  Given the extreme accuracy of modern space astrometry, a precise relativistic
modeling of observations is required. Moreover, the availability of several
models formulated in different and independent ways is a security against the
presence of systematic errors in the analysis of future experimental results,
like in the case of the Gaia mission. In this work, we simulate a series of
observations using the two models to be used for the data analysis of Gaia, the
Gaia RElativistic Model (GREM) and the Relativistic Astrometric MODel (RAMOD),
and we compare them with the results of our astrometric model based on the Time
Transfer Functions.
