Balancing Risk And Schedule

Evaluating satellite failure risk in performance-based functional availability (PFA) tools.

Technical risk is traditionally assessed based on mission impact to an individual satellite and the likelihood of occurrence. A separate analysis is required to determine the delays and additional costs to mitigate the technical risk. Tradeoffs to delay or to launch a satellite with technical risk can be difficult and usually do not include long-term impacts to the constellation or enterprise. They don’t provide a method to directly measure the differences between fly-as-is versus slipping the schedule to repair. A method was developed to make direct comparisons between the impacts to either accept the technical risk or slip the schedule. 

The likelihood of reduced performance is modeled using multistate satellite reliability models that include all possible satellite performance states, and impacts are determined by the constellation performance models. Satellite failure risk is evaluated in performance-based functional availability (PFA) tools by modifications to the satellite reliability models. These models can address risks at the piece-part level, such as suspect microelectronics as a function of reduced performance and likelihood of occurrence. Schedule slips are incorporated into PFA tools by adjusting the scheduled satellite launch times. Models for ground segments, external threats, and mitigations are also included in PFA tools.

Risk chart

PFA plots overlay the impacts from technical risk and schedule slips at the constellation, user, enterprise, and resilience levels. The constellation impacts are shown as future availability and other impacts are shown as distributions of key constellation future performance metrics. Resilience impacts are computed under the assumption of one or more external probabilistic threats and mitigations. Once a PFA tool is established for a constellation, regenerating impact plots is quickly accomplished for any given technical risk or schedule slips allowing for direct comparisons at multiple impact levels.

For more information, contact James Womack, 310.336.7647,

This story appears in the March 2020 issue of Getting It Right, Collaborating for Mission Success.

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