Sun direction determination improvement by albedo input estimation combining photodiodes and magnetometer

One common sensor used for attitude determination in CubeSats is the cosine-type photodiode Sun sensor. By placing one on each of the six sides, it is possible to have full coverage of the surroundings and determine the direction to the Sun. One flaw is that it is not possible to discriminate between different light sources. As such the Earth's albedo can negatively influence the readings. After reviewing some of existing solutions to mitigate the albedo effect and comparing their accuracies, a new method is proposed to determine the albedo inputs in the Sun sensor measurements, by taking advantage of another sensor typically employed in CubeSats: the three-axis magnetometer. With this method it becomes possible to subtract the albedo contribution to the Sun sensor measurements, greatly improving the determination of the Sun direction. A simplified version of the algorithm is discussed, useful for real-time implementation on-board CubeSats. The method is then validated using flight data of the 1KUNS-PF CubeSat, in orbit since May 2018. The results confirm the proposed method performance and the considerable improvement in estimation of the Sun direction, showing a very good correlation of the on-board estimated albedo inputs and daily cloud coverage data.