Control Propellant Minimization for the Next Generation Gravity Mission

This chapter addresses the Next Generation Gravity Mission (NGGM), a candidate Earth observation mission of the European Space Agency (ESA), currently undergoing system and technology studies. NGGM is intended to continue the series of ESA missions measuring Earth gravity from space, successfully started with the Gravity field and Ocean Circulation Explorer (GOCE) satellite which flew between 2009 and 2013. Whereas GOCE measured static gravity by a three-axis gradiometer, NGGM will monitor the temporal variations of the gravity field due to mass (primarily water) transport in the Earth system with a concept pioneered by GRACE (Gravity Recovery and Climate Experiment), with improved sensitivity, thanks to laser tracking between satellite pairs. As a monitoring mission, NGGM shall be of a long duration, 11 years according to the current scientific requirements. In addition, the laser interferometer and accelerometer payloads impose demanding requirements such as suppression of the air drag disturbances, precise pointing, and angular rate control. The long lifetime and the control requirements can only be met by using electric thrusters with high specific impulse, hence low mass consumption. Nevertheless, propellant mass minimization remains a dominant task of the mission design. This objective requires proper selection of the thruster operating ranges, as well as an optimized thruster layout and thrust dispatching algorithms. The method applied to solve the thrust dispatching problem is the subject of another chapter in this volume. The present chapter illustrates the flow-down of mission and system requirements into the proposed spacecraft implementation and operation features, focusing on the thruster layout optimization problem. The proposed design is shown to meet the mission requirements, thus validating the methodology adopted as well as the results achieved. Further research avenues opened by the current work are outlined in the conclusions.