Galileo is a Global Navigation Satellite System (GNSS) initiated by the European Union (EU) and the European Space Agency (ESA) for providing a highly accurate, guaranteed global positioning service under civilian control (cf., e.g., ESA homepage). As an independent navigation system, Galileo will meanwhile be interoperable with the two other global satellite navigation systems, GPS and GLONASS. A user will be able to position with the same receiver from any of the satellites in any combination. Galileo will guarantee availability of service with higher accuracy.
The first Galileo satellite, which has the size of 2.7 x 1.2 x 1.1 m and weight of 650 kg, was launched in December 2005, and the system will be fully operational in 2010~2012. The Galileo constellation consists of 30 Medium Earth Orbit (MEO) satellites in three orbital planes with nine equally spaced operational satellites in each plane plus one inactive spare satellite. The ascending nodes of the orbital planes are equally spaced by 120 degrees. The orbital planes are inclined 56 degrees. Each Galileo satellite is in a nearly circular orbit with semi-major axis of 29 600 km (cf. ESA homepage) and a period of about 14 hours. The Galileo satellite rotates about its Earth-pointing axis so that the flat surface of the solar arrays always faces the Sun to collect maximum solar energy. The deployed solar arrays span 13 m. The antennas always point towards the Earth.
The Galileo satellite has four clocks, two of each type (passive maser and rubidium, stabilities: 0.45 ns and 1.8 ns over 12 hours, respectively). At any time, only one of each type is operating. The operating maser clock produces the reference frequency from which the navigation signal is generated. If the maser clock were to fail, the operating rubidium clock would take over instantaneously and the two reserve clocks would start up. The second maser clock would take the place of the rubidium clock after a few days when it is fully operational. The rubidium clock would then go on stand-by or reserve again. In this way, the Galileo satellite is guaranteed to generate a navigation signal at all times.
Galileo will provide ten navigation signals in the Right Hand Circular Polarization (RHCP) in the frequency ranges 1164-1215 MHz (E5a and E5b), 1215-1 300 MHz (E6) and 1559-1592 MHz (E2-L1-E1) (cf. Hein et al. 2004). The interoperability and compatibility of Galileo and GPS is realized by having two common centre frequencies in E5a/L5 and L1 as well as adequate geodetic coordinate and time reference frames.
