Magnetometer calibration in the presence of hard magnetic torquers

Many CubeSats use magnetic torquers for attitude control. Many of these same CubeSats use a magnetometer as the primary sensor for attitude determination. This results in a conflict where the actuators in the system corrupt the feedback measurements. Two common approaches to solve this problem are to physically separate the torquers and magnetometer by putting the magnetometer on a boom, or to separate the sensing and torquing in time so that the torquers are not energized while the sensors are sensing. The Alaska Research CubeSat (ARC) is a student-built CubeSat that is being built at the University of Alaska Fairbanks. One of the missions of ARC is to test an Attitude Control and Determination System (ACDS) that uses Low Power Magnetic Torquers (LPMTs), which use a hard magnetic core instead of an air or soft magnetic core. The hard magnetic core is magnetized by pulsing current through the coil that surrounds the core. Each torquer core can be magnetized in two opposite directions along the axis of the torquer giving it two discrete states. Both states of the torquers generate a stored magnetic field, which means that the time multiplexing strategy can't be used. Due to the small size of CubeSats a deployable boom is not desirable because of the added space and complexity. Because the torquers on the satellite take on a small number of discrete states their effects can be removed from the magnetometer measurement if the torquer states are known. This paper describes the algorithm and calibration process that will remove the effects of the torquers for the ACDS on the ARC.