The sleek, elegant aerodynamic design of GOCE immediately sets it apart from most other satellites. Since it is 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 has none of the moving parts often seen in other spacecraft. Therefore, the satellite together with its instrumentation actually forms a single composite gravity-measuring device.
The satellite orbits Earth as low as possible to observe the strongest possible gravity-field signal – hence GOCE has been designed to skim above Earth at a height of just 250 km. Its slim elongated form enables it to cut through the wisps of atmosphere that are still present at this height.
An electric ion thruster at the back continuously generates tiny forces to compensate for any drag that GOCE experiences along its orbit.
The need to fly low and be ultra-stable has led to a novel satellite design that minimises air drag and torque and excludes mechanical disturbances. The result is a slim 5 metre-long satellite with a cross sectional area of about 1m2 and weighs in at about 1050 kg. The satellite is symmetrical about its horizontal plane and has two winglets that provide additional aerodynamic stability.
In orbit, the same side of the satellite remains facing the Sun. The satellite is equipped with four body-mounted and two wing-mounted solar panels, which use triple junction Gallium Arsenide solar cells. Due to the orbit and satellite configuration, the solar panels experience extreme temperature variations. The design has, therefore, had to include materials that will tolerate temperatures as high as 160°C and as low as -170°C.
One S-band communication antenna is mounted on each wing. One faces upwards and one downwards so that full spherical coverage is achieved. The wing that points towards space carries two GPS antennas.
Inside the satellite
The satellite consists of a central octagonal tube with seven internal floors that support the equipment and electronic units. Two of the floors support the gradiometer, which is mounted at the heart of the satellite close to its centre of mass. The spacecraft structure is 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 has to cope with large temperature variations - GOCE encounters two eclipse phases per year with maximum eclipses that last up to 30 minutes. Temperature control is achieved mainly through passive means such as coatings and blankets and active control by heaters where necessary. The internal equipment is protected against the hot temperatures of the solar panels by multilayer insulation blankets, which are positioned between the solar panels and the main body of the satellite.
The cold side, which faces away from the Sun, is used in part as a radiator to dissipate heat into space. The external coatings, in particular those situated in the direction of flight, are protected against high atomic oxygen flux, which would otherwise erode unprotected materials very quickly at this low orbit. Due to its stringent temperature stability requirements (in the range of milli-Kelvin for the gradiometer sensor heads), the gradiometer is decoupled thermally from the satellite, having its own dedicated thermal-control system.
|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|
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)|
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|
|Telemetry||Up to 1.2 Mbps|
Last update: 16 September 2010