  Given the extreme accuracy reached by future global space astrometry, one
needs a global relativistic modeling of observations. A relativistic definition
of astrometric observables is then essential to find uniquely coordinates,
parallax and proper motion of a stellar object. The standard procedure is to
explicitly solve the null geodesic equations to describe the trajectory of a
photon emitted by a celestial object and received by a moving observing
satellite. However, this task can be avoided if one builds an astrometric set
up using the recent formalism of time transfer functions.
