ORBIT AND FORMATION CONTROL FOR LOW-EARTH-ORBIT GRAVIMETRY DRAG-FREE SATELLITES

The paper outlines orbit and formation control of a long-distance (>100 km) two-satellite formation for the Earth gravity monitoring. Modeling and control design follows the Embedded Model Control methodology. We distinguishe be-tween orbit and formation control: orbit control applies to a single satellite and performs altitude control. Formation control is formulated as a control capable of altitude and distance control at the same time. The satellites being placed in a low Earth orbit, orbit and formation control employ the measurements of a global navigation system. Formation control is imposed by long-distance laser interferometry, which is the key instrument together with GOCE-class accelerometers for gravity measurement. Orbit and formation control are low-frequency control systems in charge of cancelling the bias and drift of the residual drag-free accelerations. Drag-free control is the core of orbit/formation control since it makes the formation to fly drag-free only subject to gravity. Drag-free is demanded by the low-Earth orbit and by the accelerometer systematic errors. Drag-free control being required to have a bandwidth close to 1 Hz, is designed as the inner loop of the formation control, but formation control must not destroy drag-free performance, which is obtained by restricting formation control to be effective only below orbital frequency. A control of this kind appears to be original: an appropriate orbit and formation dynamics is derived, discussed and compared with the classical Hill-Clohessy-Wiltshire equations. The derived dynamics is the first step to build the embedded model which is sampled at the orbit rate. Embedded model derivation is explained only for the orbit control, and briefly mentioned for the formation control. Control design is explained in some details, pointing out reference generation, state predictor, control law and main design steps. Simulated results are provided. Drag free results are compared to GOCE data

Formation Control for the Next Generation Earth-Gravimetry Missions

Bittanti S., Cenedese A., Zampieri S. eds

The control challenges for the Next Generation Gravity Mission

Several activities are on going in preparation of a "Next Generation Gravity Mission" (NGGM) aimed at measuring the temporal variations of the Earth gravity field over a long time span with high spatial resolution and high temporal resolution. The most appropriate measurement technique identified for such mission is the "Low-Low Satellite-Satellite Tracking" in which two 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 and the non-gravitational accelerations of the satellites, measured respectively by a laser interferometer and by ultra-sensitive accelerometers, are the fundamental observables from which the Earth gravitational field is obtained. The control system for the NGGM must fulfil the challenging combination of requirements for the orbit and formation maintenance, attitude stabilisation, drag compensation and microradian laser beam pointing. This paper presents the assessment and the preliminary design of the NGGM control system, performed by Thales Alenia Space Italia and Politecnico di Torino for the European Space Agency