7.4.1 Introduction

Besides the astrometric channel and the photometric one, Gaia has the unprecedented advantage to possess in addition a spectroscopic one. All three channels contribute to provide data that are potentially further combined to improve the understanding of our spatial environment. In the present section, the sole SB1 spectroscopic channel is considered.

For the brightest stars, the RVS instrument provides spectra that are used to derive physical parameters and radial velocities for stars exhibiting a normal single spectrum. The instrumental spectra obtained at each RVS transit are converted to proper physical spectra (reduced) by the RVS spectroscopic processing pipeline (see Chapter 6). The radial velocity is measured by cross-correlating the observed reduced spectra with a theoretical/synthetic spectrum computed with various stellar atmosphere models. The best tested model provides the best radial velocity. The measurements of the radial velocities are performed by the STA (Single Transit Analysis) pipeline. The list of good epoch radial velocities (RV) corresponding to a particular star are then analysed by the MTA (Multiple Transit Analysis) in order to detect and separate variable RV sets from constant RV sets . The median of the epoch RVs of constant stars are computed and appear in the catalog. Stars with variable RVs are further considered by the pipeline and are forwarded to the NSS spectroscopic binary processing. For Gaia DR3, the adopted cutting threshold corresponds to a probability of variability of 0.99. Gaia DR3 is the first release providing analysis of the RV time-series looking for orbital motion.