Principles of gradiometry
Principles of gradiometry

GOCE mission payload

A high-tech gravity mission such as GOCE required that the satellite and the system of sensor and control elements formed one 'gravity-measuring device'; this is because the satellite itself also acted as a prime sensor. In other words, in contrast to most remote-sensing missions, there was virtually no division between the satellite and the instruments.

GOCE gradiometer

The GOCE concept was 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 were 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 dictated 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.

Gradiometry
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.

GOCE tracked by GPS
GOCE tracked by GPS

Tracking
The Satellite-to-Satellite Tracking Instrument (SSTI) consisted of an advanced dual-frequency, 12-channel GPS receiver and an L-band antenna. The SSTI receiver was capable of acquiring signals simultaneously broadcast from up to 12 satellites in the GPS constellation.

The SSTI delivered, at 1Hz, pseudo-range and carrier-phase measurements on both GPS frequencies, as well as a realtime orbit navigation solution.

GOCE counteracting drag
Counteracting drag

Drag compensation
The advanced drag compensation and attitude-control system was 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 was based on ion-propulsion technology. The electric ion propulsion system comprised two thruster units (one as backup) mounted at the back of the satellite.

The thrusters could be throttled between 1 and 20 millinewtons (mN),  set automatically, depending on the actual realtime drag the satellite experienced in orbit.

A particular feature of the GOCE system design is that the drag-free and attitude-control system used the scientific payload as a sensor.

Laser ranging
The laser retroreflector allowed 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.


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