GOCE mission payload
A high-tech gravity mission such as GOCE requires that the satellite and the system of sensor and control elements form one 'gravity-measuring device'; this is because the satellite itself also acts as a prime sensor. In other words, in contrast to most remote-sensing missions, there is virtually no division between the satellite and the instruments.
The GOCE concept is unique in meeting four fundamental criteria:
- uninterrupted tracking in three spatial dimensions
- continuous compensation for the effect of non-gravitational forces such as air-drag and also radiation pressure
- selection of a low-orbital altitude for a strong gravity signal, and
- counteraction of the gravity-field attenuation by employing satellite gravity gradiometry.
Scientifically, these are the building blocks of the GOCE mission. In general, the lower the orbit the more demanding the environment for satellite structure and subsystems. In turn, these building blocks dictate the choice of technical solutions for the instrumentation namely:
- an Electrostatic Gravity Gradiometer (EGG) as the main instrument
- 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, including a very sophisticated propulsion system, and
- a laser retroreflector to enable tracking by ground-based lasers.
The principle of operation of the gradiometer relies on measuring the forces that maintain a proof mass at the centre of a specially engineered cage. Servo-controlled electrostatic suspension provides control of the proof masses in terms of linear and rotational motion. Three pairs of identical accelerometers, which form three gradiometer arms, are mounted on the ultra-stable structure. The difference between accelerations measured by each of two accelerometers (which are about 50 cm apart), in the direction joining them contains the basic gradiometric information.
The average of the two accelerations is proportional to the externally-induced drag acceleration (common mode measurement). The three arms are mounted orthogonal to one another: one aligned with the satellite's trajectory, one perpendicular to the trajectory, and one pointing approximately towards the centre of Earth. By combining these different acceleration measurements, it is possible to derive the gravity-gradient components.
The Satellite-to-Satellite Tracking Instrument (SSTI) consists of an advanced dual-frequency, 12-channel GPS receiver and an L-band antenna. The SSTI receiver is capable of acquiring signals simultaneously broadcast from up to 12 spacecraft in the GPS constellation. The SSTI delivers, at 1Hz, pseudo-range and carrier-phase measurements on both GPS frequencies, as well as a realtime orbit navigation solution.
The advanced drag compensation and attitude-control system is a key feature required to keep the accelerometer sensor heads in near 'free fall motion' and to maintain the orbit altitude at about 250 km. The system is based on ion-propulsion technology. The electric ion propulsion system comprises two thruster units (one as backup) mounted at the back of the satellite. The thrusters can be throttled between 1 and 20 millinewtons (mN), which is set automatically, depending on the actual realtime drag the satellite experiences in orbit.
A particular feature of the GOCE system design is that the drag-free and attitude-control system uses the scientific payload as a sensor.
The laser retroreflector allows GOCE's precise orbit to be tracked by a global network of ground-based stations through the Satellite Laser Ranging Service. This provides accurate positioning for orbit determination and data products.
|Three-axis diagonal gradiometer|
|Based on three pairs of electrostatic servo-controlled accelerometers|
|Design Measurement Bandwidth (MBW)||5 * 10-3 to 10-1 Hz.|
|Baseline length||0.5 m|
|Sensitivity (detection noise)|
|- Measurement bandwidth||< 10-12 m s-2 Hz-1/2|
- Extended bandwidth
(10-5 to 1 Hz)
|< 10-10 m s-2 Hz-1/2|
|Proof-mass positioning error||6 * 10-8 m Hz-1/2|
|Absolute / relative scale factors||10-3 / 10-5|
|Absolute / relative misalignment||10-3 rad / 10-5 rad|
|Geodetic quality multi-channel GPS receiver with Helix attenna|
|To allow precise tracking by the International Laser Ranging Service|
Last update: 16 September 2010