Semi-Active Attitude Control and Off-line Attitude Determination for the SSETI-Express Student Micro-satellite
- Paper number
IAC-05-E2.3.01
- Author
Mr. Lars Alminde, Denmark
- Year
2005
- Abstract
In January 2004 a group of students met at the European Space Technology and Research Centre (ESTEC) in Holland to discuss the feasibility of building a micro-satellite, dubbed SSETI-Express, from parts derived from other student satellite projects and launch it within one and a half year. The project is an initiative under the ESA education department and the Student Space Exploration and Technology Initiative (SSETI). One of the major design gabs was the Attitude Control and Determination System (ACDS), where no available systems could be adopted within time. This article describes the subsequent development of an ACDS system for SSETI-Express, which from specification to delivery was finished in 10 months. It was identified that the control part of the system is required to detumble the polar orbiting satellite following launch, provide an orientation compatible for communication over the northern hemisphere,and finally allow pictures to be taken of the northern hemisphere using the on-board camera. For attitude determination the requirement is to provide as accurate attitude information as possible in order to evaluate the functionality of the on-board cold-gas propulsion system. To conserve power and to minimize the coupling between the control part and the determination part of the system it was decided to implement a hybrid active and passive control system utilizing a passive magnet to provide pointing stability in the direction of the magnetic field of the Earth, and for velocity stabilization two orthogonal magnetotorquers provides magnetic moment to counter rotational motion of the spacecraft. The magnetotorquers are driven from input from an on-board magnetometer, which are processed using the B-dot algorithm, which provides negative control proportional to the derivative of the local magnetic field. Attitude determination is performed off-line from data from the magnetometer and experimental MOEMS (Micro Optical, Electrical, Mechanical System) sun-sensors which are downloaded with the telemetry stream. On ground the data are temporally fused in an extended Kalman filter which is able to process the data both forwards and backwards in time. The filter relies on dynamical and kinematic models of the spacecraft as well as statistical models of the sensors. Further, another Kalman filter has been developed to perform parameter identification of the magnetometer simultaneously with attitude determination. Verification of the design through simulations has shown that the control system is able to detumble the satellite within 2.5 orbits from a worst-case initial angular velocity. After detumbling the system retains pointing along the field lines with maximum pointing deviations over equator of 5 degrees. Pointing performance can be increased at the cost of more power being spend by the actuators. The attitude determination part of the system has shown itself to provide estimations with single axis errors of less than 1 degree (2 sigma value) through the sunlit part of the orbit. In the eclipse the filter cannot reliable determine the rotation around the magnetic field lines, but still accurately determines the pointing of the camera/antenna axis of the satellite. Current work aims at improving the attitude determination filters by changing the extended Kalman filter to an unscented Kalman filter, which will be better suited to handle the higher order dynamics and statistics of the non-linear system. The SSETI-Express satellite is scheduled for launch in late June 2005 atop a Russian Cosmos launch vehicle. Part of its autonomous early operations sequence will be to initiate detumbling to allow ground controller situated in Denmark to acquire the satellite and begin operations. The described system provides a minimalistic system that meets all the operational requirements and has been developed under continuous coordination with students from the other involved groups.
- Abstract document
- Manuscript document
IAC-05-E2.3.01.pdf (🔒 authorized access only).
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