of the actuator, then they  would soon be caught by the 0-diode and corrected by the re-initialization. After approximately 6 hours of scanning, 4 position errors had been detected by the 0-diode.  Initial Diagnosis Tree and  Reduced Scanning A “problem scan”  diagnosis tree was developed to identify the slew or slews causing the errors. It was also  designed to determine if  there were operational workarounds for the position errors. The first  step was to use a reduced set of scanning (Fig. 9) that only involved using a blackbody scan and a limb scan, both limited to 10° below the limb. For sun safety reasons, freeze commands were still allowed to cause occasional scanning up to 35° below the limb. This scan pattern was designed to rule out one of the three candidate scans. Additional steps were designed to differentiate between the other two scans.                            Figure 9: Range of Reduced Scanning The reduced scanning was started on Dec. 22; it ran over the holidays without producing any errors. The  next steps of the tree were started on Jan. 2, but still failed to produce additional position errors.  Concerned that the results of the steps in the diagnosis tree were misleading the full scanning pattern  was tested on Jan. 4. This did not generate any position errors until the morning of Jan. 16. Initial Ground Analysis In the intervening time, a number of analyses and ground tests were performed. Testing involved using  the MCS life-test actuator and the EM micro stepper controller to attempt to generate position errors. Unfortunately, this controller deviates from the flight controller in a number of ways, shedding some doubt about the meaningfulness of the test results. The initial effort focused on reduced bus voltages (showing a very sharp transition from scanning without errors to not scanning). Attempts to corrupt the scan control software to introduce position errors met with limited success. Failing to execute a slew step at high speed would produce a 4-step position error. Reviews of the software design indicated that short of very targeted and special corruption, there were no ways to mis-command the actuator. And if such corruption existed, position errors would occur on all scans longer than 16°. Reviews of the electronics revealed that much of the firmware and hardware was shared by the two actuators and none of the separate components had known intermittent failure modes. The mechanical review also revealed no obvious causes or mechanisms for generating position errors. During this time the detailed analysis necessary to detect small position errors on the limb was also developed.  Initial January Response and Frequency Study The next step on the instrument was to repeat the tests from December to see if they made the errors  (temporarily) disappear again. In doing this, an attempt was made to isolate the part of the process that  had eliminated the errors. The first new scanning, on Jan. 17 involved overwriting parts of MCS random access memory (RAM) to eliminate any corruption. This did not eliminate position errors. A second step  taken on Jan.18 was to return to the reduced scanning. Again, position errors occurred. In these two  tests, the error frequency seemed to have increased from 1/hour of scanning to 2/hour, although the time between errors was still highly variable.  281