Gaia is one of the most demanding of the 14 spacecraft operated by the European Space Operations Centre (ESOC) at Darmstadt in Germany. Situated 1.5 million km from us, the location of the spacecraft must be known to within 150 m every day of the nominal five-year mission, and the time on the spacecraft must be known to within 2 microseconds. At times the amount of data that needs to be downloaded even exceeds the capacity of all three ESA ground stations.
Where is Gaia and why do we need to know?
The location of the spacecraft feeds into the data analysis and the accuracy with which this position is measured has a direct impact on the precision with which the positions, motions and distances to the stars can be known.
To determine the distance to a spacecraft, ESOC performs two-way ranging measurements, which are accurate to 5 m. The speed at which the spacecraft is moving towards or away from us is obtained from Doppler measurements, which are accurate to 0.1 mm/s. These measurements are routinely carried out to check that Gaia is in the correct orbit and if not, to calculate and monitor the manoeuvres that are made to place the spacecraft back on track.
The location of Gaia on the plane of the sky is obtained using two widely separated antennas to simultaneously track the location of a transmitter on the spacecraft – this is the delta-differential one-way ranging (DDOR) method. DDOR can provide measurements accurate to about 22 m for Gaia, but since it requires regular use of two of the three ground stations that are shared by all science missions another method is also used.
Daily tracking of the spacecraft is carried out by the Gaia Ground-Based Optical Tracking (GBOT) programme, organised by the Gaia Data Processing and Analysis Consortium (DPAC), using a network of small robotic telescopes across the world, as well as the European Southern Observatory's VLT Survey Telescope. These measurements will be able to provide the location of Gaia to within 150 m, which is sufficient for the science needs. Initially only DDOR positions are used to determine the actual orbit of Gaia to high accuracy but with the Gaia data available GBOT will routinely produce daily measurements of the location of the spacecraft to the accuracy needed.
Some of Gaia's other operational requirements have meant it was necessary to enhance some operational practices.
At the ground stations, ESA upgraded the infrastructure to support a high data rate, up to 7.62 Mbit/s (about half of a good domestic broadband rate), which allows more data to be downloaded during each period of contact.
Even with this high data rate (the highest of any science mission to date), Gaia is still the biggest user of ESA's large 35m network. To optimise this use as much as possible, operators came up with a scheme that predicts precisely how much time is needed. The sky is not uniformly dense with stars and this is reflected in the amount of data that is recorded by Gaia. Since Gaia is scanning the sky many times this can be modelled so that the amount of time needed per day can be predicted. Most of the time all the data can be downloaded to ground using the three ground stations, at Cebreros (Spain), New Norcia (Australia), and Malargüe (Argentina). However, when scanning the densest regions of the sky, such as the Galactic Centre, even the high rate and all three stations are not sufficient, and an intelligent onboard scheme selects the least important data types to be deleted.
The communication protocols that are used to transmit data from the ground stations to the space operations centre also had to be adapted to cater to the demands imposed on time accuracy: ESOC must time stamp the data received on ground to an accuracy of better than 2 microseconds. This accuracy is needed to be able to reconstruct the orbits of near-Earth asteroids.