© 2018 Univelt Inc. All rights reserved.JAXA’s ERG (Exploration of Energization and Radiation in Geospace) Spacecraft, which is nicknamed Arase, was launched on 20 December 2016. Arase is a spin-stabilized and Sun-oriented spacecraft. Its mission is exploring how relativistic electrons in the radiation belts are generated during space storms. Two different on-ground attitude determination algorithms have been designed for the mission: a conventional straightforward algorithm that inherits from old missions and an advanced new algorithm. This paper discusses the design of the advanced attitude determination algorithm and presents the preliminary attitude estimation results for the spacecraft that were obtained after the launch. Results are presented along with the encountered challenges and suggested solutions.