major jump in error rate occurred when the first large position error occurred. Note that even at the most  rapid, the average was still over 20 slews between errors. One interpretation of the two tests (in addition to eliminating a large number of systems from the fault  tree) was that the elevation actuator was rapidly degrading and would soon be inoperable. The final error  rate was also significantly degrading the ability to perform good science observations. And the large  pointing errors introduced a sun safety issue. This led to the formation of an anomaly tiger team. At this  time, Starsys [7] was actively brought into the investigation. Limb Staring and Ground Testing It was not know if the progressive degradation was due to use or just due to aging. Given the fact that  staring at the blackbody provided no science, it was decided to move once (Feb. 9) to stare at the limb  and avoid using the elevation actuator until the anomaly investigation could perform a thorough analysis. The theory was that if the actuator were to remain jammed in that orientation, it provided the maximum science return (estimated to be about 70% of the planned science).  One series of key tests was a set of torque measurements and controller waveform sampling. These  were performed with the EM controller with either the life test actuator or a test motor. Testing revealed that the micro stepper controller using the flight parameters generated significantly less torque than expected. This system no longer met (and did not come close to meeting) the design requirement of a factor of 2 for torque margin at the lower end of the 26 V to 36 V S/C voltage range. There were a number of other peculiarities in the power waveforms supplied to the actuator, mostly related to limitations of the motor controller processor.  Most of the torque reduction was traced to a miscommunication over the back-EMF (electromechanical  force) constant to be used for the flight motor winding configuration. A new parameter was determined by analysis and then refined experimentally with the life test actuator. With an increased back-EMF constant, effectively supplying more current, the motor torques were still marginal, but at least now met design requirements at the nominal operating voltage of 32.5V.  In late March, MCS was power cycled due to an MRO safe mode entry. After the power cycle, MCS  returned to limb staring. This removed corruption concerns and a number of other very unlikely items  from the fault tree.  The in depth investigation determined there were no plausible catastrophic failures on the fault tree. The  investigation also developed fixes for a number of the torque and slewing issues (although not all could be fixed, some were built into the way the MCS FSW controlled the motors). But the investigation was not able to point to a root cause, or even a plausible root cause. Continued Flight Actuator Testing It was decided to resume using the elevation actuator in late May, incorporating the changes in the motor  control parameters to increase the instrument torque and clean up the waveform to the extent possible.  The scanning was further reduced to only involve the blackbody (but only twice per hour), space views  and the limb. The first four blackbodies were followed by a limb scan that had a position error, ending the test. Due to the simplicity of the scanning, the limb radiance analysis indicated all four errors appeared to be overshoots (the only other scenarios involve complex inaccuracies of the 105° diode system and scans ending exactly the “right” distance short).  The instrument returned to limb staring at this point. An evaluation of the fault tree in light of overshoots  focused on actuator resonances (the only fault on both the overshoot and undershoot branches). The blackbody pre-conditioning of all errors (as opposed to January when errors were so frequent, many were occurring without the pre-conditioning) also led to considerations of slew length. The next test (on June 14) reduced the top speed of the actuators to avoid any resonance effects. The change to the top speed  did not appear to have any effect; three errors occurred within 30 minutes and the instrument stowed. Following the end of the previous test, scanning was resumed, but only between the limb and space (~8°  total slew length). This was an attempt to see if eliminating the blackbody pre-conditioning improved the  283