3-Axis Attitude Determination and Control of the AeroCube-4 CubeSats

On 13 September 2012, three identical 10x10x10 cm (1U) AeroCube-4 satellites built by The Aerospace Corporation were ejected together as a cluster from the NROL-36 Atlas V vehicle along with 8 other CubeSats. Each identical AeroCube-4 is a fully functional satellite with three axis attitude control, likely the smallest ever with such capability. Attitude determination and control hardware flown on the previous PicoSatellite Solar Cell Testbed-2 (PSSCT-2) was enhanced and integrated with new processing and control algorithms. The sensors and actuators for this mission included two sun sensors, an Earth nadir sensor suite, three magnetic torque coils, a two-axis magnetometer and three miniature reaction wheels. Attitude determination is performed using Earth sensor, Sun sensor, and magnetometer. Spacecraft position, Sun position, Earth magnetic field, and commanded attitude quaternion profiles are calculated on the ground and uploaded to the onboard processor as a set of polynomials which are evaluated as a function of time. The calculated Earth, Sun, and magnetic field vectors in ECI coordinates are compared to measured Earth, Sun, and magnetic field vectors in body coordinates to determine ECI to body attitude. After initialization, attitude determination propagates attitude assuming constant rate, and updates attitude using gains from a fixed covariance filter. The commanded attitude and the estimated attitude are used by the control algorithm which calculates reaction wheel rate commands. A momentum control algorithm uses torque coils to dump momentum (accumulated due to magnetic dipole and atmospheric drag) to keep reaction wheels from saturating. A line of sight pointing requirement of 3.0 Degrees 3-sigma was verified by collecting images from an onboard digital camera. Both stellar and ground pointing targets were used in the verification. This report will discuss the attitude sensor processing, attitude control laws and line of sight pointing accuracy verification techniques