  4 emitters broadcasting an increasing electromagnetic signal generate a
system of relativistic coordinates for the space-time, called emission
coordinates. Their physical realization requires an apparatus similar to the
one of the Global Navigation Satellite Systems (GNSS). Several relativistic
corrections are utilized for the current precisions, but the GNSS are conceived
as classical (Newtonian) systems, which has deep implications in the way of
operating them. The study of emission coordinates is an essential step in order
to develop a fully relativistic theory of positioning systems. This talk
presents some properties of emission coordinates. In particular, we
characterize how any observer sees a configuration of satellites giving a
degenerated system and show that the trajectories of the satellites select a
unique privileged observer at each point and, for any observer, a set of 3
orthogonal spatial axes.

General Relativity and Quantum Cosmology