Line-of-sight Jitter Of SELEX Optical Terminals,Analysis And Verification Activities

Abstract

The Semi-Conductor Laser Inter-Satellite Link Experiment (SILEX) is a very innovative program of the European Space Agency. Two optical terminals, PASTEL and OPALE, respectively on the SPOT4 satellite (low Earth orbit) developed by MATRA MARCONI SPACE under CNES contract, and on the ARTEMIS satellite (geostationary) developed by ALENIA under ESA contract, must be pointed to each other to within 2 microradians in the communication phase. To achieve this very high accuracy pointing, the Pointing, Acquisition and Tracking (PAT) system of each terminal provides high bandwidth (above 100 Hz), fine control capabilities so as to decrease the pointing jitter induced by host spacecraft disturbances. In this frequency band, the spacecraft and the optical terminal are no more rigid bodies, and the coupling of structural modes is a performance driver that requires detailed analysis and verification tests. The aim of the paper is to describe the activities carried out at MATRA MARCONI SPACE to predict and verify on the ground the high frequency line-of-sight pointing performances of the SILEX terminals, essentially focusing on the case of PASTEL. Due to the complexity of the problem (flexible bodies in a high frequency range, large angle azimuth/elevation motions of PASTEL, PAT control loop, on-board disturbing environment...), the pointing verification logic was built on a combined tests/analyses approach. A series of modal tests have been performed at terminal level, completed by a microvibration test on the SPOT4/PASTEL assembly. Their exploitation has led to Finite Element Model update, and definition of worst case transmissibility envelopes. In addition, the detailed characterisation of the main on-board disturbance sources (SPOT4 Reaction Wheels and Oblique Viewing Mirror mechanism) has provided the Transactions on the Built Environment vol 19, © 1996 WIT Press, www.witpress.com, ISSN 1743-3509