Galileo (European Union)

Figure 35 Galileo Galilei
Figure 35 Galileo Galilei (Portrait by Justus Sustermans)

“Measure what is measurable, and make measurable what is not so.”

In May 1999, a mountaineering expedition carried a GPS receiver to the summit of Mount Everest, allowing them to accurately measure its elevation at 8,850 metres (29,035 ft). We think Galileo, the Italian physicist, would have been happy.

Galileo, Europe’s Global Navigation Satellite System, provides a highly accurate and guaranteed global positioning service under civilian control. The U.S. and European Union have been cooperating since 2004 to ensure that GPS and Galileo are compatible and interoperable at the user level.

Figure 36 Galileo satellite in orbit
Figure 36 Galileo satellite in orbit (European Space Agency)
Satellites 24 operational
Orbital planes 3
Orbital inclination 56 degrees
Orbit radius 23,222 km
(14,430 miles)
Table 9 Galileo satellite constellation

Galileo guarantees availability of service under all but the most extreme circumstances, and it will inform users, within seconds, of a failure of any satellite. This makes it suitable for applications where safety is crucial, such as in air and ground transportation.

The first experimental Galileo satellite, part of the Galileo System Test Bed (GSTB) was launched in December 2005, and a second in April 2008. The purpose of these experimental satellites was to characterise critical Galileo technologies, which were already in development under European Space Agency (ESA) contracts. Four in-orbit validation satellites were launched in 2011 and 2012 to validate the basic Galileo space and ground segment.

System design

The Galileo space segment is summarised in Table 9. Galileo navigation signals provide coverage at all latitudes. The large number of satellites, together with the optimisation of the constellation and the availability of the three active spare satellites, ensures that the loss of one satellite has no discernible effect on the user segment.

Two Galileo Control Centres (GCC), located in Europe, control the Galileo satellites. Data recovered by a global network of thirty Galileo Sensor Stations (GSS) is sent to the GCC through a redundant communications network. The GCCs use the data from the sensor stations to compute integrity information and to synchronise satellite time with ground station clocks. Control centres communicate with the satellites through uplink stations, which are installed around the world.

Galileo provides a global Search and Rescue (SAR) function based on the operational search and rescue satellite-aided Cospas-Sarsat3 system. To do this, each Galileo satellite is equipped with a transponder that transfers distress signals to the Rescue Coordination Centre (RCC), which then initiates the rescue operation. At the same time, the system provides a signal to the user, informing them that their situation has been detected and that help is underway. This latter feature is new and is considered a major upgrade over existing systems, which do not provide feedback to the user.

Galileo signals

Table 10 provides further information about Galileo signals.

Galileo services

Five Galileo services are available, as summarised in Table 11.

Designation Frequency Description
E1 A 1575.42 MHz Public regulated service signal.
E1 B Safety-of-life and open service signal (data).
E1 C Safety-of-life and open service signal (dataless).
E5a I 1176.45 MHz Open service signal (data).
E5a Q Open service signal (dataless).
E5b I 1207.14 MHz Safety-of-life and open service signal (data).
E5b Q Safety-of-life and open service signal (dataless).
AltBOC 1191.795 MHz Combined E5a/E5b signal.
E6 A 1278.75 MHz Public regulated service signal.
E6 B High accuracy service signal (data).
E6 C Commercial service signal (dataless).
Table 10 Galileo signal characteristics
Service Description
Free Open Service (OS) Provides positioning, navigation and precise timing service. It is available for use by any person with a Galileo receiver. No authorisation is required to access this service.
Highly Reliable Commercial Service (CS) Service providers can provide added-value services, for which they can charge the end customer. The CS signal provides high data throughput and accurate authenticated data relating to these additional commercial services.
Safety-of-Life Service (SOL) Improves on the Open Service by providing timely warnings to users when it fails to meet certain margins of accuracy. A service guarantee is provided for this service.
Government Encrypted Public Regulated Service (PRS) Highly encrypted restricted-access service offered to government agencies that require a high availability navigation signal.
High Accuracy Service (HAS) HAS is a PPP service with orbit corrections, clock corrections, between-signal biases (code, phase) and atmospheric corrections. It would include corrections for all Galileo signals and GPS L1, L2C and L5. The target is 20 cm (8 inches) 95% horizontal user error within 300 seconds, everywhere and for free. It is broadcast on E6. This service is planned for SIS testing in 2021, initial service in 2022 and full service in 2024.
Open Service Navigation Message Authentication (OSNMA) Galileo will provide OSNMA service to allow users to authenticate GNSS navigation data. This service has been tested since 2021 and is expected to be in service in 2023.
Table 11 Galileo services

3 Cospas-Sarsat is an international satellite-based Search And Rescue (SAR) distress alert detection and information distribution system, established by Canada, France, United States and the former Soviet Union in 1979.

Chapter 3: GNSS constellations