Gravity Field and Steady-State Ocean Circulation Explorer
|Operator||European Space Agency|
|Major contractors||Thales Alenia Space (Italy)|
|Launch date||March 17, 2009|
|Mass||1,100 kg (2,400 lb)|
|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. 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 continuously compensates 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 measure gravitational gradients along three different axes.
Mission objectives 
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.
Initial findings 
Initial results of the GOCE satellite mission were presented at the American Geophysical Union (AGU) 2010 Fall (Autumn) Meeting by Dr Rory Bingham from Newcastle University, UK. The maps produced from the GOCE data show ocean currents in much finer detail than previously available. Even very small details like the Mann Eddy in the North Atlantic are visible in the data, as was the effect of Hurricane Igor (2010).
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 was 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[when?] planned at 270 km. During this time, the spacecraft will be commissioned and the electrical propulsion system will be checked for reliability in attitude control[dated info].
The first launch attempt on 16 March 2009, was aborted due to a malfunction with the launch tower. 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.
In February 2010 a fault was discovered in the satellite's computer, which meant controllers were forced to switch control to the backup computer. In July 2010, GOCE suffered a serious communications malfunction, when the satellite suddenly failed to downlink scientific data to its receiving stations. Extensive investigations by experts from ESA and industry revealed that the issue was almost certainly related to a communication link between the processor module and the telemetry modules of the main computer. The recovery was completed in September 2010: as part of the action plan, the temperature of the floor hosting the computers was raised by some 7°C – resulting in restoration of normal communications.
The first Earth global gravity model based on GOCE data was presented at ESA’s Living Planet Symposium, in June 2010.
In November, 2010, it was decided to extend the mission lifetime of 18 months, till the end of 2012, in order to improve the collected data.
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 is 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.
The ion propulsion electric engine, designed and built at QinetiQ's space centre in Farnborough, 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). 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.
See also 
- CryoSat-2 ESA - launch 2010
- Gravity Recovery and Climate Experiment (GRACE) NASA - launch 2002
- ICESat, (Ice, Cloud, and land Elevation Satellite) NASA - launch 2003 / end 2010
- List of climate research satellites
- Soil Moisture and Ocean Salinity satellite (SMOS) ESA - launch 2009
- GOCE site, ESA, retrieved 2009-03-04
- "Homepage - ESA's gravity mission GOCE". European Space Agency. October 16, 2008. Retrieved 2008-10-26.
- ESA Bulletin n. 133, ESA, February 2008
- Drinkwater, M R et al (2003). "GOCE: ESA’s first Earth Explorer Core mission". Space Science Reviews, vol. 108: pp. 419–432,. Kluwer Academic Publishers. Retrieved 2011-24-8.
- Johannessen J J et al (2003). "The European hiExplorer Satellite Mission: Impact in hi". Surveys in Geophysics, 24, 4, pp. 339-386. Springer.
- "GOCE scientific objectives". esa.int.
- Bingham, R J et al (2010). "Using GOCE to estimate the mean North Atlantic circulation (Invited)". Abstract presented at 2010 Fall Meeting, AGU, San Francisco, Calif., 13-17 Dec. American Geophysical Union. Retrieved 2010-12-22. Text "761" ignored (help); Text "2560" ignored (help); Text "Using%20GOCE%20to%20estimate%20the%20mean%20North%20Atlantic%20circulation%20%28%3Ci%3EInvited%3C%2fi%3E%29" ignored (help); Text "HTML" ignored (help); Text "localhost:0" ignored (help); Text "%2fdata%2fepubs%2fwais%2findexes%2ffm10%2ffm10" ignored (help); Text "21927386%2021929946%20%2fdata2%2fepubs%2fwais%2fdata%2ffm10%2ffm10.txt " ignored (help)
- Jonathan Amos, Science correspondent, BBC News (21 December 2010 Last updated at 00:10). "Goce gravity mission traces ocean circulation". BBC News website, Science & Environment. BBC News. Retrieved 21 Dec 2010.
- "ESA missions highlighted at world's largest scientific conference". ESA GOCE website, News. European Space Agency. 17 December 2010. Retrieved 2010-12-22.
- ESA Bulletin n. 133, ESA, February 2008
- Launch of ESA’s gravity mapping satellite delayed, ESA, 16 March 2009
- ESA launches first Earth Explorer mission GOCE, ESA, 17 March 2009
- Computer blow to Europe's Goce gravity satellite, BBC News Online, 21 August 2010, retrieved 22 August 2010
- GOCE fully operational again, ESA, 29 September 2010
- GOCE giving new insights into Earth’s gravity, ESA, retrieved 29 June 2010
- ESA's gravity mission granted 18-month extension, ESA, 25 November 2010
- "GOCE – How Low Can It Go?". 19 Nov 2012.
- Amos, Jonathan (24 October 2008). "Goce gravity flight slips to 2009". BBC News. Retrieved 2008-10-26.
- "BBC News - Goce satellite views Earth's gravity in high definition". BBC. June 28, 2010. Retrieved 2010-06-30.
- 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
- 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