Latest advances in an astrometric model based on the Time Transfer   Functions formalism
  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. 
Stefano Bertone, Alberto Vecchiato, Christophe Le Poncin-Lafitte,   Mariateresa Crosta and Luca Bianchi
gr-qc astro-ph.IM