Satellites stay in contact with the Earth by means of a ground-station network. Engineers send commands to correct their trajectories, manoeuvre them into different orbits, and operate their instruments. The satellite transmits back to Earth not only the scientific data that it is gathering, but also the ‘housekeeping’ information needed by the operators to check the satellite’s performance.
For these purposes, ESA has a number of antennas located at ESAC in Villafranca, Madrid, with modern electronic telemetry, telecommand and ranging equipment that allows commands to be sent to control the satellites and their payloads, as well as the scientific data to be received on the ground.
At Villafranca there are several ground stations, located as follows:
- VIL-1: a 15-metre diameter dish antenna with reception in S-band and transmission in S-band including an auto track capability, a ranging system, a frequency and timing system, a monitoring and control system, and a communications system. Since September 2007, VIL-1 has been mothballed but it is being considered for educational purposes.
- VIL-2: a 15-metre diameter dish antenna with reception and transmission in S-band including an auto track capability, a ranging system, a frequency and timing system, a monitoring and control system, and a communications system.
- VIL-4: a 12-metre diameter dish antenna. used for technology testing with reception in both X and Ka-band and transmission in X-band.
In addition there is an antenna for the Global Positioning System - Tracking and Data Facility (GPS-TDAF). Note: the TVSAT antenna is not part of TT&C and therefore Villafranca ground station.
The Vil-2 antenna at Villafranca (the only one operational) currently support:
- The Cluster flotilla, four satellites that are studying the Earth's magnetic field.
- Integral, the gamma-ray telescope.
- Two of ESA's Earth-observation satellites, Envisat and ERS-2.