After two years in orbit, the European Space Agency’s (ESA) Gravity field and steady-state Ocean Circulation Explorer (GOCE) reached the end of its nominal lifetime in February. But GOCE has enough xenon fuel left to continue its mission until at least the end of 2012.
It has already produced the most accurate map ever of the so-called geoid – an Earth-encompassing spirit level and global reference surface.
Markedly different from a simple sphere or ellipsoid, the geoid is the mathematically ‘true’ shape of our planet. It represents a motionless global ocean but takes into account the effects of the Earth’s rotation, weight difference resulting from the position of mountains and ocean trenches, and uneven mass distribution and density variations in the planet’s interior. The resulting small variations in the Earth’s gravitational field feature on the geoid as ‘bulbs’ and ‘dips’ in an idealized ‘ocean’ surface.
ESA scientists presented the latest version of the GOCE-derived geoid map – based on eight months of data processed so far – at a user workshop this week in Munich, Germany.
Video: Animation of the global geoid. ESA
The geoid has many applications. Oceanographers can use it as a reference surface for determining steady-state ocean topography, circulation strength and sea level variations – information which climate scientists can then assimilate in their models.
Geophysicist can gain from it new insights into processes in the Earth’s crust which cause quakes and volcanism.
And geodesists can hope to finally construct a unified global height system that will allow them calculate the precise height differences between any two points on land.
‘Levelling’ errors can be substantial: For example, no less than a half meter mismatch between the British and French systems came to light when the Channel tunnel was drilled.
It will take several years until the 70 million or so GOCE measurement have been mathematically processed – an exercise that includes solving equations with some 100,000 unknowns – says Rune, Floberghagen, ESA’s mission manager for GOCE.
But a picture of the previously elusive ‘steady state’ of the global ocean circulation is already emerging.
“Major ocean currents can now be clearly identified,” says Rory Bingham, an oceanographer at the University of Newcastle.
Bingham has produced a map, filtered from GOCE-derived dynamic ocean topography, of the strength of the North Atlantic Gulf Stream. GOCE data allow estimating the strength of the Gulf Stream 50% more accurately than was previously possible, he says, adding that accuracy should improve further as the GOCE missions continues.
GOCE estimated speed of the Gulf Stream.
The GOCE mean dynamic ocean topography (MDT) of the North Atlantic. MDT is the geoid height subtracted from mean sea surface level as determined from altimetry measurements.