Airbus Defence and Space

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The Galileo navigation system will comprise 30 satellites in total, plus the associated ground infrastructure. Twenty-seven satellites will be required to operate, while three others will be kept on stand-by as reserves. The satellites will circle the Earth on three orbital planes at an altitude of approximately 23,300 km.


The Galileo system

The Galileo navigation system will comprise 30 satellites in total, plus the associated ground infrastructure. Twenty-seven satellites will be required to operate, while three others will be kept on stand-by as reserves. The satellites will circle the Earth on three orbital planes at an altitude of approximately 23,300 km. They will transmit time signals and the co-ordinates of their orbital position to the ground receivers, which use the signal propagation time to determine their distance from the satellite and therefore their position relative to the surface of the globe. To calculate its exact position, a ground receiver needs to receive signals from at least four satellites. The ability to determine a position accurately worldwide to within one metre calls for technology that has not previously existed in space. The in-orbit validation (IOV) phase is an important milestone in setting up the system and is laying the foundations for Galileo.

GIOVE-B is the first satellite to carry actual Galileo technology on board, and thus leads on directly to the IOV phase of the European satellite navigation system. It is equipped with completely new instruments and standards, which have now impressively demonstrated their distinct advantages in space.

 

The hydrogen maser



In addition to conventional rubidium clocks, a space passive hydrogen maser is being used for the first time. This atomic clock is the most precise timekeeper ever to have been used in space and is the key to the greater accuracy that Europe’s navigation system will offer compared to the American GPS. In theory, this maser only errs by one second every million years.

This atomic clock is the ideal companion for any navigation satellite when it comes to ensuring the best possible positioning accuracy. Car drivers who have so far relied on GPS to guide them can now look forward to even more reliable navigation systems. The more accurately satellites and ground stations are temporally coordinated with one another, the more precise the maps and directions become. A deviation of just one millisecond, i.e. one thousandth of a second, in space results in an error of 300 metres on Earth.

The in-orbit behaviour of the GIOVE-B maser has proven to be outstanding, boosting confidence that this and other critical new technologies will deliver the superior performance expected of the Galileo system.

 

Securing the Galileo signals

Galileo will be the first set of satellites to be sent by Europe into a medium earth orbit (MEO). Although this orbit is particularly stable, it entails a greater exposure to radiation than a traditional geostationary orbit. In order to determine the extent of this burden, GIOVE-B is also carrying radiation gauges. At the heart of GIOVE-B’s navigation payload, and that of the subsequent Galileo satellites, is a signal generator (NSGU) which produces the navigation signals required to seize and use the reserved transmission frequencies – the satellites’ main task. This will guarantee an uninterrupted signal up until the in-orbit validation phase, even after the GIOVE-A satellite launched in 2005 as a pure frequency holder has reached the end of its service life. Galileo will use the frequency bands L1, E5 and E6. GIOVE-B already possesses full broadcasting ability for the signals that are actually applicable to the complete system. The Multiplexed Binary Offset Coding, in particular, which was agreed upon by the USA and the European Union, is already being implemented by GIOVE-B today. This code will form the overall basis for ensuring the interoperability and compatibility of the Galileo system with the next generation of GPS satellites.

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