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Attitude Control System

Magnetfeldsensoren des BEESAT-2

The attitude control system of BEESAT-2 comprises of all the necessary components for complete attitude stabilization and control in three axes. The control loop is cycling at 2 Hz.

The sensor suite contains Sun sensors developed in-house that are mounted onto each of the six surface panels. Each Sun sensor is able to precisely determine the angle of incident sunlight in two axes and delivers a unit vector to the Sun in the body fixed reference frame.

Additionally, two three-axis magnetometers allow measuring the Earth's magnetic field based on the anisotropic effect in magneto-resistive materials. Three single-axis analog MEMS gyroscopes, complemented by one next generation, digital three-axis MEMS gyroscope, measure the current rotation rate of the satellite around its body axes.

MEMS Drehratensensoren entlang der drei Körperachsen des BEESAT-2

In order to obtain an attitude information from these sensors, the on-board computer needs reference vectors. From known orbital parameters and UTC time, the current position of the satellite in orbit can be calculated using the SGP4 algorithm. Besides the position vector from the Earth's center to the satellite, an inertial vector pointing from the satellite towards the Sun is also calculated. To achieve higher accuracy of the Sun vector, even the movement of the Sun around the barycenter of the solar system is taken into account. Knowing the orbit position of the satellite, another reference vector can be calculated by using the current reference model of the Earth's magnetic field (IGRF11).

Both reference vectors, the Sun vector and the magnetic field vector, are combined with measurement values from up to five sensors by a statistical estimation algorithm (QUEST), yielding attitude information of the satellite with respect to the inertial coordinate system. Using QUEST, the rotation matrix obtained shows the lowest error between the calculated and measured vectors.

Mikroreaktionsräder als Aktuatoren der Lageregelung des BEESAT-2

Regarding the actuators for attitude control, BEESAT-2 is equipped with six magnetic coils implemented into the surface panels as well as three micro reaction wheels mounted along the body axes of the satellite.

The magnetic coils are designed for damping the satellite's rotation by interacting with the Earth's magnetic field. For this purpose, knowledge of the satellite's attitude is not mandatory.

The reaction wheels are used to point the satellite towards a certain target. For this purpose, several closed-control loops are used: a damping mode, to reduce the satellite's rotation to zero, the slew-mode, to perform large angle maneuvers along the shortest available track, and finally, the fine-pointing mode that maintains the orientation of the satellite towards a specific target with high accuracy.

It is possible to point the satellite to either a predefined inertial target (inertial-pointing) or downwards to the center of the Earth (nadir-pointing).

The reaction wheel system has its own electronics for driving the brushless motors. With a 10Hz control loop cycle frequency, either angular momentum (via rotation) of the wheels or resulting torque (via acceleration) can finely be controlled. Both the magnetic coils system as well as the micro wheels system can be operated manually by the ground personnel for testing.

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