For this purpose, during the transfer phase the nominal attitude with respect to the Sun is the same as the operational one. The final commissioning of the Payload Module occurs after orbit insertion around L2, consisting of final calibration of the instrument and demonstration of instrument performance.

Operations

• Routine operations consist of a daily science data dump (of ~ 8 hours) and weekly updates of the scan law to account for densely populated sky regions
• The orbit maintenance operations are typically performed once every month
• One Earth eclipse avoidance manoeuvre is performed in the course of the mission
• All correction manoeuvres are performed with the nominal spacecraft attitude. The scanning law is not interrupted for short manoeuvres, so as to minimise the impact on observations
• The Gaia spacecraft/mission ensures science data acquisition and downlink even during eclipses of the Sun by the Moon

Ground Segment

Ground stations contacts with a daily duration of eight hours are achieved using the Cebreros 35m X-band antenna. Also the ground station in Perth (Australia)

Time Correlation

In addition to the classical corrected one-way path technique, it is proposed that on-board-to-UTC time correlation be performed by a specific two way process. This technique allows to cancel symmetrical delays of ionospheric or tropospheric origin, or delays of relativistic origin. This correlation performance is furthermore independent of the orbit.