Based on work done under Microcosm's recently completed Phase II SBIR program on X-ray pulsar based navigation (XNAV), relevant X-ray source characterization, X-ray detector measurement models, and source timing models will be developed to support eventual implementation in the GEONS flight software environment. In Phase I, an initial assessment will be made of the key model parameters and accuracy of time determination. Microcosm has recently been awarded a contract by DARPA to assess the potential for XNAV as a precision timing source for DoD missions. This work will be evaluated in Phase I for the potential to provide a precise, independent, X-ray pulsar-based time reference for NASA missions as well. The team will work with the Naval Research Lab, JHU/Applied Physics Lab, and NASA GSFC, focusing on sources and detectors to develop enhanced models, and assessing potential timing accuracy. Development of pulsar signal processing methods and prototype algorithms will continue, incorporating new source and detector models. This work, along with the previous Phase II effort, will further validate the utility of XNAV for NASA, and provide sufficient development and testing of relevant technology to provide a rapid path to a full flight software development in a Phase II program.