  This chapter describes the development of X-ray Navigation at the Naval
Research Laboratory (NRL) within its astrophysics research programs. The
prospects for applications emerged from early discoveries of X-ray source
classes and their properties. Starting around 1988 some NRL X-ray astronomy
programs included navigation as one of the motivations. The USA experiment
(1999) was the first flight payload with an explicit X-ray navigation theme.
Subsequently, NRL has continued to work in this area through participation in
DARPA and NASA programs. Throughout, the general concept of X-ray navigation
(XRNAV) has been broad enough to encompass many different uses of X-ray source
observations for attitude determination, position determination, and
timekeeping. Pulsar-based X-ray navigation (XNAV) is a special case.
