Meeting science requirements for attitude determination and control in a low-power, spinning nanosatellite

This paper describes the attitude determination and control system for a nanosatellite (30 kg), using geomagnetic field data and solar panels as sun sensors, applied to a spinning nanosatellite (Penn State University's LionSat). LionSat will map plasma densities in the ram and wake of the vehicle's path, using two hybrid plasma probes that rotate with the spacecraft. Attitude will be determined using a combination of voltage outputs from contiguous solar panels and a three-axis magnetometer (TAM). To correct for spin-axis drift due to orbital plane nodal regression (approximately five degrees per day), two magnetic torque rods will provide the necessary control actuation. To avoid corrupting the TAM and plasma probe measurements, periodic quiescent periods for the torque rods are required. Initial design concepts for this mission employed an extended Kalman filter (EKF, implemented onboard) to predict attitude during the passive control intervals; making use of digital sun sensors as part of the attitude determination scheme; however, budget limitations have resulted in the need to employ the body-mounted solar panels (sides and end caps) to estimate the sun vector. Simulation results indicate that the solar-panel sun sensors (in conjunction with the TAM) can produce the required attitude knowledge for processing the scientific data; the pointing accuracy for spin-axis control (nominally within 5 degrees of orbit-normal) is degraded to 10 degrees accuracy, but this is still within acceptable limits for the science.