Sunny side of GOCE

GOCE satellite

GOCE's sleek, elegant aerodynamic design immediately set it apart from most other satellites. Since it was vital to ensure that the measurements taken are of true gravity and not influenced by any movement of the satellite, this unique five-metre long arrow-shaped satellite had none of the moving parts often seen in other spacecraft. The satellite together with its instrumentation actually forms a single composite gravity-measuring device.

The satellite orbited Earth as low as possible to observe the strongest possible gravity-field signal – hence GOCE was designed to skim the edge of Earth's atmosphere at a height of just 250 km. Low fuel consumption meant that its altitude could be lowered to 235 km in 2012.

An electric ion thruster at the back of the satellite continuously generated tiny forces to compensate for any drag that GOCE experienced along its orbit.

The need to fly low and be ultra-stable  led to a novel satellite design that minimised air drag and torque and excludes mechanical disturbances. The result was a slim 5 metre-long satellite with a cross sectional area of about 1m2, weighing in at about 1050 kg. The satellite was symmetrical about its horizontal plane and had two winglets that provided additional aerodynamic stability.

In orbit, the same side of the satellite remained facing the Sun. The satellite was equipped with four body-mounted and two wing-mounted solar panels, which used triple junction Gallium Arsenide solar cells. Due to the orbit and satellite configuration, the solar panels experienced extreme temperature variations. The design, therefore, had to include materials that tolerated temperatures as high as 160°C and as low as -170°C.

One S-band communication antenna was mounted on each wing. One faced upwards and one downwards so that full spherical coverage was achieved. The wing that pointed towards space carried two GPS antennas.

Inside the satellite

Inside GOCE

The satellite comprised a central octagonal tube with seven internal floors that supported the equipment and electronic units. Two of the floors supported the gradiometer, which was mounted at the heart of the satellite close to its centre of mass. The spacecraft structure was built largely of carbon-fibre reinforced plastic sandwich panels to guarantee stable conditions under varying temperatures and at the same time to limit mass.

The satellite had to cope with large temperature variations - GOCE encountered two eclipse phases per year with maximum eclipses that lasted up to 30 minutes. Temperature control was achieved mainly through passive means such as coatings and blankets and active control by heaters where necessary. The internal equipment was protected from the hot solar panels by multilayer insulation blankets, which were positioned between the solar panels and the main body of the satellite.

The cold side, which faced away from the Sun, was used in part as a radiator to dissipate heat into space. The external coatings, in particular those situated in the direction of flight, were protected from high atomic oxygen flux, which would otherwise erode unprotected materials very quickly at this low orbit.

Owing to the need for stringent temperature stability  (in the range of milli-Kelvin for the gradiometer sensor heads), the gradiometer was decoupled thermally from the satellite, having its own dedicated thermal-control system.

Technical details

GOCE 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
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