The Mars Climate Sounder In-Flight Positioning Anomaly  Bruno M. Jau* and David Kass*  Abstract The paper discusses the Mars Climate Sounder (MCS) instrument’s in-flight positioning errors and  presents background material about it. A short overview of the instrument’s science objectives and data acquisition techniques is provided. The brief mechanical description familiarizes the reader with the MCS  instrument. Several key items of the flight qualification program, which had a rigorous joint drive test program but some limitations in overall system testing, are discussed. Implications this might have had for the flight anomaly, which began after several months of flawless space operation, are mentioned. The detection, interpretation, and instrument response to the errors is discussed. The anomaly prompted  engineering reviews, renewed ground, and some in-flight testing. A summary of these events, including a  timeline, is included. Several items of concern were uncovered during the anomaly investigation, the root  cause, however, was never found. The instrument is now used with two operational constraints that work  around the anomaly. It continues science gathering at an only slightly diminished pace that will yield approximately 90% of the originally intended science. Introduction The MCS instrument is one of six scientific instruments flying aboard the Mars Reconnaissance Orbiter  (MRO) spacecraft, launched Aug. 12, 2005. The instrument’s purpose is to acquire data about the Martian weather and climate. Science observations started Sept. 24, 2006. The duration of mapping  operations is scheduled for two earth years but it is hoped that MCS can continue to acquire data for an additional two years. Current information about the MRO mission and this instrument can be found in JPL’s [1] and the Planetary Society’s [2, 3] web pages. A nearly identical instrument, performing surface science investigations around the Moon, will be launched onboard the Lunar Reconnaissance Orbiter Spacecraft in October of 2008. On Dec. 11, 2006, about a day after a solar flare near Mars, the instrument’s elevation drive started to  miss its intended positioning by a few steps several times. The flight software reacts by automatically executing a saving routine after several positioning errors, stowing the instrument in a sun-safe position. Initial evaluations revealed no obvious cause; the position errors were random and happened a few minutes or many days apart. Up to now, over 200 positioning errors have occurred; all with the elevation drive. Scan image evaluations confirmed that most errors were four steps away from the commanded position. A tiger team was tasked to review all software, electronics, and mechanical aspects, to hopefully find the cause of the anomaly and to suggest remedies. This paper gives an account of the MCS in-flight anomaly and its investigations, provides technical background material about it, and lists possible causes. Lessons learned are included for future space instrument builders. The paper should also be of interest for the general public that likes to learn about the intricacy of space science. Science Objectives and Data Acquisition Techniques MCS is an infrared (IR) band radiometer, primarily performing atmospheric measurements by measuring  the radiance profile versus altitude at the limb (or horizon). One minor component of the science  objectives also involves measurements of the surface for thermal and radiative balance calculations [4]. The radiances are inverted via a retrieval process to obtain profiles of temperature, dust, clouds, and humidity versus altitude. The individual profiles are then combined in 4-dimensional “maps” (covering  space and time for each measured quantity) to examine the weather and climate of Mars. * Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA  Proceedings of the 39th Aerospace Mechanisms Symposium, NASA Marshall Space Flight  Center, May 7-9, 2008  271