  Only a causal class among the 199 Lorentzian ones, which do not exists in the
Newtonian spacetime, is privileged to construct a generic, gravity free and
immediate (non retarded) relativistic positioning system. This is the causal
class of the null emission coordinates. Emission coordinates are defined and
generated by four emitters broadcasting their proper times. The emission
coordinates are covariant (frame independent) and hence valid for any user. Any
observer can obtain the values of his(her) null emission coordinates from the
emitters which provide him his(her) trajectory.
