olerate failure are reliability and availability. Reliability, R(t), is the conditional probability that a system will remain operational (or can be restored to an operational state) over the interval 0 to t, normally the duration of the mission. Availability, A, is defined as the probability that a system is operational at any particular time, and expresses the fraction of time a system is operational. R(t) = = = a = 1 / MTBF Mean Time Between Failure =11/(2+11) u=1/MTTR Mean Time To Repair/Recover A system with high reliability (evidencing a low mean-time-between-failure) may exhibit unacceptable availability if the mean-time-to-repair/recover is unacceptably long. A system with high availability may in fact fail; if the mean-time-to-repair/recover is suitably low, acceptable availability may be achieved. Although availability is expressed as the percentage of the time the system is available to users, developing the requirement which is specified is typically based on outage frequency, outage duration, and ultimately on the cost implications of users being denied access to the system. Although degraded modes and conditions of systems create situations which infer partial or limited availability, the bottom line is that the system is available if the customer says that it is. The customer will declare that it is not available when denied access or provided a quality of service less than fully specified performance. Availability is a system characteristic pertaining to end-to-end operations which relies upon all system resources including space segment, ground operations, and user equipment. Departures from perfect system operation which contribute to availability are allocated; the consequences, detection, and recovery of space segment payload operations due to the occurrence of single event upsets receive only a portion of this allocation. Availability is often expressed as a steady-state value, either as the probability that the system is operational at any random time, or as a given amount of downtime over a