equals 90◦.

However, when scanning more or less along a great circle (as during a day or so), the accuracy with which the one-dimensional positions of stars along the great circle can be derived, as carried out in the one-day iterative solution (ODAS) as part of continuous payload health monitoring (Sect. 6.3), is poor when Γ = 360◦× m/n for small integer values of m and n (Lindegren & Bastian 2011); this can be understood in terms of the connectivity of stars Article number, page 6 of 36 subsystem (AOCS)

is therefore based on a custom design (e.g. Chapman et al. 2011; Risquez et al. 2012) including various sensors and actuators. The sensors include two autonomous star trackers (used in cold redundancy), three fine Sun sensors used in hot redundancy (i.e. with triple majority voting), three fibreoptic gyroscopes (internally redundant), and low-noise rate data provided by the payload through measurements of star transit speeds through the focal plane. Gaia contains two flavours of actuators: two sets of eight bi-propellant (NTO oxidiser and MMH fuel) newton-level thrusters (used in cold redundancy) forming the chemical-propulsion subsystem (CPS) for spacecraft manoeuvres and back-up modes, including periodic orbit maintenance (Sect. 5.3.2); and two sets of six proportional-cold-gas, micro-newton-level thrusters forming the micro-propulsion subsystem (MPS) for fine attitude control required for nominal science operations. In nominal operations (AOCS normal mode), only the star-tracker and payload-rate data are used in a closedloop, three-axes control with the MPS thrusters, which are operated with a commanded thrust bias; the other sensors are only used for failure detection, isolation, and recovery. Automatic, bi-directional mode transitions between several coarse and fine pointing modes have been implemented to allow efficient operation and autonomous settling during transient events, such as micro-meteoroid impacts (Sect. 5.1).
3.2.2. Phased-array antenna Extreme centroiding requirements of the payload prohibit the use of a conventional, mechanically steered dish antenna for science data downlink because moving parts in Gaia would cause unacceptable degradation of the image quality through microvibrations. Gaia therefore uses a high-gain phased-array antenna (PAA), allowing the signal to be directed towards Earth as the spacecraft rotates (and as it moves through its orbit around the L2 Lagrange point; Sect. 5.1) by means of electronic beam steering (phase shifting). The antenna is mounted on the Sun- and Earth-pointing face of the service module, which is perpendicular to the rotation axis. The radiating surface resembles a 14sided, truncated pyramid. Each of the 14 facets has two subarrays and each comprises six radiating elements. Each subarray splits the incoming signal to provide the amplitude weighting that determines the radiation pattern of the subarray. The overall antenna radiation pattern is obtained by combining the radiation patterns from the 14 subarrays. The equivalent isotropic radiated power (EIRP) of the antenna exceeds 32 dBW over most of the 30◦elevation range (Sect. 5.1), allowing a downlink information data rate of 8.7 megabits per second (Sect. 5.3.1) in the X band. The phased-array antenna is also used with orbit reconstruction measurements made from ground (Sect. 5.3.2).

[Figure: Fig. 1. Exploded, schematic view of Gaia. (a) Payload thermal tent (Sect. 3.3); (b) payload module: optical bench, telescopes, instruments, and focal plane assembly (Sect. 3.3); (c) service module (structure): also housing some electronic payload equipment, e.g. clock distribution unit, video processing units, and payload data-handling unit (Sect. 3.2); (d) propellant systems (Sect. 3.2.1); (e) phased-array antenna (Sect. 3.2.2); and (f) deployable sunshield assembly, including solar arrays (Sect. 3.2). Credit: ESA, ATG Medialab.]