Galileo General Introduction

GALILEO
Title	Galileo General Introduction
Edited by	GMV
Level	Basic
Year of Publication	2011

Galileo is Europe’s own global navigation satellite system, providing a highly accurate, guaranteed global positioning service under civilian control. It is inter-operable with GPS and GLONASS. Galileo receivers compute their position in the Galileo Reference System using satellite technology and based on triangulation principles.

Introduction

Galileo Constelation (artistic interpretation)

The Galileo program is Europe's initiative for a state-of-the-art global satellite navigation system, providing a highly accurate, guaranteed global positioning service under civilian control. While providing autonomous navigation and positioning services, Galileo will be interoperable with other GNSS systems such as GPS and GLONASS. The system will consist of 30 satellites, to be deployed in a staggered approach, and the associated ground infrastructure.

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, Galileo will deliver real-time positioning accuracy down to the meter range. It will guarantee availability of the service under the most extreme circumstances and will inform users of a failure of any satellite.

GPS (US), GLONASS (Russia), BeiDou (China) and the regional system developed by Japan (QZSS) are military systems under military control – indeed they provide a civil service but that civil service could be either switched off or made less precise when desired (e.g. in case of conflict). The world has become so dependent on services provided by satellite navigation in our daily lives that should a service be reduced or switched off, the potential disruption to business, banking, transport, aviation, communication, etc., to name but a few, would be very costly (e.g. in terms of revenues for business, road safety, etc.).

The combination of Galileo and GPS signals (inter-operability) in dual receivers will open the door to new GNSS applications that require a higher level of precision than currently available with GPS alone. From most locations, six to eight Galileo satellites will be visible which, in combination with GPS signals, will allow positions to be determined up to within a few centimetres. Examples of these applications are: guide the blind, increase the success rate of rescue operations in the mountains, monitor the whereabouts of people suffering from Alzheimer Disease, etc.

In addition, Galileo will improve the overall availability and coverage of GNSS signals. For example, the higher number of satellites will improve the availability of the signals in high rise cities where buildings can obstruct signals from satellites that are low on the horizon.

With Galileo, Europe will be able to exploit the opportunities provided by satellite navigation to the full extent. GNSS receiver and equipment manufacturers, application providers and service operators will benefit from novel business opportunities.

History and Development

As far back as the 1990s, the European Union saw the need for Europe to have its own global satellite navigation system. The conclusion to build one was taken in similar spirit to decisions in the 1970s to embark on other well-known European endeavours, such as the Ariane launcher and the Airbus. The European Commission and European Space Agency joined forces to build Galileo, an independent European system under civilian control.

The definition phase and the development and In-Orbit Validation phase of the Galileo program were carried out by the European Space Agency (ESA) and co-funded by ESA and the European Union. The Full Operational Capability phase of the Galileo program is fully funded by the European Union and managed by the European Commission. The Commission and ESA have signed a delegation agreement by which ESA acts as design and procurement agent on behalf of the Commission.

The Galileo program has been structured according to three main phases: In-Orbit Validation (IOV), Initial Operational Capability (IOC) and Full Operational Capability (FOC) phases.

GALILEO Services

The Galileo mission and services have been elaborated during the initial definition phase in consultation with user communities and the Member States. The services that are planned to be provided by Galileo are the following:

• Open Service (OS): With positioning accurate to one metre, the freely accessible Open Service targets the mass market and is intended for motor vehicle navigation and location-based mobile telephone services. Free to the user, it provides positioning and synchronization information intended for high-volume satellite radio navigation applications;
• Safety-of-Life Service (SoL): The Safety of Life service will automatically inform users of a failure of any satellite or similar problem affecting performance. This service is already available for aviation following the ICAO standard thanks to EGNOS, Galileo will further improve this service performance and will be made available for safety-critical applications such as running trains, guiding cars, navigation and aviation. This service also fulfills the requirements of certain sectors for continuity, availability and accuracy and includes an integrity function alerting the user of any failure in the system;
• Commercial Service (CS): Encrypted and accurate to the nearest centimetre, the Commercial Service allows for development of applications for professional or commercial use owing to improved performance and data with greater added value than that obtained through the open service;
• Public Regulated Service (PRS): The Public Regulated Service is restricted to government-authorised users, for sensitive applications which require a high level of service continuity. It will be encrypted and designed to be more robust, with anti-jamming mechanisms and reliable problem detection. This service is intended for security and strategic infrastructure (e.g. energy, telecommunications and finance);
• Search and Rescue Service (SAR): Galileo's worldwide search and rescue service will help to forward distress signals to a rescue coordination centre by detecting emergency signals transmitted by beacons and relaying messages to them.

GALILEO Architecture

Galileo Space Segment

To ensure these Galileo services, a specific architecture is deployed. The Galileo system is divided into three major segments: Space Segment, Control Segment and User Segment. For details see Galileo Architecture.

The main functions of the Galileo Space Segment are to generate and transmit code and carrier phase signals with a specific Galileo signal structure, and to store and retransmit the navigation message sent by the Control Segment. These transmissions are controlled by highly stable atomic clocks on board the satellites.

When Galileo is fully operational, there will be 30 satellites in Medium Earth Orbit (MEO) at an altitude of 23,222 kilometres. The satellites will occupy each of three orbital planes inclined at an angle of 56° to the equator. The satellites will be spread evenly around each plane and will take about 14 hours to orbit the Earth. One satellite in each plane will be a spare; on stand-by should any operational satellite fail.

The Galileo Control Segment (also referred to as Ground Segment) is the responsible for the proper operation of the GNSS system. Its basic functions are:

• To control and maintain the status and configuration of the satellite constellation.
• To predict ephemeris and satellite clock evolution.
• To keep the corresponding GNSS time scale (through atomic clocks).
• To update the navigation messages for all the satellites.

The Galileo Ground Segment constitutes the major system element controlling the entire constellation, the navigation system facilities and the dissemination services. It is composed of two Ground Control Centres (GCC), a network of Telemetry, Tracking and Control (TT&C) stations, a network of Mission Uplink Stations (ULS), and a network of Galileo Sensor Stations (GSS).

The Galileo user segment is composed of Galileo receivers. Their main function is to receive Galileo signals, determine pseudoranges (and other observables), and solve the navigation equations in order to obtain their coordinates and provide a very accurate time.

Galileo Signal Characteristics

The Galileo navigation Signals are transmitted in the four frequency bands indicated in next figure. These four frequency bands are the E5a, E5b, E6 and E1 bands. They provide a wide bandwidth for the transmission of the Galileo Signals.

Galileo Frequency Plan

The Galileo frequency bands have been selected in the allocated spectrum for Radio Navigation Satellite Services (RNSS). In addition to that, E5a, E5b and E1 bands are included in the allocated spectrum for Aeronautical Radio Navigation Services (ARNS), employed by Civil-Aviation users, and allowing dedicated safety-critical applications. The names of the Galileo signals are the same as the corresponding carrier frequencies. Note that E5a and E5b signals are part of the E5 bandwidth.

GALILEO Performances

The Galileo performances are different for each service. For the Galileo Open Service (OS) no specific requirements of integrity are applicable. The performances for horizontal positioning accuracy at 95% for a dual-frequency receiver are 4 m (8 m for vertical accuracy), with an availability of the service of 99%.

In the case of the Galileo Safety of Life (SoL) and the Galileo Public Regulated Service (PRS), the performance requirements include horizontal and vertical accuracy, integrity, continuity and time to alert for different service levels. The availability of the service should be 99.5% for both services.

See the article Galileo Performances for further information.

Credits

Edited by GMV, using information from ESA and European Union as indicated through the references.