Drag-free and attitude control for the GOCE satellite

Abstract

The paper concerns Drag-Free and Attitude Control of the European satellite Gravity field and steady-state Ocean Circulation Explorer (GOCE) during the science phase. Design has followed Embedded Model Control, where a spacecraft/environment discrete-time model becomes the real-time control core and is interfaced to actuators and sensors via tuneable feedback laws. Drag-free control implies cancelling non-gravitational forces and all torques, leaving the satellite to free fall subject only to gravity. In addition, for reasons of science, the spacecraft must be carefully aligned to the local orbital frame, retrieved from range and rate of a Global Positioning System receiver. Accurate drag-free and attitude control requires proportional and low-noise thrusting, which in turn raises the problem of propellant saving. Six-axis drag-free control is driven by accurate acceleration measurements provided by the mission payload. Their angular components must be combined with the star-tracker attitude so as to compensate accelerometer drift. Simulated results are presented and discussed.Key words: satellite, drag-free, attitude, control, Embedded Model.inertia, and i , aligned with the cylinder axis, points toward v . O J j J i J k LEO sun-synchronous orbit O k k i O j v Sun O i Earth rotation centred equatorial frame { } , , , J J J

Differential optical shadow sensor for sub-nanometer displacement measurement and its application to drag-free satellites

“…It is feasible to use it as stand-alone displacement sensor for drag-free applications such as geodesy. A number of missions [4,34] have already mapped the Earth's gravity field with sufficient precision to detect the effects of droughts and mass loss of ice sheets. The device described here can be used to obtain improved measurements from future missions with only minor upgrades for launch qualification.…”

“…Uses range from navigation [6] to geodesy [4] to fundamental physics [7][8][9]. The detection of very low frequency gravitational waves in space is probably the most challenging application for drag-free satellites, while geodesy and navigation have much less stringent requirements.…”

“…Spherical TMs have been used in the TRIAD I and Gravity Probe B missions [8,11], while cubic TMs are used in Laser Interferometer Space Antenna (LISA) Pathfinder (LPF) [12] and Gravity field and steady-state Ocean Circulation Explorer (GOCE) [4]. In general, spherical TMs have the advantage of a less complex control system as only the translational degrees of freedom need to be controlled [13], while disadvantages include the need to spin the sphere or fully map its surface in order to handle geometric imperfections and reflectivity variations over the surface for interferometric readouts [14].…”

“…One aspect of mission design is the use of drag-free spacecraft to reduce orbit perturbations that affect the signal/noise ratio. Drag-free technology also has more immediate application to measurements of the Earth's geoid [3,4]. Over the last few years we have developed a novel optical technique to simplify the instrumentation with the aim of reducing cost and improving mission reliability.…”

Atmospheric Drag, Occultation ‘N’ Ionospheric Scintillation (ADONIS) mission proposal

“…Recently the European Space Agency's (ESA) Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) satellite has also performed indirect drag measurements due to its sophisticated drag-free mode control system, at an altitude of about 250 km (Canuto 2008). In the near future, the QB50 mission, which will use a constellation of 50 CubeSats, plans to investigate the drag in the lower thermosphere over a period of 3 months (Gill et al 2013).…”