GOCE Mission Objectives
The GOCE mission objectives were to measure Earth's gravity field and model the geoid with unprecedented accuracy and spatial resolution. This resulted in a unique model of the 'geoid', which is the surface of equal gravitational potential defined by the gravity field – crucial for deriving accurate measurements of ocean circulation and sea-level change, both of which are affected by climate change. GOCE helped to make significant advances in geodesy and surveying. GOCE data will also facilitate one global system for tide-gauge records, so that sea levels can be compared all over the world. This also contributes to observing and understanding sea-level change as a result of melting continental ice-sheets associated with a changing climate and postglacial rebound.
A better understanding of Earth's gravity field and its associated geoid significantly advances our knowledge about how the Earth system works. In particular, an accurate model of the geoid advances our understanding global ocean circulation patterns.
Ocean circulation plays a crucial role in climate regulation by transporting heat from low to high latitudes in surface waters, while currents cooled at high latitudes flow in deeper waters back towards the Equator. The Gulf Stream, which carries warm surface waters northwards from the Gulf of Mexico, is a good example of the important role ocean currents play in moderating the climate. Thanks to this current, the coastal waters of Europe are 4°C warmer than waters at equivalent latitudes in the north Pacific. However, knowledge of the role that the oceans play in the Earth system is currently insufficient for the accurate prediction of climate change.
In order to study ocean circulation more effectively it is necessary to have an accurate map of Earth’s geoid. The geoid represents the shape of a hypothetical ocean surface at rest in response to variations in Earth's gravity field. External forces such as the wind cause the actual sea surface to deviate from the geoid. Previous ESA ocean altimeter systems such as those on ERS and Envisat measured sea-surface height and typically show +/- 1 metre variations relative to the geoid. Importantly, the large-scale current systems flow along the lines of equal topography and are focused around the strongest gradients in sea-surface height.
The degree to which altimetry data can be used to make precise estimates of the transport of heat, salt and freshwater, is limited by the quality of the geoid at short length scales. It is therefore the combination of sea-surface height mapped by altimeters and the knowledge of the precise ocean geoid that improve our understanding of surface currents and leads to a better knowledge of general ocean circulation patterns – crucial for understanding climate change.
Given the GOCE 1-2 cm global geoid, satellite-altimetry data records spanning the last few decades can be used to provide a detailed retrospective picture of ocean circulation patterns and variations, and their consequences for the global water and energy cycles.
Solid Earth Physics
Since the gravity measurements taken by GOCE reflected density variations in Earth's interior, the resulting data led to new insights into processes occurring in the lithosphere and upper mantle - down to a depth of about 200 km. This detailed mapping along with seismic data shed new light on the processes causing earthquakes and volcanic activity.
GOCE also furthered our knowledge of land uplift due to post-glacial rebound. This process describes how Earth's crust is rising a few centimetres in Scandinavia and Canada as it has been relieved of the weight of thick ice sheets since the last Ice Age - when the heavy load caused the crust to depress. As a result, there is global redistribution of water in the oceans. Hence, a better understanding of these processes help in assessing the potential dangers of current sea-level change.
Geodesy is concerned with the measurement of Earth's shape and the mapping of its regions. Its products are used extensively in all branches of Earth sciences. In addition, they are applied to many areas of civil engineering, exploration, mapping and cadastral work and are the basis of all geo-information systems. Whereas positioning on Earth's surface in two- or three-dimensional co-ordinates is based on purely geometric techniques, height determination requires knowledge of the planet’s gravity field. Only through knowledge of differences in gravity potential is it possible to decide on the direction of the flow of water or the direction of 'up' and 'down'.
Since it represents a surface along which no water would flow, the geoid defines our sense of the horizontal and is the classical reference surface for establishing height. However, there is currently no globally unified height-reference system. There are numerous practical implications of having any number of nationally accepted benchmark references, such as how to define the true height of a mountain.
Data from GOCE led to a global unification of height systems, so that mountain ranges in the Americas will be able to be measured against those in Europe or Asia. In the construction industry, an accurate geoid can be used for levelling, for example, to ensure that water flows in the direction intended. It will also aid such things as the building of bridges over water and tunnels through mountains – especially those linking different countries currently using different reference benchmarks. This issue was illustrated in the 1990s with the construction of the Oresund Bridge, which now links Denmark and Sweden. Much effort was taken in connecting two national height systems and precise levelling over the 22 km span of the bridge.
GOCE data will in time facilitate one global system for tide-gauge records, so that sea levels can be compared all over the world. This will also contribute to observing and understanding sea-level change as a result of melting continental ice-sheets associated with a changing climate and postglacial rebound.
The GOCE geoid will provide a global standard that will greatly simplify all these height-related issues.
Gravity is a fundamental force of nature that influences many dynamic processes within Earth’s interior, and on and above its surface. It was Sir Isaac Newton who, more than 300 years ago, explained the basic principles of gravity and the concept more commonly known as the 'g' force.
The value for gravitational acceleration - g = 9.8 m/s2 - was, for a long time, assumed to be constant for the entire planet. However, as more sophisticated and sensitive tools have been developed to measure g, it has become apparent that the force of gravity varies from place to place on Earth's surface. The standard value of 9.8 m/s2 refers to Earth as a homogeneous sphere, but in reality there are many reasons for this value to range from a minimum of 9.78 m/s2 at the Equator to a maximum of 9.83 m/s2 at the poles. We can now measure how g varies to more than eight decimal places, but what causes these small but significant changes?
The most significant deviation from the standard value of g is a result of the Earth’s rotation. As the planet spins, its shape is slightly flattened into an ellipsoid, so that there is a greater distance between the centre of Earth and the surface at the Equator, than the centre of Earth and the surface at the poles. This greater distance, coupled with the world's rotation, results in the force of gravity being weaker at the Equator than at the poles.
Secondly, the surface of Earth is very uneven; high mountains and deep ocean trenches cause the value of gravity to vary.
Third, the materials within Earth’s interior are not uniformly distributed. Not only are the layers within the crust and mantle irregular, but also the mass distribution within the layers is not homogeneous. Petroleum and mineral deposits or ground-water reservoirs can also subtly affect the gravity field, as can a rise in sea level or changes in topography such as ice-sheet movement or volcanic eruptions. Even large buildings can have a minor effect. Of course, depending on location, these factors are often superimposed upon each other, and can also change with time.
The irregular gravity field shapes a virtual surface at mean sea-level called the 'geoid'. This is the surface of equal gravitational potential of a hypothetical ocean surface at rest and so defines 'the horizontal' - meaning that if a ball were placed on this hypothetical surface it would not roll, despite the presence of virtual slopes. The geoid is important for understanding more about ocean currents and is also employed as a reference for traditional height systems and monitoring sea-level change. In addition, the geoid is used for levelling and construction.