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’.
In other words, in contrast to most remote-sensing missions, there is virtually no division between the satellite and its instruments.
GOCE’s main instrument is the Electrostatic Gravity Gradiometer (EGG), a set of six 3-axis accelerometers mounted in a diamond configuration in an ultra-stable structure.
Each accelerometer pair forms a ‘gradiometer arm’ 50 cm long, with the difference in gravitational pull measured between the two ends. Three arms are mounted orthogonally: along-track, cross-track and vertically.
The gradiometer is 100 times more sensitive than any sensor of the same type previously flown in space.
The gradiometer will for the first time measure gravity gradients in all directions. It is specifically designed for the stationary gravity field – measuring the geoid and gravity anomalies with high accuracy and high spatial resolution.
GOCE also carries a GPS receiver to be used as a Satellite-to-Satellite Tracking Instrument (SSTI) to supplement the gradiometer measurements. The SSTI consists of an advanced dual-frequency, 12-channel GPS receiver and an L-band antenna.
The SSTI receiver is capable of simultaneously acquiring signals broadcast from up to 12 satellites in the GPS constellation. The SSTI instrument delivers, at 1 Hz, so-called pseudo-range and carrier-phase measurements on both GPS frequencies, as well as a real-time orbit navigation solution.
GOCE also has a Laser Retroreflector to allow its precise orbit to be tracked by a global network of groundstations through the Satellite Laser Ranging Service. This provides accurate positioning for orbit determination and data products.