The Gaia satellite (Fig. 1) has been built under an ESA contract by Airbus Defence and Space (DS, formerly known as Astrium) in Toulouse (France). It consists of a payload module (PLM; Sect. 3.3), which was built under the responsibility of Airbus DS in Toulouse; a mechanical service module (M-SVM; Sect. 3.2), which was built under the responsibility of Airbus DS in Friedrichshafen (Germany); and an electrical service module (E-SVM; Sect. 3.2), which was built under the responsibility of Airbus DS in Stevenage (United Kingdom).
3.1. Astrometric measurement principle and overall design considerations The measurement principle of Gaia is derived from the globalastrometry concept successfully demonstrated by the ESA astrometric predecessor mission, Hipparcos (Perryman et al. 1989). This principle of scanning space astrometry (Lindegren & Bastian 2011) relies on a slowly spinning satellite that measures the crossing times of targets transiting the focal plane. These observation times represent the one-dimensional, along-scan (AL) stellar positions relative to the instrument axes. The astrometric catalogue is built up from a large number of such observation times, by an astrometric global iterative solution (AGIS) process (e.g. Lindegren et al. 2012, 2016), which also involves a simultaneous reconstruction of the instrument pointing (attitude) as a function of time, and of the optical mapping of the focal plane detector elements (pixels) through the telescope(s) onto the celestial sphere (geometric calibration). The fact that the nuisance parameters to describe the attitude and geometric calibration are Article number, page 5 of 36 along the circle (Makarov 1998).

[Figure: Fig. 2. Measurable, along-scan (AL) angle between the stars at P and F depends on their parallaxes ϖP and ϖF in different ways, depending on the position of the Sun. This allows us to determine their absolute parallaxes, rather than just the relative parallax ϖP −ϖF. Wide-angle measurements also guarantee a distortion-free and rigid system of coordinates and proper motions over the whole sky. Image from Lindegren & Bastian (2011).]

Taking this into account, several acceptable ranges for the basic angle remain, for instance 99◦.4±0◦.1 and 106◦.5±0◦.1.

Telescope accommodation aspects identified during industrial studies favoured 106◦.5 as the design value adopted for Gaia.

During commissioning, using Tycho-2 stars, the actual in-flight value was measured to be 1′′.3 larger than the design value.

For the globalastrometry concept to work, it is important to either have an extremely stable basic angle (i.e. thermally stable payload) on timescales of a few revolutions and/or to continuously measure its variations with high precision.

Therefore, Gaia is equipped with a basic angle monitor (Sect. 3.3.4).
– Ideally, ξ equals 90◦.

However, this would mean that sunlight would enter the telescope apertures.

To ensure optimum thermal stability of the payload, in view of minimising basic angle variations, it is clear that ξ should be chosen to be constant.

For Gaia, ξ = 45◦represents the optimal point between astrometric-performance requirements, which call for a large angle, and implementation constraints, such as the required size of the sunshield to keep the payload in permanent shadow and solar-array-efficiency and sizing arguments, which call for a small angle.
derived simultaneously with the astrometric source parameters from the regular observation data alone (without special, calibration data) means that Gaia is a self-calibrating mission.

Following in the footsteps of Hipparcos, Gaia is equipped with two fields of view, separated by a constant, large angle (the basic angle) on the sky along the scanning circle.

The two viewing directions map the images onto a common focal plane such that the observation times can be converted into small-scale angular separations between stars inside each field of view and large-scale separations between objects in the two fields of view.

Because the parallactic displacement (parallax factor) of a given source is proportional to sin θ, where θ is the angle between the star and the Sun, the parallax factors of stars inside a given field of view are nearly identical, suggesting only relative parallaxes could be measured.

However, although scanning space astrometry makes purely differential measurements, absolute parallaxes can be obtained because the relative parallactic displacements can be measured between stars that are separated on the sky by a large angle (the basic angle) and, hence, have a substantially different parallax factor.