ESA's gravity mission GOCE
ESA's Gravity field and steady-state Ocean Circulation Explorer (GOCE) mission was launched on 17 March 2009 and ended on 11 November 2013.
This novel mission delivered a wealth of data to bring about a whole new level of understanding of one of Earth's most fundamental forces of nature – the gravity field.
Dubbed the 'Formula 1' of satellites, this sleek high-tech gravity satellite embodied many firsts in its design and use of new technology in space to map Earth's gravity field in unprecedented detail.
As the most advanced gravity space mission to date, GOCE data are realising a broad range of fascinating new possibilities for oceanography, solid Earth physics, geodesy and sea-level research, and significantly contributing to furthering our understanding of climate change.
Although invisible, gravity is a complex force of nature that has an immeasurable impact on our everyday lives. It is often assumed that the force of gravity on the surface of Earth has a constant value, but in fact the value of 'g' varies subtly from place to place.
These variations are due to a number of factors such as the rotation of Earth, the position of mountains and ocean trenches and variations in density of Earth's interior.
GOCE mapped these variations in the gravity field with extreme detail and accuracy.
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-derived data are also being used to understand more about processes occurring inside Earth and for use in practical applications such as surveying and levelling.
To gain the best possible gravity measurements, the sleek aerodynamic satellite was designed to fly in an extremely low orbit of 255 km above Earth – about 500 km lower than most Earth observation satellites.
However, low fuel consumption and relatively quiet solar activity allowed GOCE to be lowered to 235 km in 2012. This improved its sensitivity to Earth’s gravity even further and thus produced even more accurate data.
To help avoid drag and ensure that the gravity measurements are of true gravity, the satellite had to be kept stable in ‘free fall’. Any buffeting from residual air at this low altitude could have potentially drowned out the gravity data. To help achieve this, the satellite carried an electric ion thruster system that continuously generated tiny forces to compensate for any drag the satellite experienced at this low altitude.
This low-orbiting satellite was the first mission to employ the concept of 'gradiometry' – the measurement of acceleration differences over short distances between an ensemble of proof masses inside the satellite.
GOCE was equipped with three pairs of ultra-sensitive accelerometers arranged in three dimensions that responded to tiny variations in the 'gravitational tug' of Earth along its orbital path. Because of their different positions in the gravitational field they all experienced gravitational acceleration slightly differently. The three axes of the gradiometer allowed the simultaneous measurement of six independent, but complementary, components of the gravity field.
In order to measure gravity, there can be no interference from moving parts so the entire satellite was actually one extremely sensitive measuring device.
- to determine gravity-field anomalies with an accuracy of 1 mGal (where 1 mGal = 10–5 ms–2).
- to determine the geoid with an accuracy of 1-2 cm.
- to achieve the above at a spatial resolution better than 100 km.