1.1.5 Ground segment and operations

Mission operations are conducted from the European Space Agency (ESA) Mission Operations Centre (MOC), located at the European Space Operations Centre (ESOC), Darmstadt, Germany. Mission operations include mission planning, regular upload of the planning products to the mission time line 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, performing and monitoring operational time synchronisation, anomaly investigation, mitigation, and recovery, orbit prediction, reconstruction, monitoring, and control, spacecraft calibrations (e.g., star-tracker alignment, micro-propulsion offset calibration, etc.), and on-board software maintenance. Details are provided in Gaia Collaboration et al. (2016).

Science operations are conducted from the ESA Science Operations Centre (SOC), located at the European Space Astronomy Centre (ESAC), Madrid, Spain. Science operations include generating the scanning law, including the associated calibration of the representation of the azimuth of the Sun in the scanning reference system in the VPU software (Section 1.3.3), generating the science schedule, i.e., the predicted on-board data rate according to the operational scanning law and a sky model, to allow for adaptive ground-station scheduling, generating the avoidance file containing time periods when interruptions to science collection would prove particularly detrimental to the final mission products, generating payload operation requests (PORs), i.e., VPU-parameter updates (e.g., TDI-gating scheme or CCD-defect updates; Section 1.3.3), tracking the status and history of payload-configuration parameters in the configuration database (CDB; Section 1.2.3) through the mission time line and telecommand history, hosting the science-telemetry archive, generating event anomaly reports (EARs) to inform downstream processing systems of ’bad time intervals’, outages in the science data, or any (on-board) events which may have an impact on the data processing and/or calibration, monitoring (and recalibrating as needed) the star-packet-compression performance, monitoring (and recalibrating as needed) the BAM-pattern location inside the readout windows (Figure 3.5), reformatting the optical observations of Gaia received from Gaia’s Ground-Based Optical Tracking (GBOT; Section 3.2.2 and Altmann et al. 2014) programme for processing in the orbit reconstruction at the MOC, and disseminating meteorological ground-station data – required for delay corrections in the high-accuracy time synchronisation / on-board clock calibration – from MOC to DPAC (Section 3.1.6). Details are provided in Gaia Collaboration et al. (2016).

The use of all three 35-meter deep-space dishes in ESA’s tracking station network (ESTRACK) ensures a high-quality telemetry link budget and an optimum science data rate. These stations are located at Malargüe (Argentina), Cebreros (Spain), and New Norcia (Australia) and hence provide (close to) 24-hour coverage. The daily telecommunications period is adjusted to the expected data volume to be down-linked each day. This volume is predicted based on a sky model, itself based on the Gaia DR1 Catalogue, combined with the operational scanning law. The typical, daily down-link time of $\sim$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’, or GPS), required down-link 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 using Gaussian minimum-shift keying (GMSK) modulation. Error correction in the down-link 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 down-link information data rate (including packetisation and error-correction overheads) is some 8.7 megabits per second. The typical amount of (compressed) science data down-linked to ground is around 40 gigabytes per day.

Stars brighter than $\sim$3 mag in the Gaia $G$ band are not properly detected on-board at each transit (e.g., Sahlmann et al. 2016). Special sky-mapper (SM) SIF images (Section 1.1.3) are therefore acquired for all of them in the sky-mapper (SM) CCDs. See Gaia Collaboration et al. (2016) for details. These special SIF data are not part of Gaia DR2.

Gaia cannot cope with extremely dense areas on the sky. As explained in Gaia Collaboration et al. (2016), the crowding limit is a few $\mathrm{100\hspace{0.17em}000}$ objects per square degree for astrometry and photometry; for spectroscopy, the limit is around $\mathrm{35\hspace{0.17em}000}$ objects per square degree. For a handful of selected, dense areas (Table 1.2), special SIF images (Section 1.1.3) are acquired in the sky- mapper (SM) CCDs to support the management of overlapping windows (deblending of images) in the ground processing. More details are provided in Gaia Collaboration et al. (2016). These special SIF data are not part of Gaia DR2.