ariance comprehends the changes in the severity of the charged particle environment as a function of the satellite position within its orbit. The concentration of proton induced single event upsets during a low earth orbit system's traverse through the South Atlantic Anomaly region, and heavy ion single event effects during polar traverse of highly inclined orbits are typical examples of spatial variance. Figures 3-2 and 3-3 provide description of the flux contour levels of trapped protons present in the SAA region. Section 3.5 provided description of the shielding effects which the earth's geomagnetic field provide to galactic and solar heavy ions, resulting in little shielding effects over the high latitude polar regions and significant shielding at lower latitudes. Figure 5-2 indicates the single event upset rates estimated for a memory device exhibiting SEU susceptibility to both heavy ions and protons in a 750 kilometer near polar orbit. This figure indicates the wide spatial variance in SEU rate for orbits which traverse through the SAA region and for those which do not, and for both high latitude polar regions and low latitude segment of the orbit. Indicated in this figure is the overall orbit average single event upset rate which represents the long term average over numerous orbit 1E-03 Trapped Proton 1E-04 Orbit Average 1E-05 Galactic Cosmic 1E-06 Heavy lon 1E-07 1E-08 Time (sec) Figure 5-2. Single Event Upset (SEU) Rate, One Orbit, SAA Traverse periods. The extremely large range in single event upset rates and the limited value which orbit averaged SEU rate provides in establishing fault tolerance design requirements are indicated in this figure. The propensity for SEU events to occur in the SAA region and during polar traverse establish requirements for the effectiveness of fault mitigation and tolerance strategies for low earth orbit systems. Fault Tolerant Systems Engineering System Reliability and Availability [Wood97] [Somani97] [Nelson90] The two most common expressions of a system's ability to t