3.4.6 Barycentric radial velocity correction (BVC)

Author(s): Javier Castañeda

The measurement of the barycentric radial velocity correction in Gaia can be obtained from:

• Gaia ephemeris or orbit; allowing one to compute Gaia position and velocity in the BCRS for any moment of time covered by observations (see Section 4.2.3).

• Spacecraft attitude; providing the orientation of the satellite and thus the viewing direction of both field of views (see Section 3.4.5).

In practice, the module in charge of the barycentric radial velocity correction computation populates a table with values for the two fields of view of Gaia at regular time knots (typically of 5 minutes). For each time knot, the Gaia velocity is retrieved from its ephemeris which is then projected over the viewing directions of the two field of views at the reference zero point of the RVS instrument (measured before launch). These viewing directions are obtained from the attitude and some basic transformations between the different Gaia reference systems accounting for the basic angle between the two field of views and focal plane geomatry among others (see Figure 3.15).

The precision of even the most preliminary orbit data ($\sim$mm/s) and attitude data (5-mas level) exceed the needs of the barycentric velocity correction consumers by orders of magnitude (see Chapter 6). This scheme provides an RMS precision of about 0.05 km/s. The corrections can range from $+30$  km s${}^{-1}$ to $-30$  km s${}^{-1}$ as a function of Gaia’s six hour revolution as shown in Figure 6.6.

The barycentric radial velocity correction table was initially produced by IDT just after the computation of OGA1 (see Section 3.4.5). However, its production in the Daily pipeline was discontinued around mid 2017 (when the third data segment started) as it was considered more practical to compute it on the fly given its small computational cost. Furthermore, consumers of this data can benefit with this new approach from the latest orbit and attitude updates.