We developed a domain engineering approach and the supporting tools to define software product lines. The approach is composed of a process and a notation. The notation is an extension of UML {Unified Modeling Language), the process is influenced by Synthesis [1] and FAST [2]. Supporting tools have been built on top of a commercial CASE tool for UML This paper focuses on the domain analysis part of the approach and specifically on the notation and tools to support it. The approach is being applied to sub-domains of the Guidance Navigation and Control domain, a set of functionalities used on board of satellites developed by the NASA Goddard Space Flight Center.