DARPA recently initiated the XNAV program to undertake development of GPS independent, precision navigation and time determination based on observations of certain extremely stable millisecond X-ray pulsars for near Earth military applications. The groundwork for this effort comes from more than a decade of high-energy astrophysics research at NRL and data from space-based surveys of galactic X-ray sources (e.g., ARGOS/USA, Chandra). These signals can be exploited as a "natural GPS" to provide position information and an extremely stable time reference. Anticipated NASA Applications: 1. Accurate clock synchronization (solar system wide) 2. Fully autonomous, precision navigation (solar system wide) 3. DSN Augmentation 4. Planetary GPS enabler 5. LEO GPS backup Microcosm has worked with NRL on exploitation of this technology for 2.5 years, and we anticipate continued access to their work during Phase I which will examine the utility, and feasibility of developing X-ray pulsar based navigation and time determination technology for candidate NASA applications. Requirements will be determined, approaches developed, and performance potential projected. The Phase II study will develop the most promising applications. Detailed designs will be created, performance simulations developed, utility assessed, a technology roadmap developed, and a mission planning guide developed.