  This paper is devoted to a practical model for relativistic reduction of
positional observations with an accuracy of 1 \muas which is expected to be
attained in the future space astrometry missions. All relativistic effects
which are caused by the gravitational field of the Solar system and which are
of practical importance for this accuracy level are thoroughly calculated and
discussed. The model includes relativistic modeling of the motion of observer,
modeling of relativistic aberration and gravitational light deflection as well
as a relativistic treatment of parallax and proper motion suitable for the
accuracy of 1 \muas. The model is formulated both for remote sources (stars,
quasars, etc.) and for Solar system objects (asteroids, etc.). The suggested
model is formulated within the framework of Parametrized Post-Newtonian
Formalism with parameters $\beta$ and $\gamma$. However, for general relativity
($\beta=\gamma=1$) the model is fully compatible with the IAU Resolutions
(2000) on relativity in celestial mechanics, astrometry and metrology. The
model is presented in a form suitable for implementation in a software system
for data processing or simulation. The changes which should be applied to the
model to attain the accuracy of 0.1 \muas are reviewed. Potentially important
relativistic effects caused by additional gravitational fields which are
generated outside of the Solar system are also briefly discussed.
