both actuators (as well as the life test actuator). Keystoning, if it manages to create position errors, would  most likely produce large errors (stalling), not 4-step errors, and would never create overshoots.  - Excessive Friction/Debris:  Excessive mechanical Impedance in transmission or driven load, specific to  the flight elevation system, is a possibility. Its origin might be foreign particles/debris in the actuator, parts  rubbing each other due to positional shifts, a somewhat loose thermal blanket, workmanship issues, etc.  It could explain some positional dependence and subsequent disappearance of the errors with the higher  current tables now in place. However, it is difficult to imagine how this failure mode could generate the  short four-step nature of most errors, nor would it produce overshoots.  - Drive Electronics Parts Intermittent Failures : It is possible that the errors are due to a (partial) failure of  an electronics part of the elevation motor driver. This might be due to the solar flare. But the motor driver is completely position insensitive. Besides, none of the known failure modes of any electronics part used has a history of intermittent behavior, nor would it self-correct over time.- Controller-Motor Interactions:  The errors could be caused by a subtle interplay between the waveforms  generated by the motor controller software, the interpretation of them by the motor drive electronics and the momentum, motion, and normal phase lag of the rotor. Testing and reviews were unable to identify any specific categories of interactions (e.g. resonances) that might produce this behavior, but due to the complex and poorly modeled interfaces involved, there might be an interaction that was not noticed or did not exist with the test equipment, that is slightly dissimilar to the flight hardware. This type of phenomena might be expected to affect both actuators as well. This scenario has no obvious mechanism for generating position dependent errors either. It is also not obvious why it would suddenly appear after several months of perfect operations, get significantly worse, then disappear again.  References [1] http://mars.jpl.nasa.gov/mro/  [2] http://www.planetary.org/programs/projects/mars_exploration/mars_climate_sounder/[3] http://www.planetary.org/programs/projects/mars_climate_sounder/objectives.html [4] McCleese, D. J., J. T. Schofield, F. W. Taylor, S. B. Calcutt, M. C. Foote, D. M. Kass, C. B. Leovy, D.  A. Paige, P. L. Read, and R. W. Zurek (2007), Mars Climate Sounder: An investigation of thermal and  water vapor structure, dust and condensate distributions in the atmosphere, and energy balance of  theolar regions. J. Geophys. Res., 112, E05S06, doi:10.1029/2006JE002790.  [5] Pennzane is a product of Nye Lubricants Inc., Fairhaven, MA.  [6] Jau, B. M., "Mechanical Description of the Mars Climate Sounder Instrument." IEEE Aerospace  Conference, Big Ski, Montana, March 1-8, 2008.  [7] The Joint Drives were supplied by Starsys Inc., Louisville, CO. [8] Bearings supplied by Timken, Keene, NH.  Picture Credit  Figures 6 and 7: Courtesy of Starsys Inc.  Acknowledgement The research described in this paper was carried out at the Jet Propulsion Laboratory, California Institute  of Technology, under a contract with the National Aeronautics and Space Administration.  288