  Emission relativistic coordinates are a class of spacetime coordinates
defined and generated by four emitters (satellites, pulsars) broadcasting their
proper time by radio signals. They are the main ingredient of the simplest
conceivable relativistic positioning system. The emission coordinates are
independent of any observer. Receiving directly the proper time at emission of
four satellites, any user or observer can measure the values of the emission
coordinates, from which he/she can obtain his trajectory and hence, in
particular, his position. Moreover, if and only if the four satellites also
broadcast to the users the proper times they are receiving by cross-link
autonavigation from the other emitters, the positioning system is called
autolocated or autonomous. In an autolocated positioning system the
trajectories of the satellites of the constellation can also be known by the
users and they can also obtain the metric of the spacetime (the gravitational
field) on the constellation.
  The study of autolocated relativistic positioning systems has been initiated
by Coll and coll. several years ago and it has been aimed for developing an
exact fully relativistic theory of positioning systems and gravimetry, based on
the framework and concepts of General Relativity. This exact relativistic
framework is the alternative to considering post-newtonian relativistic
corrections in a classical Newtonian framework, which is the customary approach
yet now used in GPS and GLONASS.
