Fault-based analysis is an early lifecycle approach to improving software quality by preventing and/or detecting pre-specified classes of faults prior to implementation. It assists in the selection of verification and validation techniques that can be applied in order to reduce risk. This paper presents our methodology for requirements-based fault analysis and its application to National Aeronautics and Space Administration (NASA) projects. The ideas presented are general enough to be applied immediately to the development of any software system. We built a NASA-specific requirement fault taxonomy and processes for tailoring the taxonomy to a class of software projects or to a specific project. We examined requirement faults for six systems, including the International Space Station (ISS), and enhanced the taxonomy and processes. The developed processes, preliminary tailored taxonomies for Critical/Catastrophic High-Risk (CCHR) systems, preliminary fault occurrence data for the ISS project, and lessons learned are presented and discussed.