Galileo Architecture

GALILEO
Title	Galileo Architecture
Author(s)	GMV
Level	Basic
Year of Publication	2011

The Galileo System will be an independent, global, European-controlled, satellite-based navigation system and will provide a number of guaranteed services to users equipped with Galileo-compatible receivers.

To ensure GALILEO services, a specific architecture is deployed, that will consist of 30 satellites, to be deployed in a staggered approach, and the associated ground infrastructure. The Galileo system is divided into three major segments: Space Segment, Ground Segment and User Segment.

Introduction

Galileo Architecture

The Galileo infrastructure will be composed of:

• A constellation of 30 satellites in Medium-Earth Orbit (MEO). Each satellite will contain a navigation payload and a search and rescue transponder;
• a global network of Galileo Sensor Stations (GSS) providing coverage for clock synchronisation and orbit measurements;
• two Control Centers and two Launch Early Operations (LEOP) Centers;
• a network of Mission Uplink stations;
• several Telemetry, Tracking and Control (TT&C) stations.

This infrastructure is organized in two segments, the Space Segment and the Ground Segment, to be complemented by the users receivers, which compose the User Segment.

GALILEO Space Segment

Galileo Space Segment

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 kilometers. 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.

Based on the award of the contracts for the first order of satellites, the launch services, the system support services and the operations, the European Commission announced the Initial Operational Capability (IOC) with 18 operational satellites and three initial services to be provided in 2014/2015: an initial Open Service, an initial Public Regulated Service and an initial Search and Rescue Service. At this stage, accuracy and availability will not yet have reached their optimum level, the Safety-of-Life Service and the Commercial Service will be tested and will be provided as the system reaches full operational capability with the 30 satellites.

Highly accurate atomic clocks are installed on these satellites. Each of the 30 satellites in the Galileo system will have two of each type of clock on board, a rubidium and a hydrogen maser clock. The frequency is at around 6 GHz for the rubidium clock and at around 1.4 GHz for the hydrogen clock. The Galileo system uses the clock frequency as a very stable reference by which other units can generate the accurate signals that the Galileo satellites will broadcast. The broadcast signals will also provide a reference by which the less stable user receiver clocks can continuously reset their time.

GALILEO Ground Segment

Galileo Ground Segment

The Galileo 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 Centers (GCC), five Telemetry, Tracking and Control (TT&C) stations, nine Mission Uplink Stations (ULS), and about thirty Galileo Sensor Stations (GSS).

The Galileo Ground Segment is decomposed into the Galileo Control System and the Galileo Mission System. The Galileo Control System (GCS), as the responsible for satellite constellation control and management of Galileo satellites, provides the telemetry, telecommand and control function for the whole Galileo satellite constellation. The Galileo Mission System (GMS) is responsible for the determination and uplink of navigation and integrity data messages needed to provide the navigation and UTC time transfer service.

GALILEO User Segment

The Galileo User Segment is composed by 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.

Three receiver development activities have been initiated within the Galileo programme, addressing the different needs of the system development process and covering the range of signals and services that will be offered. Activities in receiver development are in the following areas:

1. test user segment;
2. receivers for the signals transmitted by the first experimental satellites;
3. receivers for the Galileo receiver chain.