| 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 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.
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To increase resolution, the satellite will fly in an unusually low orbit.
The final gravity map and model of the geoid will provide users worldwide with well-defined data product that will lead to:
Initial results of the GOCE satellite mission were presented at the American Geophysical Union (AGU) 2010 Fall 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. [7] Even very small details like the Mann Eddy in the North Atlantic are visible in the data,[8] as was the effect of Hurricane Igor (2010). [9]
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 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.[10]
The first launch attempt on 16 March 2009, was aborted due to a malfunction with the launch tower.[11] 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.[12]
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.[13] 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.[14]
The first Earth global gravity model based on GOCE data was presented at ESA’s Living Planet Symposium, in June 2010.[15]
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.[16]
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.
GOCE has fixed solar panels, which will produce 1,300 W of power and cover the Sun-facing side of GOCE.[17]
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.[18]
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).
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