Gravity Field and Steady-State Ocean Circulation Explorer

GOCE

Operator	European Space Agency
Mission type	Orbiter
Satellite of	Earth
Launch date	March 17, 2009
Launch vehicle	Rockot
Homepage	www.esa.int
Mass	1,100 kg (2,400 lb)
Orbital elements
Eccentricity	Near circular
Inclination	96.70°
Periapsis	270 km (170 mi)

The Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) is an ESA satellite that was launched on March 17, 2009.^[1] It is a satellite carrying a highly sensitive gravity gradiometer which detects fine density differences in the crust and oceans of the Earth.

GOCE data will have many uses, probing hazardous volcanic regions and bringing new insight into ocean behaviour. The latter, in particular, is a major driver for the mission. By combining the gravity data with information about sea surface height gathered by other satellite altimeters, scientists will be able to track the direction and speed of geostrophic ocean currents. The low orbit and high accuracy of the system will greatly improve the known accuracy and spatial resolution of the geoid (the theoretical surface of equal gravitational potential on the Earth).

The satellite's arrow shape and fins help keep the GOCE stable as it flies through the wisps of air still present at an altitude of 260 km. In addition, an ion propulsion system will continuously compensate for the deceleration of air-drag without the vibration of a conventional chemically-powered rocket engine, thus restoring the path of the craft as closely as possible to a purely inertial trajectory. The craft's primary instrument is three pairs of highly sensitive accelerometers which will measure gravitational gradients in three different axes.

Mission objectives

• To determine gravity-field anomalies with an accuracy of 10⁻⁵ ms⁻² (1 mGal).^[2] To increase resolution, the satellite will fly in an unusually low orbit.
• To determine the geoid with an accuracy of 1–2 cm.
• To achieve the above at a spatial resolution better than 100 km.

Discoveries and applications

The final gravity map and model of the geoid will provide users worldwide with well-defined data product that will lead to:

• A better understanding of the physics of the Earth's interior to gain new insights into the geodynamics associated with the lithosphere, mantle composition and rheology, uplift and subduction processes.
• A better understanding of the ocean currents and heat transport.
• A global height-reference system, which can serve as a reference surface for the study of topographic processes and sea-level change.
• Better estimates of the thickness of polar ice-sheets and their movement.^[3]

Launch and operations

GOCE was launched from the Plesetsk Cosmodrome in northern Russia with a Rockot vehicle at 15:21 CET (14:21 UT). The Rockot is a modified SS-19 intercontinental ballistic missile that was decommissioned after the Strategic Arms Reduction Treaty. The launcher uses the two lower liquid fuel stages of the original SS-19 and is equipped with a Briz-KM third stage developed for precise orbit injection. GOCE will be launched into a Sun-synchronous dusk-dawn orbit with an inclination of 96.70° and an ascending node at 18:00. Separation from the launcher was at 295 km. The satellite’s orbit will then decay over a period of 45 days to an operational altitude, currently planned at 270 km. During this time, the spacecraft will be commissioned and the electrical propulsion system will be checked for reliability in altitude control.^[4]

The first launch attempt on 16 March 2009, was aborted due to a malfunction with the launch tower.^[5] Liftoff occurred successfully at 14:21 GMT on 17 March 2009. The Rockot launcher delivered the satellite northward over the Arctic. About 90 minutes later, after one orbital revolution and two Briz-KM upper-stage burns, the spacecraft was successfully released into a circumpolar orbit at 280 km altitude with 96.7° inclination to the Equator. Soon after the separation, contact was successfully established with the satellite.^[6]

In February 2010, a fault was discovered in the satellite's computer, which meant controllers were forced to switch control to the backup computer, which failed in July 2010, preventing data from being sent back to Earth. The mission team attempted to regain partial control of both computers, which would allow normal operations to resume, although the attempt was not expected to be completed until September 2010. By this time, the satellite had collected around two thirds of the total data it was expected to measure.^[7]

Payload

The satellite's main payload is the Electrostatic Gravity Gradiometer (EGG) to measure the gravity field of Earth. They are arranged in three pairs of ultra-sensitive accelerometers arranged in three dimensions that respond to tiny variations in the 'gravitational tug' of the Earth as it travels along its orbital path. Because of their different position in the gravitational field they all experience the gravitational acceleration of the Earth slightly differently. The three axes of the gradiometer allow the simultaneous measurement of the five independent components of the gravity gradient tensor.

Other payload will be an onboard GPS receiver used as a Satellite-to-Satellite Tracking Instrument (SSTI); a compensation system for all non-gravitational forces acting on the spacecraft. The satellite is also equipped with a laser retroreflector to enable tracking by ground-based lasers.

Power

GOCE (09-013A) flaring briefly to magnitude +2 on this image as the 67.5 degree solar panel briefly mirrors sunlight to the observer. This photograph was taken by Marco Langbroek (Leiden, the Netherlands), the flaring occurred at 17:24:23.15 UTC (Jan 3, 2010).

GOCE has fixed solar panels, which will produce 1,300 W of power and cover the Sun-facing side of GOCE.^[8]

The ion propulsion electric engine ejects xenon ions at velocities exceeding 40,000 m/s, which will compensate for the orbital decay losses. GOCE's mission will end when the 40 kg xenon fuel tank empties (with a predicted lifetime of about 20 months).^[2] However, the ESA has reported that unusually low solar activity (meaning a calmer upper atmosphere, and hence less drag on the craft) may mean the mission could extend past its predicted 20 months due to fuel savings - possibly into 2014.^[9]

The 5 m × 1 m frame incorporates the fixed solar panels as fins to stabilise the spacecraft while it orbits through the residual air in the ionosphere (also called thermosphere).

See also

• List of climate research satellites

• Gravity Recovery and Climate Experiment (GRACE) NASA - launch 2002
• Ice, Cloud, and land Elevation Satellite (ICESat) NASA - launch 2003

• Soil Moisture and Ocean Salinity satellite (SMOS) ESA - launch 2009
• CryoSat-2 ESA - launch 2010

References

External links

• ESA GOCE site
• ESA's gravity mission GOCE
• GOCE page at ESA Spacecraft Operations
• GOCE Mission Profile by NASA's Solar System Exploration
• Jonathan Amos (27 July 2007), 'Space arrow' to map Earth's tug, BBC News, http://news.bbc.co.uk/1/hi/sci/tech/6919016.stm 
• 24 October 2008 BBC News story
• 6 April 2009 BBC News story
• Goce satellite views Earth's gravity in high definition BBC News (includes a copy of the new gravity map) 29 June 2010
• http://www.earthzine.org/2007/10/29/esa’s-earth-observation-programmes-advancing-earth-science-through-new-sensing-technology