Payload
The satellite's main payload is the Electrostatic Gravity Gradiometer (EGG) to measure the gravity field of Earth. They are arranged in three pairs of ultra-sensitive accelerometers arranged in three dimensions that respond to tiny variations in the 'gravitational tug' of the Earth as it travels along its orbital path. Because of their different position in the gravitational field they all experience the gravitational acceleration of the Earth slightly differently. The three axes of the gradiometer allow the simultaneous measurement of the five independent components of the gravity gradient tensor.
Other payload is 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. The satellite is also equipped with a laser retroreflector to enable tracking by ground-based lasers.
Read more about this topic: Gravity Field And Steady-State Ocean Circulation Explorer