GOCE ready to measure Earth’s gravity field

After 6 months of intensive testing in space, the GOCE satellite is ready to make a long series of precise measurements of the Earth’s gravity field. SRON plays an important role in converting the raw measurement data into ‘gravity maps’ of the Earth. These maps might throw new light on issues such as climate change and rising sea levels.

GOCE – launched by ESA on 17 March 2009 – is a completely novel concept: not a satellite packed full of instruments with unfolding solar panels, but instead an aerodynamic projectile. And that is just as well because the gravity satellite is now rapidly orbiting at just 255 kilometres above the Earth, mapping the gravity field with unprecedented accuracy. The gradiometer is the most important instrument onboard GOCE. This refined meter measures small variations in the Earth’s attractive force. In effect, the gradiometer continuously measures the difference in gravitational force between two points in space that are no more than 0.5 metres apart. GOCE has been extensively tested over the past 6 months to make sure that it has successfully survived the launch and it is now ready to start making its gravity measurements.

GOCE is flying in a relatively low orbit around the Earth, mapping the gravity field with unprecedented accuracy

Precise measurements
SRON researcher Sietse Rispens: "Now GOCE is flying in a relatively low orbit around the Earth, it can commence a long series of precise measurements. The tests results revealed that the instruments are functioning superbly. We were particularly concerned about the highly sensitive gradiometer. How the gradiometer responds to differences in gravitational force was determined with considerable accuracy to ensure the correct interpretation of the real measurements. Because the Earth is not a perfect sphere, the gravity field differs slightly from place to place. With the measurement results from GOCE, earth scientists can produce detailed maps of the variation in the gravity field. That could provide new information about ocean currents, the rise or fall of sea levels, climate change or Earth’s internal processes."

Gravity maps
The measurement data soon to be transmitted by GOCE to Earth will still have to be converted into data that scientists can use. This will be done using the High Level Processing Facility (HPF), a scientific processing system developed by a European consortium led by Munich University of Technology. The precise measurements of GOCE – full name Gravity Field and Steady-State Ocean Circulation Explorer will reach SRON via a number of intermediate stations of ESA.

The data chain of the GOCE mission

SRON shall perform the qualtity control and external calibration of the gradiometer and distributes the data over nine other European institutions, including Delft University of Technology. These institues will take care of the further data processing. In the end everything will be sent back to SRON so that the final product of GOCE’s measurements can be delivered to ESA in the form of ‘gravity maps’ of the Earth. ESA will make these maps available to interested scientists. The first ‘gravity map’ shall be presented during the course of 2010.


The institutes involved in the GOCE mission

SRON and its consortium partners, Delft University of Technology and Munich University of Technology, have been involved in GOCE right from the definition phase. Under the management of SRON for example end to end performance simulations have been carried out: simulations taking into account the instruments, satellite, orbit, a hypothetical gravity field and the environment, resulting in the estimates of the accuracy of the gravity field measurements. These parties played an important role during the selection of GOCE in ESA’s space programme (1999). Dutch Space from Leiden and Bradford Engineering are the other two Dutch parties involved in GOCE.