Push-broom Earth Scanner (ea, EGPMl  The now postponed EGPM mission requires an earth-scanner using the push broom scan approach. The  main instrument rotor and reflector (mass >70 kg) rotate at a constant speed up to 32 revs per minute  over a nominal 15 year mission, resulting in >250 million revolutions. The instrument rotor must be  supported rigidly during launch and rotated about a precise axis in-flight.  The main issue is related to the high instrument rotor mass and the relatively large C of G offset of the  instrument rotor in relation to the scan mechanism. This leads to combination of high lateral and axial  forces and very high moment loading during launch. Therefore the current instrument configuration  incorporates a dedicated hold-down system that supports the rotor at its outer extremities at four locations  for launch. These are released once on-orbit and then the instrument rotor is supported purely by the scan  mechanism. While the primary launch protection is able to rigidly secure the bulk instrument structure, due  to the size and supported mass of the instrument equipments mounted within the instrument there are  also local panel modes between the scan mechanism interfaces with the instrument lower panel and the  spacecraft side wall. It is not possible to accommodate these modes by allowing relative internal  movements within the mechanism due to a number of critical internal operating gaps (e.g. at the motors,  sensors and power/signal transfer devices) Therefore the scan mechanism must react the local forces  and moments associated with constraining these local modes, which leads to significant local forces and  moments across the mechanism. The bearings of the scan mechanism are required to have a bore of  greater than 100 mm due to internal design constraints and so in principle the mechanism is capable of  reacting significant loads. The use of BAPS technology allows the bearings to react these launch loads  without introducing any significant internal movements and provides a relatively stiff fifth hold down point  at a location where it is most mass effective. This is done without compromising the mechanism’s on orbit  performance, in particular the long life and the critical on orbit nodding frequency.  Note that although a bi-stable BAPS is currently base-lined, there would be some benefits in this  application in the use of a fully variable BAPS that would enable on-orbit tuning of instrument rotor preload  in order to achieve an ideal rotor stiffness.  Conclusions  This paper demonstrates that the novel BAPS device can be used in a wide variety of the more  demanding spacecraft mechanism applications to improve both system level and mechanism  performance and lifetime. The examples used demonstrate that for maximum benefit, the BAPS concept  needs to be incorporated within the design from an early stage in the system development.  The BAPS is currently at the pre-Qualification stage of development, but it is expected that it will be  Qualified during 2006.  References  1. Roberts E.W. (Ed.) ESTL Space Tribology Handbook  2. Performance Guide - Self-lubricating Bearings Prepared by NCT, June 1976  3. Anderson M.J. “Duroid Replacement Material Requirements Specification” €STL/TM/20 (July 1998)  4. Lewis S.D. “GERB Phase 3b Post-Test Review” (Unpublished Experimental Work) ESTL (2000)  5. Stevens K.T. & Todd M.J. “Parametric Study of Solid Lubricant Composites as Ball Bearing Cages”  Tribology International Oct. 1 982 (pp293-302)  6. Felici F. “The Giotto Mechanisms and Their Functions” Roc. Is‘€SMATS, Neuchatel 7983, ESA-SP-  196  140