99 A Coarse Pointing Assembly for Optical Communication 

G. Székely1, D. Blum*, M. Humphries*, A. Koller*, D. Mussett*, S. Schuler* and P. Vogt *  

Abstract 

In the framework of a contract with the European Space Agency, RUAG Space are developing a Coarse Pointing Assembly for an Optical Communication Terminal with the goal to enable high-bandwidth data exchange between GEO and/or LEO satellites as well as to earth-bound ground stations. This paper  describes some development and testing aspects of su ch a high precision opto-mechanical device, with  emphasis on the influence of requirements on the final  design, the usage of a Bearing Active Preload  System, some of the lessons learned on the BAPS implem entation, the selection of a flex print design as  rotary harness and some aspects of functional and environmental testing.   Introduction 

In the framework of the Europ ean Space Agency’s (ESA) Artes V long-term technology development  initiative, RUAG Space is developing a Coarse Po inting Assembly (CPA) for an Optical Communication  Terminal.  The main requirements for the CPA can be summarized as follows:   • full hemispherical pointing range  • high absolute pointing accuracy: ± 270 µrad under TV conditions  • very low jitter: < 2 µrad RMS  • high reflectivity, minimal wave front distortion, as  well as maximum stray light suppression of the  optical components in the CPA  • challenging environmental requirements in the form of  o high accelerations during take-off on various launchers  o very large temperature ranges derived fr om a broad spectrum of thermal load cases  o optics and electronics com ponents sustaining a tough radiation environment during 15  years in-orbit life   Full hemispherical pointing is achieved by a tw o-axis azimuth/elevatio n design – a commonly  encountered solution for such pointing devices. Arranged  at a 90° angle to each other, each actuator  stage carries a flat mirror mounted at 45° with respect to  its axis of rotation. With this arrangement, each  mirror deflects the optical beam by 90° effectiv ely resulting in the required pointing range.   From the very beginning, it was clear that the jitte r and pointing performance could only be realized by  minimizing friction and stick-slip effects. Beside s utilizing ultra-precision bearings and brushless DC  motors operated in closed-loop control with a high-re solution optical encoder, the CPA’s actuator stage  performance was pivota lly enhanced by integrat ing a Bearing Active Preload System (BAPS).   The BAPS offers on one hand a high and stiff preload state for launch, providing high tolerance to the  above mentioned challenging launch lo ads. On the other hand, the BAPS c an be actively transitioned to a  low and soft bearing preload state for in-orbit opera tion, enabling smooth, low-jitter movements during  beam tracking, as well as allowing for relatively la rge thermal gradients across the bearings, which is  especially important for GEO applications, where the interior of the CPA might be exposed to the sun.  Finally, minimization of friction was tackled by implem enting the harness from the azimuth to the elevation                                                    1 RUAG Space AG, Zürich, Switzerland 

Proceedings of the 40th Aerospace Mechanisms Symposium, NASA Kennedy Space Center, May 12-14, 2010  NASA/CP-2010-216272