The circular dish attached to one face of the spacecraft bus is a 1.6 m diameter high gain antenna for receiving and transmitting radio signals when the spacecraft is a long way from Earth. When it is close to Earth at the beginning of its journey, communication is via a low gain antenna which is a 40 cm aerial protruding from the spacecraft bus.
For up to six hours during the spacecraft's 7.5 hour Martian orbit, the high gain antenna will point towards Earth for communications between the spacecraft and three ground stations. During the remaining 1.5 hours, the spacecraft will point towards the Martian surface so that the on-board instruments can make observations. Each time the spacecraft passes over Beagle 2 on the Martian surface, the lander will automatically relay data collected by its instruments to a special UHF antenna on the spacecraft.
The Beagle data, together with that collected by the instruments on the orbiter, will be sent back to Earth during the communications phase at a rate of up to 230 kbit/s. The European Space Operations Control Centre (ESOC) in Darmstadt will communicate with the spacecraft via the ESA ground station in Perth, Australia. The spacecraft will send housekeeping data on instrument temperatures, voltages and spacecraft orientation, for example, and the ground station will send back software commands to control the spacecraft and its instruments over the following few days.
Signals to Earth will be in the X-band (7.1 GHZ) and those from Earth will be the S band (2.1 GHZ).
As scientific data cannot be transmitted back to Earth as soon as it is collected, it will be stored on the spacecraft's computer which has 12 Gbits of solid state mass memory. The computer will control all aspects of the spacecraft's functioning including switching instruments on and off, assessing the spacecraft's orientation in space and sending commands to change it. The control and data management software is being developed for the Rosetta mission.
To communicate with a 34 m satellite dish on Earth up to 400 million km away and conduct sensitive scientific experiments, Mars Express must maintain a pointing accuracy of 0.15o. So it is essential that the spacecraft knows not just where it is but in which direction it is pointing. There are three on-board systems to help:
Small corrections to the spacecraft's orientation can be achieved by altering the rotation of spinning (off-the-shelf) reaction wheels attached to the underside of the bus. Such changes are necessary, for example, to correct jitter which could disturb observations when the thrusters are fired. The reaction wheels are also used to rotate the spacecraft slowly as it moves round its orbit so that the instruments or antenna are kept pointing in the right direction.