Attitude Determination & Control

The purpose of the Attitude Determination and Control (ADC) system is to determine the attitude of the satellite in reference to the Earth and correct any deviation from the expected attitude. The ADC can be viewed as two separate subsystems where one subsystem determines the attitude of the satellite in reference to Earth and the other subsystem corrects deviatons from the expected attitude.

                       The ADC board resting on one of the magnetourqer coils

The requirement for the ADC system is governed by the need to avoid wake effects onto the payload sensor probes. It has been determined that an attitude control with less than +/-10 degrees deviation along the pitch and yaw axis is sufficient to perform undisturbed scientific measurements.

Sensors
In order to be able to determine the attitude of the satellite, sensors are needed to sense the orientation of the satellite. The attitude determination system collects the position of the sun, the magnetic field from the Earth and the rotation of the satellite in all axes. The system uses sun sensors, gyroscopes (3 axes) and magnetometers (3 axes).

Detumbling
The detumbling is the process of stabilizing the angular rate of the satellite after orbital insertion. The system uses a B-dot algorithm to detumble. The principle of the B-dot algorithm is to reduce and minimize the derivative of the angular rate. The B-dot controller uses magnetometers to derive the angular rates and manipulate the actuator coils to reduce the angular rate.

Attitude Determination
Attitude determination is a meticulous process requiring more complex algorithms and processing power than is necessary for detumbling.
Attitude determination is performed using a simplified Kalman filter to process the sensor outputs using a high speed controller to perform the calculations. 

Attitude Control
The attitude control is performed using actuator coils. Three coils (magnetorquers) are used to control the attitude, one for each axis. The magnetorquers generates a magnetic field that interacts with the Earths magnetic field and creates a force that is used to stabilize the satellite.