Stellar Reference Units
Attitude determination is achieved in the Attitude and Articulation Control Subsystem using input from three Inertial Reference Units (IRUSs) and from one of two Stellar Reference Units (SRU), also called star trackers. The inertial reference units use solid-state gyroscopes developed by the Delco Division of the Hughes Aircraft Company. The stellar reference unit essentially navigates by the stars. It detects stars in its field of view and it compares the view with its onboard catalog of 5,000 stars.
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Attitude Control in the Basics of Space Flight