based on spacecraft (S/C) pointing and assuming the pointing is correct. The red and green families are  the two measurements before the error occurred. The light blue profiles are after the pointing error and the brown family is after the actuator was re-initialized. The most accurate determination is done where the radiance is changing the fastest with height. In this case, the 50 to 60 km region was used. It is obvious that the light blue family is mis-pointed. The dotted horizontal line indicates detectors that should be at the same altitude and have the same radiance. The vertical dotted line shows the actual family of detectors that have the same radiance, and are pointed at the same altitude. The two detectors between the actual and expected alignments indicate a pointing error of two detectors. Since each detector is approximately 0.2° tall (or 5 km on the limb), this is a pointing error with the actuator pointing 0.4° (equivalent to 4 steps).  This provides additional information on the timing of many position errors by indicating that the actuator  was already mis-pointed while observing the limb, thus reducing the number of candidate scans for the  error. The limb view radiances also sometimes (as in Fig. 8) provide knowledge of the magnitude and  direction of the error. Based on experience, we have found that for small errors (< 1°), looking at the limb radiance provides a pointing accuracy of one actuator step (0.101°).  There are cases (especially over the poles) when the atmosphere is changing too fast with latitude and it  is not possible to accurately measure the pointing error. The profiles are too different to determine which detectors are actually pointing at the same location. In many of these cases, it is still fairly obvious that the actuator is mis-pointed. There are also cases where there are no limb views during the scanning sequence before the error is fixed by re-initializing the actuator. In these cases, the size of the position error cannot be determined. Actuator Anomaly Timeline  Initial Flight Errors MCS started primary science phase operations on Sept. 24, 2006. After an initial flurry of scanning  changes, routine operations settled in. On the evening of Dec. 11, 2006, MCS FSW detected a position error and re-initialized the actuators. Over the next 4 hours, three additional position errors occurred and caused the instrument fault protection to stow the instrument as designed. This occurred at ~5% of the tested life of the actuator.  Over the next two days, an analysis of the telemetry during the position errors showed no other signs of a  problem. The initial study of the science observations by the team indicated that there were no large  pointing errors in the data. The first four errors were very intermittent and all showed preconditioning by  the black body viewing (see below). The last instrument ground commanding prior to the error (a routine health status check) had been 3 days before. The last change to the scanning had been 11 days before. All of the commanding was verified to be correct and not the cause of the position errors.  Initial Response and Diode System Testing An initial fault tree was defined. The most likely candidate on the tree was the elevation 105°-diode  system (since this diode had reported all four errors). The other early candidate was the occurrence of a solar particle event about a day before. Three initial actions were taken: Another health status check was performed (it was unchanged). Second, on Dec. 14, MCS fault protection was initialized with a count of 6 position errors (stow on the 7 th error) and scanning resumed. This was to see if the errors were transient.  And if not, the scanning would hopefully provide additional information on the actuator health. After 7 hours of scanning, MCS FSW had detected 7 more position errors and again stowed the instrument. This scanning did show that the blackbody preconditioning (see below) was not required for an error to occur. It also indicated that errors could occur in sequences other than the limb scan.  In order to test that the problem was the 105°-diode system, a quick patch was developed to disable its  use by the FSW. On Dec. 19, this was ready, installed, and scanning resumed with the position error  counter set back to its default value of three. The expectation was that this would stop the FSW from  reporting position errors, allowing the investigation to focus on the 105°-diode system while the instrument continued to collect science observations. If the position errors were due to a real mis-pointing  280