elongation of the error ellipse. As the error ellipse collapses to a line, the confidence
level of the 2 drms measurement approaches 95 percent; as the error ellipse becomes
circular, the confidence level approaches 98 percent. The GPS 2 drms accuracy will
be at 95 percent probability.

DOD specifies horizontal accuracy in terms of Circular Error Probable (CEP—the
radius of a circle containing 50 percent of all possible fixes). For the FRP, the
conversion of CEP to 2 drms has been accomplished by using 2.5 as the multiplier.

Specifications of radionavigation system accuracy generally refer to one or more of
the following definitions:

• Predictable accuracy: The accuracy of a radionavigation system’s position
  solution with respect to the charted solution. Both the position solution and
  the chart must be based upon the same geodetic datum. (Note: Appendix B
  discusses reference systems and the risks inherent in using charts in
  conjunction with radionavigation systems).
• Repeatable accuracy: The accuracy with which a user can return to a position
  whose coordinates have been measured at a previous time with the same
  navigation system.
• Relative accuracy: The accuracy with which a user can measure position
  relative to that of another user of the same navigation system at the same
  time.

The availability of a navigation system is the percentage of time that the services of
the system are usable by the navigator. Availability is an indication of the ability of
the system to provide usable service within the specified coverage area. Signal
availability is the percentage of time that navigational signals transmitted from
external sources are available for use. It is a function of both the physical
characteristics of the environment and the technical capabilities of the transmitter
facilities.

The coverage provided by a radionavigation system is that surface area or space
volume in which the signals are adequate to permit the navigator to determine
position to a specified level of accuracy. Coverage is influenced by system geometry,
signal power levels, receiver sensitivity, atmospheric noise conditions, and other
factors which affect signal availability.