude of the satellite down to a centimetre.
Secondly, variations in the orbit must be clearly traced back to irregularities in the terrestrial gravitational field. This is very difficult because, in spite of its streamlined shape, GOCE is very slightly slowed by the atmosphere. This negative acceleration is in the same order of magnitude as the gravitational "dents." However, there is a key difference between these two disturbances: If friction causes the satellite to drop, there is no equilibrium between the gravitational and centrifugal forces and the satellite accelerates. This effect is continually measured with three instruments, called gradiometers, perpendicularly positioned to each other. The onboard computer evaluates this data and directs an ion propulsion unit, which at a given moment delivers exactly the amount of thrust needed to compensate for the friction. In this way the satellite always stays in the same orbit. The remaining "surf-movements" are caused by the irregularities in the gravitational field which are just what the researchers want to measure.
This procedure is called drag-free control. "The interaction in this way, between drag-free control and the ion power unit is completely new", explained the GOCE project manager, Albrecht Woelker. "This system is, for us, one of the most demanding tasks of the project." Ion power units were often used in the past, especially to regulate the position of satellites. What is new here is the permanent thrust control of the power unit.
EADS Astrium was also able to benefit from previous developments in the construction of other parts of the GOCE platform and thereby come up with a more cost effective solution. For instance the company is currently building a similar platform for the ESA environmental satellite, CryoSat. "However because of the special demands with GOCE we had to further develop many of the components", Woelker said.
Special demands are placed above all by the incredible orbital precision to which the satellite must adhere. Each force that may disturb the satellite must therefore be avoided. For instance the satellite structure must not be deformed by strong variations in temperature. These occur whenever GOCE emerges from the Earth shadows and flies into the sunshine, and vice versa. "In order to avoid this effect a thermally stable structure made of plastic re-enforced with carbon fibre and a shrewdly-conceived thermal control system are important. These systems can hardly be tested in the laboratory and have to be created with theoretical models", explained Woelker.
In addition the operation of relays and other moving parts during measurement phases is "verboten", because they exert undesirable forces on the satellite, which would disturb the measurements. This is no small task, given the multitude of devices which the EADS Astrium engineers have to install in the platform, including electronic controls for the solar generator, star sensor, solar sensor, Earth sensor, magnetometer and magnetic coils for the positional control, as well as S-Band antennas and communication transponders.
EADS Astrium is on schedule with its development programme and will deliver the completely assembled and tested platform to the prime contractor Alenia by May, 2005.
EADS Astrium is Europe’s leading satellite system specialist. Its activities cover complete civil and military telecommunications and Earth observation systems, science and navigation programmes, and all spacecraft avionics and equipment. EADS Astrium is a wholly owned subsidiary of EADS SPACE, which is dedicated to providing civil and defence space systems. In 2003 EADS SPACE had a turnover of €2.4 billion and 12,000 employees in France, Germany, the United Kingdom and Spain. EADS is a global leader in aerospace, defence and related services. In 2003, EADS generated revenues of € 30.1 billion and employed a workforce of more than 100,000.
Friedrichshafen, May 2004 /04008
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