This paper presents an analysis of the unit testing approach developed and used by the Core Flight Software System (CFS) product line team at the NASA Goddard Space Flight Center (GSFC). The goal of the analysis is to understand, review, and recommend strategies for improving the CFS' existing unit testing infrastructure as well as to capture lessons learned and best practices that can be used by other software product line (SPL) teams for their unit testing. The results of the analysis show that the core and application modules of the CFS are unit tested in isolation using a stub framework developed by the CFS team. The application developers can unit test their code without waiting for the core modules to be completed, and vice versa. The analysis found that this unit testing approach incorporates many practical and useful solutions such as allowing for unit testing without requiring hardware and special OS features in-the-loop by defining stub implementations of dependent modules. These solutions are worth considering when deciding how to design the testing architecture for a SPL. © 2012 Elsevier B.V. All rights reserved.