Airbus Defence and Space

More about GOCE and Airbus Defence and Space

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.

The potato-shaped gravitational field 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. For one thing, 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, caused for instance by high mountains and 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 gravitational field is less strong. Such anomalies arise, for instance, in regions where continental plates collide or drift apart.

These irregularities in the structure of the Earth are directly mirrored in the structure of the gravitational field. If the field is mapped in three dimensions, the Earth looks like a potato. An atlas of gravitational fields is as valuable to a geophysicist as a topographical map is to 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 recognise 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 such as seismology can the causes be separated.

 Geophysicists particularly want to study two aspects with GOCE:

 1) 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.

 2) 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 collide 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 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.

 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’. The values for this ‘mean sea level’ currently vary by up to a metre between continents, so the sea level in one part of the world cannot be compared with that in another. Yet this is what is required to prove global changes. GOCE is intended to establish this reference surface worldwide to within one centimetre and in certain areas to within a few millimetres.

 This global ‘calibration’ is also required for other operations. For instance, ocean currents can be studied much more easily. 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 more accurately relative to a clearly defined reference surface (the geoid).

 Last but not least, the new reference system also aids geodesy and thereby the creation of topographic maps given that these, too, are based on the mean sea level.


GOCE surfs the gravitational field

Satellites offer the only possible way to survey the entire gravitational field of the Earth uniformly. This works as follows: 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’ (referred to by experts as an ‘anomaly’) 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 will slightly drop and speed up, whereas in a region of weaker gravitational force it will slightly climb up and slow down. The terrestrial gravitational field can be reconstructed from the orbital deviations by exactly following the path of the satellite.

Using the two satellites CHAMP (launched in July 2000) and GRACE (launched in March 2002), this technique has enabled researchers to chart the Earth’s gravitational field more accurately than ever before over the past few years. Both satellites were designed and manufactured under the leadership of Airbus Defence and Space. GOCE will carry on the work of these two successful missions and deliver even more precise data. It is designed to measure details in the terrestrial gravitational field down to a spatial resolution of 100 kilometres, and variations in its strength down to a millionth of the average gravitational field of the Earth. The precision of these measurements can only be achieved with new, costly technology.

Since the gravitational field weakens with increasing distance from the Earth, GOCE orbits at an altitude of only 250 kilometres. However, at that altitude there is still a partial atmosphere. 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 the body lengthwise and rigidly mounting the solar panels almost parallel to the direction of flight.


Airbus Defence and Space delivers the satellite platform

At the heart of the satellite is an acceleration-measuring device known as a ‘gradiometer’, which measures acceleration in all three spatial directions. It is made up of six crystals which hover in pairs 50 centimetres apart within a container and form the ends of three spatial axes running perpendicularly to one another. An electrostatic field ensures that the distance between each pair of crystals remains constant. Normally, this distance would change if the satellite flew over a gravitational anomaly. However, the gradiometer registers this and counteracts the effect by modifying the given electrostatic force by exactly the right amount. From the strength of this electrostatic force, it is possible to calculate the strength of the gravitational anomaly.

The effect of gravity is overlaid by the effect of atmospheric friction. However, the constant deceleration caused by the air is likewise registered by the gradiometer, which is able to distinguish it from acceleration by gravitational anomalies. The on-board computer analyses these acceleration data and actuates an ion engine which corrects friction-related errors by varying its thrust accordingly.

This procedure of keeping the satellite in its precise orbit with the help of an active regulation system is called attitude control. The special feature of the GOCE satellite is that it offsets deceleration in the flight direction caused by residual atmosphere by means of ‘drag-free control’. “This system, which works with a regulated ion engine, is completely new in this form,” explains GOCE platform manager Karl-Otto Hienerwadel. “It is one of the most demanding tasks that we face in this project.” Although ion power units have been used in the past, the permanent thrust control is an entirely new feature.


Airbus Defence and Space has also been able to benefit from previous developments in the construction of other parts of the GOCE platform and thereby come up with a cost-effective solution. For instance, the company is currently building a similar platform for ESA’s environmental satellite, CryoSat. Because of the special requirements of the GOCE programme, many of the components had to be further developed.


Special demands are made particularly by the immense precision with which the Earth’s gravitational field has to be measured. Any force that might disturb the satellite must therefore be avoided. For instance, the satellite structure must not be deformed by strong temperature fluctuations. These occur whenever GOCE emerges from the Earth’s shadow and flies into the sunshine, and vice versa. In order to avoid this effect, a thermally stable structure made of carbon-fibre-reinforced plastic and a shrewdly conceived thermal control system has been implemented. This system can only be tested to a limited extent in the laboratory and therefore has to be built and verified using theoretical models.

In addition, the operation of relays and other moving parts during measurement phases is prohibited 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 Airbus Defence and Space engineers have to install in the platform, including electronic controls for the solar generator, a star sensor, a solar sensor, an Earth sensor, a magnetometer and magnetic coils for attitude control, as well as S-band antennas and communication transponders.

Airbus Defence and Space and the Earth Explorers

Airbus Defence and Space is also considerably involved in other satellites of the Earth Explorer Missions which are currently being built. Airbus Defence and Space Germany, for example, is industrial prime contractor for the Cryosat-2 ice satellite, the clouds and aerosol mission EarthCARE and for SWARM, a three-spacecraft constellation to investigate the Earth’s magnetic field.

Airbus Defence and Space UK is the prime contractor for the ADM-Aeolus wind mission, for which Airbus Defence and Space France is developing the Aladin instrument. Airbus Defence and Space Spain is developing and building the Miras payload of the SMOS mission for the acquisition of data on soil moisture and ocean salinity.