Facts and figures

Full name: Gravity field and steady-state Ocean Circulation Explorer

Launched: 17 March 2009 from the Plesetsk Cosmodrome, Russia

Duration: After 4 years and 8 months orbiting Earth, the mission came to an end on 11 November 2013, far exceeding its 20-month design life

Mission control: European Space Operations Centre (ESOC), Darmstadt, Germany

Data download: Kiruna (Sweden) and Svalbard (Norway) ground stations

Processing & archiving: ESA’s Earth observation centre in Italy (ESRIN) and the distributed High-level Processing Facilities

Number of instruments: 3

Mission cost: €350 million (including launcher and operations)

Orbit: about 260 km altitude, polar, Sun-synchronous

Mass: 1100 kg

Size: 5.3 m long, about 1 m body diameter

Propulsion tank: 40 kg of xenon

Mission objectives
– to determine gravity-field anomalies with an accuracy of 1 mGal (where 1mGal = 10–5 ms–2).
– to determine the geoid with an accuracy of 1-2 cm.
– to achieve the above at a spatial resolution better than 100 km.

Geoid accuracy: 1 - 2 cm vertically with 100 km spatial resolution

Gravitational acceleration at Earth’s surface is about 9.8 m/s², varying from a minimum of 9.788 m/s² at the equator to a maximum of 9.838 m/s² at the poles.

The six accelerometers (three pairs in three orthogonal directions) are some 100 times more sensitive than any previously flown in space.

Industrial Core Team
Thales Alenia Space (Italy) – satellite prime contractor
EADS Astrium GmbH (Germany) – platform contractor
Thales Alenia Space (France) – gradiometer
ONERA (France) accelerometer & system support

The Core Team led a consortium of 41 companies distributed over 13 European countries.

Technical details:

Satellite
Configuration Minimum cross-section in the direction of motion (1.1 m2)
Approx. 9 m2 solar array aligned in the orbit plane
1050 kg launch mass 
Structure Several carbon fibre, reinforced plastic structural compartments; load-carrying external structure
Structural dynamics: 110 Hz axial, 18 Hz lateral 
Thermal control Passive with heaters
High-thermal-stability gradiometer compartment (10 mK @ 5 mHz)
Electrical power 24-32 Vdc unregulated bus; protected and redundant lines
Fixed Gallium Arsenide (GaAs) cell solar array, 1300 W 
Lithium Ion (Li-Ion) battery, 78 Ah, made of 52 strings with 8 cells each 
Attitude control Nadir pointing
Only magnetorquers for attitude control 
Wide-field star trackers hybridised with gradiometer angular acceleration measurement 
Coarse Sun sensors and magnetometer for acquisition and safe mode 
AOCS/DFACS application software run in central computer 
Drag control Ion thrusters commanded in closed loop, based on gradiometer common-mode acceleration measurements
Reaction Control System (RCS) 20 mN Kaufman-type ion thrusters (2x)
Data handling Packet telemetry
Flexible and reallocatable packet sizes
High-rate (10 Hz) gradiometer-to-computer link via 1553 bus for drag control command synthesis

Telemetry and Command
RF S-band up- and down-link and ranging
2 hemispherical antennas on solar array edges 
Telecommand 4 Kbits/s
Telemetry Up to 1.2 Mbps
Copyright 2000 - 2015 © European Space Agency. All rights reserved.