Optimized Spacecraft Fault Protection fortheWISE

TheWISEproject isaNASA-funded medium- class Explorer mission tomaptheentire skyinfour infrared bandsduring thecourse ofa6-month survey. Because of themission's limited financial resources, a traditional robustness strategy offullblock-redundancy wasnot feasible. Byleveraging aspects ofthemission design that tendtoreduce therisk associated withcertain failures, the project hasbeenabletoadoptarobustness strategy of mitigating high-risk failures, whileaccepting theriskof low-impact faults orunlikely faults inheritage equipment withproven reliability. Theresulting WISEflight system designisprimarily single-string withsome select functional- andblock-redundancy andincludes fault tolerance measures targeted atachieving themostcost- effective risk reduction possible forthesystem design. Thefault protection teamhasbeenchallenged with balancing theriskoffaults, cost, anddown-time with GroundSegmentcapabilities, heritage, andeffectively designed fault mitigations. Faults wereidentified viaa collection ofanalyses, andacriticality rating wasapplied to eachfault toassess itsimpact tothemission. Taking into consideration eachfault's impact andtimecriticality, mitigations toeachpossible fault wereconsidered inareas suchason-board autonomy, theaddition oruseof functional andblockredundancy, andgroundsystem detection. Through this exercise, theproject hasrealized a robust andreliable system design inline withtheproject's risk posture andcostconstraints.12