Galileo Orbit Constellation

Galileo Overview

Galileo will be Europe’s own global navigation satellite system, providing a highly accurate, guaranteed global positioning service under civilian control. It will be inter-operable with GPS and GLONASS, the two other global satellite navigation systems. A user will be able to take a position with the same receiver from any of the satellites in any combination. By offering dual frequencies as standard, however, Galileo will deliver real-time positioning accuracy down to the metre range, which is unprecedented for a publicly available system. It will guarantee availability of the service under all but the most extreme circumstances and will inform users within seconds of a failure of any satellite. This will make it suitable for applications where safety is crucial, such as running trains, guiding cars and landing aircraft.

Galileo Experimental Satellites (GIOVE) Overview

The first experimental satellite, GIOVE-A, was launched on 28 December 2005. The objective of this satellite is to characterize the critical technologies, which have already been developed under ESA contracts.

The second experimental satellite, GIOVE-B, was launched on 27 April 2008. This satellites closely resembles the envisioned Galileo satellites. Its objective is the further characterize and test the critical technologies. In the case of GIOVE-B the special highlight is the passive hydrogen maser (PHM). An ultra-stable clock which holds a lot of promise for future GNSS systems in general and for Galileo in particular.

One further experimental satellites is in the pipeline: GIOVE-A2. The actual launch date of this satellite will be decided later, taking into account the situation of GIOVE-A and GIOVE-B.

Galileo Constellation Overview

The fully deployed Galileo system will consist out of 30 satellites, 27 operational satellite and 3 active in orbit spares. The Galileo satellites will be positioned in three circular Medium Earth Orbital (MEO) planes with an altitude of 23222 km above the Earth. This means the Galileo satellites are above the GPS and Glonass satelites. The so called "repeat cycle" for the Galileo satellite orbits is 10 days. The orbital revolution period being 14 hours and 7 minutes. The inclination of the orbital planes will be 56 degrees with reference to the equatorial plane. In full orbit constellation (FOC) the Galileo navigation signals will provide good coverage even at latitudes up to 75 degrees north, which corresponds to the North Cape, and beyond. The large number of satellites together with the optimisation of the constellation, and the availability of three active spare satellites, will ensure that the loss of one satellite does not effect the end users of the system.

Galileo Signal Overview

Just like for GPS and GLONASS all Galileo signals transmitted by the satellite are derived from the fundamental frequency (f0) of the satellite oscillator. However, Galileo will transmit signals on three different frequencies, f1, f2, and f5. On these frequencies are relatively large set of different signals is broadcasted giving rise the a relatively large set of pseudo range and carrier phase observations. Some signals are modulated to contain the navigation messages to transmit information such as the readings of the satellite clocks, the orbital parameters, etc. 

Altogether Galileo will provide five levels of services with guaranteed quality which marks the difference from this first complete civil positioning system.

As an interesting new feature compared to GPS and GLONASS, Galileo will provide a global Search and Rescue (SAR) function, based on the operational COSPAS-SARSAT system. Each Galileo satellite will be equipped with a transponder that is able to transfer the distress signals from the user transmitters to the Rescue Coordination Centre, which will then initiate the rescue operation. At the same time, the system will provide a signal to the user, informing him that his situation has been detected and that help is under way. This latter feature is new and is considered a major upgrade compared to the existing system, which does not provide feedback to the user.