Balancing, Turning, Saving; Special AOCS Operations to extend the GRACE Mission

Abstract

The two GRACE satellites were successfully launched on March 17, 2002 by a Russian Rockot launcher. The main objective of the mission is the measurement of the Earth’s gravity field and its time dependencies. It was the first dual-satellite mission operated by GSOC and also the first formation-flying occurring at an altitude below 500 km. The mission is extremely successful from a scientific point of view and the originally envisaged duration of 5 years has more than doubled by now. A follow-on mission is planned by the same partners for 2016 and JPL projects a new generation in the twenties, so there is a strong incentive to prolong GRACE and try to bridge the gap. A number of special AOCS operations and analyses have evolved over the years in order to extend the mission as long as possible. This encompasses such obvious measures as the minimisation of fuel usage and thruster cycles, but also the continuous optimisation of parameter settings and the balancing of several consumables. Close interaction between the science- and operation- teams is required throughout, because the satellites themselves are part of the experiment.