Earth-pointing and polar-orbiting spacecraft, the Advanced Land Observing Satellite (ALOS), has a precision attitude determination system exploiting measurements of a precision star tracker and an inertial reference unit, and controls its attitude, based on the attitude estimates. The star tracker provides the positions and magnitudes of stars. The attitude control system's computer identifies stars which enter the star tracker's FOV and move across it, and applies the extended Kalman filter. The ALOS precision attitude determination system consisting of the star tracker and the onboard computer was tested at the Usuda Deep Space Center in the real night sky environment. The tests using a two-axis motion table demonstrated end-to-end performance, capability, and robustness of the attitude determination system. This paper presents the overview and results of the real sky tests. It also proposes test settings for Earth-pointing spacecraft and a method for alignment error assessment and calibration. Limit of performance due to the test site and configuration are also evaluated.
