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The Instrument Pointing System is a versatile three axis gimbal system providing pointing capability in the arc-second range to any type of experiment, requiring precision orientation towards celestial objects. Aside from this prime objective, the system is designed to execute automated acquisition procedures and to follow a pre-defined trajectory, e.g. for landmark tracking. A summary description of the IPS mechanical and electrical hardware configuration with special considerations of the optical sensor package (OSP) consisting of three Fixed Head Star/Sun Tracker (FHST) is given, followed by a presentation of the IPS operational modes, the control loop design, and functional modes and performance data of a FHST. The further chapters of this paper deal mainly with the inertial attitude acquisition of the IPS strapdown system, i.e. the initial attitude acquisition and the operational star identification. The method used for star identification realized in an IPS flight software application program and the selection criteria of candidate stars for the IPS star catalogue are discussed. The operations necessary for initial attitude acquisition as control unit switch on sequence, IPS deployment, slewing to celestial objective direction, and the subsequent attitude acquisition with the related software functions are presented. The approach of the initial attitude achievement using unique bright stars for line of sight (LOS) and roll attitude acquisition is described in detail together with back up methods. Finally the present tracking capabilities of the system are outlined and methods for improvements are indicated.
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