  We present here a method for the relativistic positioning in spacetime based
on the reception of pulses from sources of electromagnetic signals whose
worldline is known. The method is based on the use of a four-dimensional grid
covering the whole spacetime and made of the null hypersurfaces representing
the propagating pulses. In our first approach to the problem of positioning we
consider radio-pulsars at infinity as primary sources of the required signals.
The reason is that, besides being very good clocks, pulsars can be considered
as being fixed stars for reasonably long times. The positioning is obtained
linearizing the worldline of the observer for times of the order of a few
periods of the signals. We present an exercise where the use of our method
applied to the signals from four real pulsars permits the reconstruction of the
motion of the Earth with respect to the fixed stars during three days. The
uncertainties and the constraints of the method are discussed and the
possibilities of using moving artificial sources carried around by celestial
bodies or spacecrafts in the Solar System is also discussed.
