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The Advanced Land Observing Satellite "Daichi" (ALOS) was successfully injected into its mission orbit on March 26, 2006. ALOS' missions require the orbit to be precisely recurrent regarding both the ground track and altitude. Therefore, the ALOS mission orbit is a sun-synchronous sub-recurrent frozen orbit. A sun-synchronous orbit has to be maintained by compensating inclination change caused by lunisolar gravity perturbation. In the ALOS case, inclination adjustment is planned every two and a half years. The inclination adjustment should be done by out-of-plane maneuvers. During the inclination adjustment, it is also required that the ground track and altitude error should be minimized. However, it is difficult because there are some restrictions on the out-of-plane maneuvers; such as necessity of attitude change maneuver, extra in-plane accelerations by the attitude change maneuver. JAXA planned and performed the inclination adjustment which kept the ground track and altitude error as small as possible by estimating extra in-plane accelerations and exploiting rotation of eccentricity vector caused by gravity perturbations. In this paper, we review the ALOS requirements for orbit keeping, and discuss the challenges of ALOS inclination adjustment maneuvers due to the restrictions. Our strategies and the theoretical background are also presented with subsequent evaluation of flight results.

Inclination Adjustment Maneuver and Frozen Orbit Keeping of The Advanced Land Observing Satellite (ALOS)