 specified interval. Steady-state availability is computed as the mean-time-to-failure divided by the sum of mean-time-to-failure and mean-time-to-recovery. Statistical mean values of system failure and recovery are often used in system evaluation. However, statistical mean values can be misleading, given the order-of-magnitude changes which are present in the spatial and temporal variances in the charged particle environments that produce single event upsets and transients. Previous paragraphs have described the non-uniformities which heavy ion particle flux in high latitude orbit segments and the concentration of trapped proton flux in the SAA region present to Low Earth Orbit (LEO) systems, and the short term increases in particle flux resulting from solar events and geomagnetic field disturbances. Consideration must be given to the availability and quality of service which are provided to specific regions of the earth and for short periods of time in which increased particle flux levels produce increased propensity for faults and resultant detection and correction / recovery actions. Fault Tolerant Systems Design [Somani97] [Avizienis97 ] [Nelson90] The general subject of fault tolerant systems addresses numerous causes of faults, including incorrect specifications, design errors, manufacturing defects (and inadequate testing to detect them), human operator actions (accidental and intentional), component damage or failure, and interaction with the operating environment. A fault may be classified by its nature, duration, and extent; the duration of a fault may be transient, intermittent or permanent. The context in which fault tolerance techniques are discussed herein addresses transient faults introduced by single event upset effects due to the operating environment. In this sense we are addressing a subset of the broader subject of system fault tolerance, that of transient effects. Not related to the subject of this short course or NSREC conference are the other sources of faults which are indicated; however