ABSTRACTThis paper will bean overview of the critical orbital flight missions for which navigation performance is well defined relative to the various mission phases and key events (e.g. ascent, orbital coast, deorbit, entry, drag acquisition, post blackout, Terminal Area Energy Management and Autoland). Special emphasis will be given to those missions, events and performance parameters that are drivers of navigation accuracy. The navigation sensors will be described and their usefulness in keeping navigation accuracy within specified limits will be discussed by mission phases.