Delfi-C³ Attitude Determination & Control System
The Attitude Determination & Control Subsystem (ADCS) of Delfi-C³ comprises two separated sections: attitude determination and attitude control.
The attitude determination onboard Delfi-C³ is kept very simple and is not intended for onboard operations. The autonomous wireless sun sensor payload from TNO provides information on the sun vector when the sun is in their field of view. As this is not omni-directional and to avoid cross-dependencies between payloads, each solar panel is equipped with a reference photo diode. This photodiode provides a value of the solar flux to correlate this with the thin film solar cell payload measurements. It can also provide a simple means of determining the attitude of the satellite towards the sun.The in-orbit results provide insight in the solar angle at different times. It also provides a rough estimation of the rotational rate of the satellite.
The attitude control is designed as a completely passive system. The spacecraft does not have any active attitude control and is free to rotate about all its axes. However, in order to limit the rotation rates of the satellite to about two rotations per minute, magnetic hysteresis material and a permanent magnet is be employed to limit rotation rates using the Earth's magnetic field. This Passive Magnetic Attitude Stabilization (PMAS) system consists of a small permanent magnet with a dipole moment of 0.3 A·m2 and two Permenorm 5000 H2 hysteresis rods of each 769 mm3. The intention of this system was to dampen the rotational rate to a oscillation with an average rotational speed of 0.1 o/s.
The damping should have taken place in a few hours from a maximum of 10 o/s. The real in orbit results shows that the damping takes place, but in a much slower pace than modelled: from about 5o/s to 0.1 o/s in three months. A plausible hypothesis for this is the close presence of the permanent magnet causing partial saturation of the hysteresis rods. A paper on the results of this system has been published in the proceedings of the International Astronautical Congress of 2009.