Optimized Spacecraft Fault Protection for the WISE Mission

The WISE project is a NASA-funded medium- class Explorer mission to map the entire sky in four infrared bands during the course of a 6-month survey. Because of the mission's limited financial resources, a traditional robustness strategy of full block-redundancy was not feasible. By leveraging aspects of the mission design that tend to reduce the risk associated with certain failures, the project has been able to adopt a robustness strategy of mitigating high-risk failures, while accepting the risk of low-impact faults or unlikely faults in heritage equipment with proven reliability. The resulting WISE flight system design is primarily single-string with some select functional- and block-redundancy and includes fault tolerance measures targeted at achieving the most cost- effective risk reduction possible for the system design. The fault protection team has been challenged with balancing the risk of faults, cost, and down-time with Ground Segment capabilities, heritage, and effectively designed fault mitigations. Faults were identified via a collection of analyses, and a criticality rating was applied to each fault to assess its impact to the mission. Taking into consideration each fault's impact and time criticality, mitigations to each possible fault were considered in areas such as on-board autonomy, the addition or use of functional and block redundancy, and ground system detection. Through this exercise, the project has realized a robust and reliable system design in line with the project's risk posture and cost constraints.