  We investigate a relativistic positioning system where the coordinates of the
users are determined by the proper times broadcasted by clocks in motion in
spacetime: these are the so-called emission coordinates. In particular, we
focus on emission coordinates in flat spacetime: in this case, we show that
these coordinates can be defined using an approach based on simple geometrical
properties of geodesic triangles. We analyse the 2-dimensional case and then we
show how the whole procedure can be applied to 4 dimensions, also in terms of
ordinary three-dimensional spheres. An analytic solution for the coordinates of
the receiver is obtained. The solution remains valid at almost any place, in
particular when redundancy in the number of emitters can be exploited.
