20. Next Generation Gravity Mission

After the successful experience of the gravity missions GRACE and GOCE, several activities are on going in preparation of a “Next Generation Gravity Mission” (NGGM) aimed at measuring the temporal variations of the Earth’s gravity field over a long time span (up to ~11 years) with high spatial resolution (comparable to that provided by GOCE) and high temporal resolution (weekly or better). Its data will find wide application in geodesy, geophysics, hydrology, ocean circulation and many other disciplines. The most appropriate measurement technique identified for such mission is Low-Low Satellite-Satellite Tracking in which two (or more) satellites flying in “loose” formation in a low Earth orbit act as proof masses immersed in the Earth gravity field. The distance variation between the satellites (measured by a laser interferometer) and the non-gravitational accelerations of each satellite (measured by ultra-sensitive accelerometers) are the fundamental observables from which the gravity field is obtained. Suitable satellite formations for this mission include the “In-line” (the simplest one), the “Cartwheel” and the “Pendulum” (more complex but also scientifically more fruitful), with an inter-satellite distance up to 100 km. Polar, circular orbits with altitudes between ~340 and ~420 km are suitable candidates for the NGGM, providing all-latitude coverage, short repeat cycles/sub-cycles and a still excellent gravity signal compatibly with a long lifetime. Each satellite shall be endowed with a complex control system capable of carrying out several tasks in close coordination: orbit maintenance, formation keeping, provision of a “drag-free” environment to the accelerometers, laser beam pointing and attitude control.