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The Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite, currently planned to he launched in the course
of 2006, will require a precise drag compensation and a fine attitude control along the Local Orbiting Reference Frame (LORF)
of a polar Sun-synchronous low orbit, allowing the Earth gravity field to be recovered with unprecedented accuracy by post-processing
the scientific telemetry. To this aim, the spectral density of the spacecraft linear and angular accelerations must be limited
below 0.025
respectively, in the frequency range from 5 mHz to 0.1 Hz, the gradiometer measurement bandwidth. In the same range, the
orientation errors of the spacecraft in the LORF and of the LORF in the inertial frame must be kept below 10
. The Drag-Free Mode, encharged of drag-free and attitude control (DFAC) during measurement phases, determines the spacecraft
state vector using a very precise gradiometer, one large Field-of-View Star Tracker and a Satellite-to-Satellite Tracking
Instrument. Force and torque commands are actuated by two assemblies of thrusters: a single ion-thruster acting along the
orbital direction, a set of eight micro-thrusters acting along the other five degrees of freedom. To cover every mission scenario,
other control modes have been studied and designed: the Coarse Pointing Mode dedicated to rate damping and Sun acquisition,
the Fine Pointing Mode handling the transition to Drag-Free Mode and the Ultimate Safe Mode, a survival operative mode to
improve mission reliability. Results presented in this paper give a positive perspective on the solidity of the current DFAC
design.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
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