Reliability measurement of fault-tolerant onboard memory system under fault clustering

Advances in spaceborne vehicular technology have made possible the long-life duration of the mission in harsh cosmic environments. Reliability and data integrity are commonly emphasized requirements of spaceborne solid-state mass storage systems, because faults due to the harsh cosmic environments - such as extreme radiation - can be experienced throughout the mission. Acceptable dependability for these instruments have been achieved by using redundancy and repair. Reconfiguration (repair) of memory arrays using spare memory lines is the most common technique for reliability enhancement of memories with faults. Faulty cells in memory arrays are known to show spatial locality. This physical phenomenon is referred to as fault clustering. This paper initially investigates a quadrat-based fault model for memory arrays under clustered faults to establish a sound foundation of measurement. Then, long-life dependability of a fault-tolerant spaceborne memory system with hierarchical active redundancy, which consists of spare columns in each memory module and redundant memory modules, is measured in terms of reliability (i.e., the conditional probability that the system performs correctly throughout the mission) and mean-time-to-failure (MTTF i.e., the expected time that a system will operate before it fails).