Defying Gravity to Define Gravity – GOCE’s heart starts beating

What a fantastic day…aside the news arriving from the Kepler front regarding the acquisition of its first light it was also the day that the heart of ESA’s GOCE started beating…on this occasion the right person to tell us about what happened returned to Beyond the Cradle, Mark Drinkwater, GOCE Mission Scientist is here one more to share his thoughts with the readers:  

Over the course of the last weeks, since the successful launch from Plesetsk, Russia on 17 March, teams of flight operations staff have been feverishly working to commission the GOCE satellite. GOCE was injected into orbit at an altitude of 283 km on 17 March. Since then, it has been freefalling at a rate of 150 to 200 m a day and will be allowed to continue to do so in a controlled manner until we enter the so-called “drag-free mode” of ion thruster operation – before starting the first science measurement operations phase.

goce22The satellite has now been placed in a stable operating state with all attitude monitoring systems functional. We have demonstrated over the course of the last 3 weeks that all of the key payload components are fully functional, including the star trackers (STR), Satellite-to-Satellite Tracking Instrument (SSTI). Last week culminated in the commissioning of the sophisticated ion propulsion system which is verified to operate normally. This has allowed us to move on to commissioning of the primary instrument at the heart of the satellite – the Electrostatic Gravity Gradiometer. 

Over the last days GOCE’s highly sensitive Gradiometer instrument has been switched on and is producing data! Forming the heart of GOCE, the gradiometer is specifically designed to measure Earth’s gravity field with unprecedented accuracy. We are all extremely excited and pleased with what we have seen from the gradiometer from the moment it was switched on. All accelerometer sensor heads are working in very good health and provide meaningful data. 

The gradiometer consists of three pairs of identical ultra-sensitive accelerometers, each mounted to point in orthogonal directions to allow the simultaneous measurement of the spatial variations of the gravity field in 3 dimensions. With the switching on of the gradiometer, all systems on the satellite have now been activated.  

In order to get the maximum performance from the gradiometer, GOCE was designed to provide a highly stable and undisturbed environment. However, GOCE has to orbit Earth close enough to measure the tiny differences in gravity, which forces it to endure significant drag from the uppermost layers of the atmosphere.  The atmospheric drag is compensated for by the ion propulsion system, which is able to deliver between 1 and 20 milliNewtons of thrust (the force being equivalent to a human exhaling).  

Over the next months the Gradiometer instrument will need to be fully commissioned and calibrated, prior to start of mission operations in summer 2009. Meanwhile, the satellite will carefully descend to an altitude of around 270 km. At that point the satellite will actively compensate for the effect of air drag and its payload will undergo some weeks of commissioning and calibration. Once the gradiometer is calibrated, the first science measurement phase can begin.

Data from GOCE will be used to create an extremely accurate map of Earth’s gravity field. Together with altimetry data from missions such as ESA’s Envisat, and the NASA/CNES/NOAA/Eumetsat Jason-2, the GOCE gravity map will allow scientists to measure sea-surface height more accurately and to better understand sea-level change and ocean circulation.  

A Happy Easter to all GOCE followers! 

Mark Drinkwater

GOCE Mission Scientist and Head of Mission Science Division, ESA Earth Observation Programmes  


For further updates please see:  

To track GOCE on a map – see:

(and click on the Earth Observation –> GOCE link)

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