above equations, converted from heliotropic angles ν and Ωto celestial coordinates, and finally approximated by piecewise polynomials using the Chebyshev representation.

During the first weeks of nominal science operations (between 25 July and 21 August 2014), Gaia was operating with a special scanning law, known as the ecliptic poles scanning law (EPSL).

In this mode, ν(t) stayed constant at 180◦, which means that the spin axis followed the Sun on the ecliptic (the alternative EPSL solution, with ν(t) = 0◦, was not used).

This means that the fields of view scan through both the south and the north ecliptic poles in every six-hour scan, and that the direction of scanning changes by the same rate as the Sun (and the spin axis) moves along the ecliptic, which is ∼1◦per day.

Advantages of the EPSL are that a limited set of objects near the ecliptic poles are observed very frequently and with a (four times) smaller across-scan image motion than nominally.

The EPSL observations have hence been used to bootstrap astrometric and photometric calibrations and for performance verification during the commissioning phase (Sect. 4.2).

The scanning law returns maximum sky-coverage uniformity after the nominal, five-year mission.

Nonetheless, the number of times an object is observed (Nobs) depends on its position in the
173 mas s−1.

Given the apparent path of the Sun on the celestial sphere and the fixed value of ξ, the scanning law, i.e. the orientation of the instrument in the heliotropic frame, which has the nominal ecliptic as its fundamental plane and the Sun as origin, is described by two heliotropic angles.

First, the revolving phase ν(t) (also known as precession phase), which is the angle between the ecliptic plane and the plane containing the Sun and the instru-
Article number, page 17 of 36 sky, in particular on its ecliptic latitude β (see Tab.

1 in Sect. 8.1).

For high-priority bright stars, data losses are small (Sect. 5.3.1) and the sky average, end-of-mission number of focal plane transits, for both fields of view combined, is around 80 in AF, BP, and RP (and a factor 4/7 = 0.57 smaller in RVS; Sect. 3.3.7).

At the faint end (G ≳20 mag or GRVS ≳14 mag), however, data losses can grow to ∼20% such that the number of transits reduces to ∼70 (and ∼40 in RVS).
5.3.

Mission operations The mission operations of Gaia are conducted by the Mission Operations Centre (MOC) located at the European Space Operations Centre (ESOC) in Darmstadt, Germany.

The core spacecraft operations of Gaia, which are similar to other ESA missions, include:
– mission planning, also based on input from the SOC (Sect. 6);
– regular upload of the planning products to the mission timeline of Gaia;
– acquisition and distribution of science telemetry;
– acquisition, monitoring and analysis, and distribution of health, performance (voltage, current, temperature, etc.), and resource (power, propellant, link budget, etc.) housekeeping data of all spacecraft units via ∼40 000 telemetry parameters;
– performing and monitoring operational time synchronisation (time correlation; Sect. 5.3.3);
– anomaly investigation, mitigation, and recovery;
– orbit prediction, reconstruction, monitoring, and control (Sect.5.3.2);
– spacecraft calibrations (e.g. star-tracker alignment, micropropulsion offset calibration, etc.); and
– onboard software maintenance.

The Gaia avionics module (AVM), with flight-representative electronics hardware, is located at MOC to support spacecraft operations and trouble shooting.
5.3.1.

Ground stations In order to have a high-quality link budget and high science data rate, the three 35-meter deep-space dishes in the ESA tracking station network (ESTRACK) are used.

These stations are located at Malargüe (Argentina), Cebreros (Spain), and New Norcia (Australia) and hence provide nearly 24 h coverage.

The daily telecommunications period is adjusted to the expected data volume to be downlinked each day, which is predicted based on a sky model combined with the operational scanning law.

The typical downlink time of ∼12.5 hours is normally covered by two of the three antennae.