  Current GNSS systems rely on global reference frames which are fixed to the
Earth (via the ground stations) so their precision and stability in time are
limited by our knowledge of the Earth dynamics. These drawbacks could be
avoided by giving to the constellation of satellites the possibility of
constituting by itself a primary and autonomous positioning system, without any
a priori realization of a terrestrial reference frame. Our work shows that it
is possible to construct such a system, an Autonomous Basis of Coordinates, via
emission coordinates. Here we present the idea of the Autonomous Basis of
Coordinates and its implementation in the perturbed space-time of Earth, where
the motion of satellites, light propagation, and gravitational perturbations
are treated in the formalism of general relativity.
