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.

In times of enhanced data rates, typically when the scanning law makes Gaia scan along, or at small angles to, the Galactic plane (loosely referred to as a Galactic plane scan), required downlink times increase, up to, and exceeding, the maximum possible 24 hours per day, which means that three antennae are used sequentially.

The science data is telemetered to ground in the X band through the high-gain phased-array antenna (Sect. 3.2.2) using Gaussian minimumshift keying (GMSK) modulation.

Error correction in the downlink telemetry stream is achieved through the use of concatenated convolutional punctured coding.

In practice, a 7/8 convolutional encoding rate is used as baseline so that the downlink information data rate (including packetisation and errorcorrection overheads) is 8.7 megabits per second.

The typical amount of (compressed) science data downlinked to ground is Article number, page 18 of 36
some 40 gigabytes per day.

Actual onboard data losses caused by shortage of ground-station contact periods in times of Galactic plane scans are modest: for astrometry and photometry (star packet 1, SP1; Sect. 3.3.8), the data loss is zero for bright objects (G < 16 mag), a few percent between 16 and 20 mag, around 10% between 20 and 20.5 mag, and ∼25% for fainter objects.

For spectroscopy (star packet 2, SP2), the data loss is zero for stars brighter than GRVS = 10.5 mag, a few percent between 10.5 and 14 mag, and grows to 5–10% around 16 mag.
5.3.2.

Orbit prediction, reconstruction, monitoring, and control The spacecraft must periodically undergo orbit maintenance (station-keeping) manoeuvres to ensure that it does not escape from L2.

These events occur a few times per year and necessitate roughly a one-hour pause of the science operations because they involve the chemical propulsion system.

Velocity changes (∆V) are at the level of a (few) dozen cm s−1 and are aimed at removing the escape component from the spacecraft orbit, so that the actual orbit only drifts over a few 1000 km over five years compared with the reference orbit.

The amount of chemical propellant available for orbit maintenance manoeuvres allows the spacecraft to be kept at L2 for several decades.

In contrast, the cold-gas consumption of the micro-propulsion subsystem (Sect. 3.2.1) will limit the lifetime of Gaia to 10 ± 1 years, under the assumption of no hardware failures.

The requirements for the reconstructed orbit of Gaia are stringent: its near-Earth asteroid science case requires the positional error to be smaller than 150 m whereas aberration corrections for astrometric data collected for bright stars require the
velocity error to be smaller than 2.5 mm s−1.

To ensure that these requirements are met at all times, four types of data enter the orbit reconstruction process at MOC:
1.

Two-way ranging data, typically acquired both at the beginning and at the end of a communications period with the spacecraft (pass); 2.

Doppler data, acquired continuously during spacecraft contact with the ground station; 3.

Delta differential one-way-range (∆-DOR) data, occasionally obtained while tracking the spacecraft position on the sky with respect to a background quasar with known coordinates in the ICRF using two ESA ground stations simultaneously; 4.

Optical, plane-of-sky measurements of the spacecraft location with respect to the background stars Gaia itself is measuring.

These data are routinely being collected through the Ground-Based Optical Tracking (GBOT) programme (Altmann et al. 2014) and are critical when the spacecraft is at low declinations (|δ| <∼15◦).

The GBOT uses a network of small-to-medium telescopes (the ESO VLT Survey Telescope, which has contributed 400 hours of time to date thanks to an agreement between ESA and ESO, the Liverpool Telescope, and the Las Cumbres telescopes) and aims to deliver a daily measurement with 20 mas precision.