ON-ORBIT RESOLUTION OF RADIATION INDUCED LOSS-OF-TRACK ANOMALIES FOR THE CT601 STAR TRACKER

Due to concern over frequent loss-of-track events common to several near-Earth NASA spacecraft employing a popular charge-coupled device (CCD) star tracker, an on-orbit experiment was conducted to determine and then correct its root cause. Similar to CCD trackers from other manufacturers, the Ball Aerospace CT601 star tracker has consistently experienced upsets while operating in both the polar regions during active solar periods and the Earth's trapped radiation belts. Using NASA's Wide-field Infrared Explorer (WIRE) spacecraft, high frequency raw pixel data from the CT601 tracker was obtained during multiple loss-of-track events occurring over a four-week period. Experimental data indicated that loss-of-track events were precipitated by the impact of high-energy particles at the start of a read cycle on the pixel region used for a bias offset calculation. A corrective software patch was then written and uploaded to the spacecraft. The patch delays the bias offset update for one read cycle if new and past values are inconsistent. Additional high rate raw pixel data was then collected over several weeks. Tracker anomalies were eliminated, even though the data showed the occurrence of several high-energy particle impact events. The patch has since been implemented successfully on several other NASA missions.