Lesson Learned in the Operation of a CubeSat ADCS in the Presence of Sensor Anomaly

Small satellites and CubeSats often employ commercial off-the-shelf (COTS) components to achieve cost effectiveness, but these components are prone to anomalies that may jeopardize mission success. It is thus desired to learn from operational experience for continuous improvement. In the paper, lessons learned from the operation of the Lilium-1 CubeSat attitude determination and control subsystem (ADCS) are discussed. Lilium-1 is a 3U CubeSat designed and operated by the NCKU team that was launched in 2023. It has essentially accomplished its missions on S-band communication, space IoT, and Earth observation. The ADCS consists of three-axis gyroscope, three-axis magnetometer, fine Sun sensor, and Earth infrared sensor for attitude estimation. In late 2025, the magnetometer experienced complete signal loss, leading to the loss of attitude control. Following this failure, even the use of a Kalman filter with the remaining sensors proved insufficient for reliable operation. To address this anomaly, a ground-based recovery strategy is proposed. An SGP4 orbit propagator combined with IGRF data is used on a ground computer to calculate local magnetic field vectors at a target orbital location T and n preceding points, where T and n stand for the look-ahead (or predicted) locations and time epochs. These vectors are converted into hard-coded magnetorquer command sequences such that the equilibrium zero-torque attitude aligns the magnetorquers with Earth’s magnetic field at each step. The commands, each associated with a specific execution time, are uploaded to the onboard computer (OBC) to perform an increasingly accurate step-wise attitude alignment, culminating at T. The proposed methodology and its limitations are characterized through testing with a different number of n points and time spacing; attitude is confirmed through images taken at T. The results indicate that by-passing onboard attitude estimation by ground-based prediction appears to be able to recover the satellite from catastrophic magnetometer loss.