  After briefly reviewing the relativistic approach to positioning systems
based on the introduction of the emission coordinates, we show how explicit
maps can be obtained between the Cartesian coordinates and the emission
coordinates, for suitably chosen set of emitters, whose world-lines are
supposed to be known by the users. We consider Minkowski space-time and the
space-time where a small inhomogeineity is introduced (i.e. a small
"gravitational" field), both in 1+1 and 1+3 dimensions.
