lity, frequency of system mode change, and the frequency and procedures for the use of external agent support for recovery operations must be provided. Fault Avoidance Fault avoidance addresses all endeavors that seek to eliminate the occurrence of specific fault conditions, and to reduce the frequency of occurrence of the remaining faults. In the context of faults arising from single event upset caused by exposure to the energetic charged particles in the space environment, avoidance addresses reduction in the severity of the environment (through choice of orbit architecture and shielding), reduction in charge generation and collection in sensitive device structures, and elimination or reduction of circuit response to collected charge. 6.3.1 System Operations and Orbit Architecture [LaBel962] Selection of the orbit architecture identifies the opportunities to present to system engineering, options which exist to reduce the severity of the environment through selection of orbit altitude, inclination, and type. System operations considerations include the method of fault avoidance achieved by not performing critical tasks in segments of the orbit in which single event effects are more probable due to elevated particle flux levels. Programming of non- volatile memory in a system vulnerable to proton induced single event effects during South Atlantic Anomaly traverse, and the conduct of critical operations in a heavy ion susceptible system at high latitude and during solar events are undesirable system operating provisions. Fault avoidance shall define system operating constraints which minimize the opportunity for critical faults to occur. Figure 6-1 describes the integral flux of galactic cosmic ray heavy ions as a function of orbit altitude and inclination. The effect of the earth's geomagnetic shielding results in low levels of exposure for altitudes out to 6000 kilometers and inclination less than 40°; however, few telecommunications systems operate in this orbit. The exposure to increased levels of heavy ion f