Thursday 9 September 1999 2.00pm EST
|Two related events occurred during the last 24 hours that lead to an interruption and re-start of the load.
During observation of an offset pointing at PKS0312-770 an anomalous dither pattern was noticed during a real-time pass. This was traced to the aspect camera locking-on and tracking a fiducial light rather than a star in a guide star window. An increased roll error is thought to have been induced during this pointing and when the next slew occurred, the error was translated into a pitch error sufficiently large to result in the aspect camera failing to lock on the new stars. This resulted in the spacecraft going to bright star hold and stopping the load. The flight software performed these functions nominally.
The bright star hold was detected at 6.30pm and following an assessment, a plan was developed to safe the science instruments prior to radiation belt entry, re-determine attitude and perform a fine attitude update. The plan was executed and completed successfully at 9.30pm.
The mission planning team performed a re-plan for the current load starting upon radiation belt exit at 6.30am 9/9 and the load began operation at approx 7am. The spacecraft was commanded to normal maneuver mode for 2 hours prior to load start in order to meet the constraint for angle to bright Earth limb. The load was operating as expected when the spacecraft came into communication at 1:45pm 9/9/. The loss of time due to the event was approx 10 hours since no science was planned for the radiation belt passage.
The root cause of the initial fid-light/star confusion by the aspect camera has been traced to an error in the ground software. A software patch is in work and expected by the weekend.
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