Overview of Galileo System

Abstract

This chapter provides an introduction to the Galileo program and architecture. It starts by presenting the program context, rationale and history, including the early definition phases and test beds and the GIOVE experimental satellites. It then presents an overview of the Galileo services. Later, an architectural overview is provided, including the Galileo segments: the Space Segment, the Ground Mission Segment, and the Ground Control Segment. The chapter also provides a description of Galileo’s contribution to the Search And Rescue services through COSPAS/SARSAT, and finalizes with an overview of the user segment and highlighting interoperability and compatibility issues with other GNSS.

Full text

Chapter 2
Overview of Galileo System
Javier Pérez Bartolomé, Xavier Maufroid,
Ignacio Fernández Hernández, JoséA. López Salcedo
and Gonzalo Seco Granados
Abstract This chapter provides an introduction to the Galileo program and
architecture. It starts by presenting the program context, rationale and history,
including the early definition phases and test beds and the GIOVE experimental
satellites. It then presents an overview of the Galileo services. Later, an architectural
overview is provided, including the Galileo segments: the Space Segment, the
Ground Mission Segment, and the Ground Control Segment. The chapter also
provides a description of Galileo’s contribution to the Search And Rescue services
through COSPAS/SARSAT, and finalizes with an overview of the user segment
and highlighting interoperability and compatibility issues with other GNSS.
Galileo Program Context, History and Implementation
Phases
Institutional Context of the Galileo Program
As the reader can imagine, the process of developing Galileo has not been easy. In
exchange, the Galileo program is, apart from a satellite navigation system whose
signals will be received worldwide, the evidence that different sovereign nations
can collaborate for the benefit of the EU and the world. Galileo has pioneered the
cooperation in EU space industries to develop a highly complex operational and
globally distributed infrastructure.
The context of the European Union under which Galileo has been conceived and
developed is a very important aspect that has driven the program significantly.
Saying that Galileo has been developed by the twenty-seven Member States that
J.P. Bartolomé (&)X. Maufroid I.F. Hernández
European Commission, DG Enterprise and Industry, Brussels, Belgium
e-mail: Javier.PEREZ-BARTOLOME@ec.europa.eu
J.A. López Salcedo G.S. Granados
Universitat Autònoma de Barcelona, Barcelona, Spain
©Springer Science+Business Media Dordrecht 2015
J. Nurmi et al. (eds.), GALILEO Positioning Technology,
Signals and Communication Technology 182, DOI 10.1007/978-94-007-1830-2_2
9

form the European Union would already be a simplification of the facts. Galileo is
therefore different to any other system like GPS, GLONASS and Beidou, which
are developed under a single nation (the United States of America, the Russian
Federation and the People’s Republic of China, respectively). Perhaps this does not
represent a difference from the technical point of view or from a usage perspective,
but it represents a big challenge for the program. The program management had to
reconcile the views of many sovereign states, liaise with independent intergov-
ernmental bodies, and deal with intricate decision processes.
The European Union is the main sponsor and the owner of Galileo, according to
the current regulation (Europeon Commission 2008). Whereas the program was
financed jointly with ESA in the first stages, its full deployment has been financed
with European Union budget funds. The European Commission, as the executive
arm of the European Union, is the program manager. As any other undertakings of
the European Union, the EU Member States are the ultimate stakeholders of the
program and play a major role in the decision-making process. The European
Commission reports to them regularly on the program developments through a
dedicated forum called the GNSS Programmes Committee. Many European
Member States also support the Galileo program through dedicated activities per-
formed by their national space agencies, as the French Centre National d’Etudes
Spatiales (CNES), the German Deutsches Zentrum für Luft-und Raumfahrt (DLR),
the Italian Agenzia Spaziale Italiana (ASI) or the recently created National Space
Agency in the UK, just to name some. Other national ministries, as the Transport or
Defence ones, are also involved. More details about the EU functioning and its
Member States can be found in Europeon Commission (2007).
The European Space Agency has the leading role in the technical direction of the
program. After pioneering research in satellite navigation in Europe in the late 1990s
and successfully demonstrating the program’s technical feasibility over the first dec-
ade of the 21st century, ESA is currently responsible for the development, deployment,
integration and operational validation of the Galileo system infrastructure.
As the European Commission’s main activities are related to policymaking and
EU budget administration, as part of its normal functioning it is assisted by
Community Agencies which deal with more specific tasks. In case of the GNSS
programs, the agency in charge of assisting the EC is the GSA (former GNSS
Supervisory Authority, now European GNSS Agency). The role of the GSA and the
EC have slightly evolved over the years, and at the time of writing the GSA is
foreseen to be in charge of the Galileo service provision and exploitation, market
development and security.
The Early Stages
In the early nineties, the European Union started to consider the development of its
own satellite navigation system, first through the deployment of a regional infra-
structure, called GNSS-1 at the time, and later by developing its own global system,
10 J.P. Bartolomé et al.

GNSS-2. A formal agreement was concluded on 18 June 1996 between the
European Community, Eurocontrol and ESA for the development of GNSS-1,
which would become European Geostationary Navigation Overlay Service
(EGNOS), an SBAS or satellite-based augmentation system aimed at augmenting
GPS to improve air navigation operations. GNSS-2 would later become Galileo. In
1995, at the time the GNSS-2 program was being outlined, GPS had just declared
its Full Operational Capability, and the U.S. Government had already committed to
provide the GPS signals to the civil user community (Kaplan and Hegarty 2006;
Pace et al. 1995), although the selective availability, a functionality to intentionally
degrade GPS position accuracy for unauthorized users, was still on. In this context,
the European Commission’s Communication “Towards a Trans-European Posi-
tioning and Navigation Network—including a European strategy for GNSS”
(Kaplan and Hegarty 2006) issued in 1998 formally opened a serious debate for
Europe to develop its own GNSS.
In 1999, the European Commission, with the support of the European Space
Agency (ESA), prepared the Communication “Galileo—Involving Europe in a New
Generation of Satellite Navigation services”(Europeon Commission 1999) that was
seminal to the GNSS program development in Europe. Three reasons for the EU to
develop its own system were stated in this Communication:
•To increase control on satellite-based safety-critical navigation systems.
•To ensure a positioning service for European users in the long term, not subject
to the risk of potential U.S. policy changes affecting GPS.
•To support EU industry competitiveness in the global market of satellite navi-
gation and grant access to the system’s technological developments.
The Communication presented the results of consultations with worldwide
stakeholders to define how the EU satellite navigation system would look alike.
Influenced by ESA GNSS-2 Comparative System Studies (ESA 1998) and EC
GNSS-2 Forum (Fairbanks 1999), it proposed to develop a system very similar to
GPS or GLONASS to minimize technical risk and provide the highest value for
money. This included a signal structure compatible and interoperable with GPS as
much as possible.
Once the political drive to build Galileo was clear, and its basic principles
outlined, the EC and ESA embarked in the first studies to define the Galileo mission
and system requirements that would ultimately determine the Galileo system and
receiver technologies. The EC formed the Galileo Task Force (GTF) and launched
the GALA project to define the future Galileo service levels and receiver functional
concepts, and ESA launched the GalileoSat program to support the definition of
ground and space infrastructure and study the signal design and transmission per-
formance (Schweikert et al. 2000; De Gaudenzi et al. 2000).
In parallel, a considerable effort in international cooperation and research was
carried out in the early 2000s to agree on the spectrum allocation, carrier fre-
quencies selection, signal design, code selection and timing and geodetic references
for Galileo, in order to make Galileo and GPS, the only operational system at the
2 Overview of Galileo System 11

time, as compatible and interoperable as possible. In the frame of the ONU Inter-
national Telecommunications Union (ITU), these efforts eventually led to the
allocation of the Galileo frequency plan in June 2000 by the World Radio Com-
munications Conference held at Istanbul, granting protection until June 2006. Some
years later, in 2004, the EU and the U.S. signed a Cooperation Agreement “On The
Promotion, Provision And Use Of Galileo And GPS Satellite-Based Navigation
Systems And Related Applications”(United States of America and European
Community 2004) that set the framework to achieve full interoperability and radio
frequency compatibility between both systems.
Galileo Early Technology Demonstrator
Several years of development and qualification of critical technologies have been
necessary for the deployment of an operational system like Galileo. This is par-
ticularly true for the satellite on-board clocks. In the late 1990s, the European Space
Agency started the development of the Rubidium Atomic Frequency Standard
(RAFS) and the Passive Hydrogen Maser (PHMs) that would be integrated in the
satellites and, in early 2000s, these two technologies were environmentally quali-
fied on-ground.
Later on, ESA launched in 2002 the GSTB-V1 (Galileo System Test Bed 1)
program aimed at developing an experimental ground mission segment for the
validation of Galileo navigation and integrity determination algorithms and products
based on raw GPS measurements collected by a global network of sensor stations.
In 2003, the European Space Agency began the development of two test sat-
ellites, GIOVE-A and GIOVE-B (Galileo In-Orbit Validation Element), as part of
the GSTB-V2 (Galileo System Test Bed 2) program.
The GIOVE-A satellite was built by SSTL and launched in December 2005. The
satellite launch mass was about 600 kg for a total power of 700 W. The satellite was
designed to transmit simultaneously 2 out of the 3 frequencies allocated to Galileo
and included 2 cold redundant RAFS with a stability of 10 ns per day. A major
Galileo program milestone was achieved on the 12th January 2006, when GIOVE-
A transmitted for the first time a Galileo-like signal in space (SIS) towards the Earth
from orbit, several months in advance of the ITU filing protection expiration date.
The GIOVE-B satellite was built by a consortium led by Astrium GmbH and
was launched in April 2008. The satellite launch mass was about 530 kg for a total
power of 1100 W. The satellite included 1 PHM and 2 RAFS. It was also the first
satellite to transmit the Multiplexed Binary Offset Carrier modulation (MBOC), the
latest signal waveform agreed between the European Union and the United States.
Both satellites were initially designed for a lifetime of about 2 years but thanks
to very good performances at the end of their lifetime, their missions were exten-
ded. GIOVE-A satellite lasted more than 6 years in-orbit before being finally
decommissioned on 30 June 2012. GIOVE-B was decommissioned on 23 July
2012 (Fig. 2.1).
12 J.P. Bartolomé et al.

Galileo Implementation Phases
During the early definition phase, a staggered approach was adopted for the
development, deployment, integration and validation of the Galileo system infra-
structure. Two major implementation phases were considered, namely:
1. The In-Orbit Validation phase (IOV),
whose main goal was the end-to-end validation of the Galileo service concept
based on a mini constellation with four operational Galileo spacecraft and a
limited ground system configuration.
2. The Full Operational Capability phase (FOC),
intended to complete the deployment of the Galileo constellation and ground
infrastructure and achieve full operational validation and service performance.
More details on the satellites and ground infrastructure of Galileo IOV and FOC
phases are provided in the following sections on the Galileo segments.
Galileo Services
Like other GNSS systems such as GPS or GLONASS, the Galileo navigation
concept relies on the measurement of the time of arrival (TOA) of electromagnetic
signals transmitted from Medium Earth Orbiting (MEO) satellites. As the signals
are synchronously transmitted by the satellites, the minimum number of indepen-
dent measurements (i.e. transmitters) required to compute a 3D position is four, to
account for the three-dimensional position unknowns plus the unknown offset of
the receiver clock, which is not supposed to be synchronized with the GNSS and
affects all measurements equally.
In addition to the time of arrival of the synchronized signals, the receiver needs
to know the position of the transmitters at the exact time the signals were
Fig. 2.1 GIOVE-A (left) and GIOVE-B (right), artist’s impression, courtesy of ESA
2 Overview of Galileo System 13

transmitted, in order to compute a position. More details on GNSS navigation
principles and the related equations can be found in most satellite navigation ref-
erences in the literature, such as (Kaplan and Hegarty 2006; Misra and Enge 2011
or Spilker and Parkinson 1996).
In order to maximize the potential user base and the potential benefits that
Galileo could offer, Galileo has been developed to provide different services, all
based on the TOA positioning method as described above, and some of them with
additional features, such as signal encryption, digital signature authentication or
search and rescue services. The Galileo services are briefly introduced hereafter:
•Open Service: The Open Service (OS) provides positioning and timing infor-
mation worldwide through ranging signals and data broadcast by the Galileo
constellation. The detailed definition of the Galileo OS signals is publicly
available and can be found in Europeon Union 2010. The OS will be accessible
free of charge by any user equipped with a Galileo compatible navigation
receiver. It will be provided in the E1 and E5 bands, and it will be comparable to
the service offered by GPS open civil signals L1C/A, L2C or L5.
•Public Regulated Service: The Public Regulated Service (PRS) provides
positioning and timing information worldwide through ranging signals and PRS
data broadcast by the Galileo constellation. The access to the PRS will be
restricted to government-authorised users, for sensitive applications. The control
access policy is implemented through the encryption of the PRS signals and the
management of decryption keys. The PRS will only be accessible through
receivers equipped with a PRS security module loaded with a valid PRS
decryption key. It will be provided in the E1 and E6 bands and will be similar to
the services offered in the L1 and L2 through the P(Y) and L2M signals by GPS.
•Commercial Service: The Commercial Service (CS) is intended to provide
‘added value’data with respect to the Open Service. At the time this chapter is
being written, the Commercial Service is still under definition. However, it is
already foreseen that these ‘added-value’services are related to high accuracy
and authentication (Fernandez et al. 2014). One of the main features the CS will
bring, with respect to other GNSS, is the capability to broadcast globally
external data in real time. It will be provided in the E6 band.
•Search and Rescue Service: The Search And Rescue (SAR) service is intended
to support the Cospas-Sarsat Program in search-and-rescue operations. Due to its
particularities, more details about this service are given in the following sections.
In addition to the above, the Galileo program foresaw a Safety-Of-Life (SOL)
service, which included the provision of worldwide system integrity. The imple-
mentation of this service has been postponed until later phases of the program, and
will rely on the reuse of regional solutions and a collaborative approach with other
global constellation providers. Another chapter in this book explains in more detail
the Galileo Integrity Concept behind the SOL service, as it was initially conceived.
The Galileo signals will be described in Chap. 3of the book. The Galileo signals
include all advanced features that are considered in modern GNSS such as trans-
mission at several frequencies, secondary codes and pilot components, and usage of
14 J.P. Bartolomé et al.

higher power levels, larger bandwidths and error correction codes. Other features
include higher chip rates, longer pseudo-noise codes and Binary Offset Carrier
(BOC) modulations and variants thereof. All these aspects and their implications
will be reviewed in a subsequent chapter.
Galileo Architecture Overview
The provision of the Galileo signals and services relies on the continuous and
coordinated operation of a network of specialized system facilities covering dif-
ferent functional needs. Figure 2.2 presents the most relevant elements of the
Galileo system infrastructure in an end-to-end service context.
These facilities can be grouped into three main categories: the Galileo Core
Infrastructure, the Galileo Service Facilities and the Galileo Support Facilities.
The Galileo Core Infrastructure (CI) comprises a Medium Earth Orbit (MEO)
satellite constellation continuously transmitting Galileo Signal-in-Space (SIS), i.e.
the Galileo Space Segment, and a global ground system infrastructure providing all
the functionality required to sustain the provision of Galileo navigation services in
an independent manner.
The Galileo CI ground infrastructure comprises two main subsystems or seg-
ments, the Galileo Ground Control Segment (GCS) and the Galileo Ground Mission
RELEVANT EXTERNAL ACTORS
Galileo Programme System Infrastructure
GNSS USER COMMUNITIES
Galileo Compatible User Equipment
End User
Galileo
CS Content
Providers
Competent
PRS
Authority
COSPAS/
SARSAT
RCC
INT. TERRESTRIAL REFERENCE
FRAME STDS
(ITRF)
INT. TIME REFERENCE STDS
(UTC)
GALILEO CORE INFRASTRUCTURE
GALILEO SERVICE
FACILITIES
Galileo
Control
Segment
TSP
GRSP
SAR/Galileo
GS
GSMC
GSC
GALILEO SUPPORT FACILITIES
Galileo Space Segment
Galileo
Mission
Segment
EXTERNAL
GNSS
CS
Users
PRS
Users
SAR
Service
Users
Galileo OS, PRS & CS
SIS
External GNSS
SIS Galileo User Support
Services
MMI
SAR Distress
Signals
OS
Users
Fig. 2.2 Galileo architectural overview
2 Overview of Galileo System 15

Segment (GMS). While the GCS provides the Galileo constellation monitoring and
control functions, the GMS supports the generation and distribution/uplink of
navigation products and other mission data required for the onboard generation of
the navigation messages modulated on some of the Galileo SIS components.
The operations of the Galileo CI are centrally managed from two fully redundant
Ground Control Centres (GCC) located in Oberpfaffenhofen (Germany) and in
Fucino (Italy) respectively.
The principal service offered by Galileo to the end users is the Galileo SIS,
which can be processed by Galileo compatible receiver equipment for accurate
positioning and time determination in the Galileo terrestrial reference frame
(GTRF) and Galileo System Time (GST) scale respectively. Two Galileo Service
Facilities, the so called Geodetic Reference Service Provider and Time Service
Provider, monitor the alignment of GTRF and GST with the international metro-
logical standards (ITRF and UTC) and provide the Galileo CI steering corrections
to ensure a very high level of consistency between reference systems.
Further to the GTRF and the TSP, there are other Galileo Service Facilities to
provide Galileo-related services to the wide public and to specific user communi-
ties. They are the GNSS Service Centre (GSC), the Galileo Security Monitoring
Centre (GSMC) and the SAR Galileo Ground Segment.
The Galileo Support Facilities is a further category of facilities not directly
involved in the routine provision of services but playing an essential role in the
deployment, commissioning and maintenance of Galileo. These include among
others two external satellite control centres supporting the Launch and Early
Operations Phase (LEOP) of each Galileo spacecraft and a ground In Orbit Test
(IOT) station for satellite commissioning operations.
The Galileo end users are represented at the bottom of the Fig. 2.2. Although not
explicitly indicated, most Galileo users will have multi GNSS interoperable
receivers able to track signals from other navigation systems such as GPS. Figure 2.2
indicates as well the main links between the Galileo system facilities, the Galileo
service end users and other actors outside the Galileo system perimeter.
The following sections provide a more detailed description of the Galileo Space
Segment, Ground Mission Segment and Ground Control Segment.
Galileo Space Segment
Galileo Satellite Constellation
At the time of writing, the Galileo reference constellation or constellation standard
foresees 24 nominal orbital positions or operational slots in Medium Earth Orbit
(MEO) homogeneously distributed in three orbital planes (i.e. 8 slots equally
spaced per plane). At the end of the FOC phase all the orbital positions defined in
the constellation standard will be populated with operational Galileo satellites.
Besides the core reference constellation, additional satellites will be deployed on
16 J.P. Bartolomé et al.

each orbital plane in order to ensure the maintenance of the Galileo services upon
satellite outages. At present, there are not reference orbital positions defined for
these in-orbit spare satellites.
It shall be noted that the Galileo constellation standard has experienced some
changes since the Galileo early definition phase. During the constellation design
trade-offs analysis, the number of satellites in the core constellation was essentially
driven by the Safety of Life (SoL) service of Galileo, initially defined as global
integrity service with demanding requirement in terms of time-to-alert, availability
and continuity. In order to meet the stringent SoL service requirements, the initial
Galileo reference constellation was based on 27 orbital positions (Walker 27/3/1).
Following the program decisions in 2012 to re-profile the SoL service, the
impact on the constellation design has been reassessed and several analyses have
shown that a reduced constellation with 24 operational satellites deployed in
Walker 24/3/1 configuration can meet the Galileo services requirements in terms of
accuracy and availability.
The nominal trajectory followed by the operational Galileo satellites is a circular
orbit with a radius of approximately 29,600 km (equivalent to 23,229 km altitude
over the Earth surface) and an orbital period of approximately14 h. This choice
ensures a satellite ground-track repeat cycle every 17 orbits (or 10 days) (Fig. 2.3).
The main orbital parameters of the reference Galileo constellation are summa-
rized below:
•Semimajor axis: 29,600 km
•Eccentricity: 0.0001 (i.e. circular orbit)
•Inclination 56°
•Argument of perigee: ±180°(i.e. not defined for circular orbits)
•RAAN: 0º, 120ºand 240º
Fig. 2.3 Galileo reference
constellation representation—
24 operational satellites
(courtesy of EC)
2 Overview of Galileo System 17

More specifically, the satellites are positioned in a Walker 24/3/1 configuration,
which means that satellites in each plane are equally spaced by 45°and satellites in
adjacent planes are phased by 15°between each other. The satellite plane incli-
nation for Galileo is 56º.
The Right Ascension of the Ascending Node (RAAN) defines the relative
angular phasing between the constellation orbital planes and the vernal equinox.
The True Anomaly indicates the angular position of each satellite within a given
orbital plane.
The first Galileo operational satellites, resulting from the Galileo IOV phase,
were launched respectively on 21st October 2011 (IOV PFM and FM2) and on the
12th October 2012 (FM3 and FM4). Their nominal position can be represented by
the following orbital elements (Table 2.1) for the reference time 1 May 2013 at
00:00:00 UTC (note that the difference in the argument of perigee between 180ºand
0ºis irrelevant as the orbits are almost circular). It must be noted that the orbital
positions assigned to these first 4 IOV Galileo satellites correspond to the former
Walker 27/3/1 reference constellation geometry.
The coordinate reference frame used is the Celestial Intermediate Reference
System CIRS (McCarthy and Petit 2003) (true equator). In order to represent the
RAAN precession, the RAAN has to be modified at a rate of −0.027644°/day while
the True Anomaly evolves at a rate of around 613.7°/day, equivalent to 1.7 revo-
lutions per day, or 17 in 10 days, as mentioned above.
Galileo Satellites
At the time of writing, the four Galileo IOV operational satellites launched from
Kourou (French Guyana) and Soyuz launchers and manufactured by ASTRI-
UM GmbH (now renamed Airbus Defense and Space) have been deployed in their
nominal orbits, as mentioned above. The other 22 additional Galileo operational
satellites are being manufactured by OHB-System AG as part of the Galileo FOC
phase and will be launched with Soyuz and Ariane 5 launchers from Kourou as well.
Table 2.1 Galileo satellite position on May 1st, 2013, 00:00:00 UTC
S/C Position Semi-major
axis
Eccentricity Inclination RAAN Arg.
perigee
True
anomaly
Plane Slot km deg deg deg deg
GSAT0101 B 5 29599.8 0.0001 56.0 113.6 0.0 295.9
GSAT0102 B 6 29599.8 0.0001 56.0 113.6 0.0 335.9
GSAT0103 C 4 29599.8 0.0001 56.0 233.6 0.0 269.2
GSAT0104 C 5 29599.8 0.0001 56.0 233.6 0.0 309.2
Source European GNSS Service Centre website—www.gsc-europa.eu
18 J.P. Bartolomé et al.

The remaining constellation satellites as well as replenishment satellites will be
ordered in 2015 and are expected to be available in the 2018–2019 timeframe.
The Galileo IOV satellites perform essentially the same functions as the FOC
satellites and their respective designs share a number of common elements dealing
with mission critical technologies (e.g. atomic frequency standards, navigation
signal generators, or mission data receivers). However, the transition from IOV to
FOC has also been used to improve the signal performances in terms of effective
radiated power and bandwidth, leading to a change of technology in some units, in
particular the high power amplifiers. The satellite description presented in the
following sections is based on the Galileo FOC satellites, which will constitute the
main part of the Galileo constellation when fully deployed (Table 2.2).
The Galileo satellite in flight configuration and its high-level decomposition
block diagram is shown on Fig. 2.4. The Galileo satellites are made of two main
components: the platform and the payload, which are further decomposed into
modules as shown in the figure. The first four Galileo satellites (called IOV satellites)
exhibit some small differences with regards to the Galileo FOC satellites but are very
similar in terms of functionalities, overall budget envelope and performances.
The Galileo satellites are designed to be launched in dual launch configuration
on Soyuz and in a quadruple launch configuration on Ariane 5. The satellites are
directly injected by the launcher into their final orbit and as a result the propulsion
capabilities of the satellites can be limited to small out-of-plane orbital corrections,
in-plane slot adjustment for constellation spare relocation strategy and graveyarding
operations at spacecraft’s end of life.
In addition to the main navigation and SAR payloads, the Galileo satellite design
includes also the following secondary payloads:
•The Laser Retro-Reflector Array (LRA), a passive instrument that allows
Galileo satellites to be tracked by Satellite Laser Ranging (SLR) stations on the
ground.
•The Environmental Monitoring Unit (EMU) whose main purpose is to measure
the number of heavy ion sand the electric charges in the MEO orbit over the
whole 11-year solar cycle.
Table 2.2 Evolution of galileo satellites from GIOVE-A to Galileo FOC
Parameter GIOVE-A GIOVE-B IOV (PFM-FM4) FOC
(FM5-FM26)
Launch mass 600 kg 530 kg 730 kg 732.8 kg
Total power 700 W 1100 W 1980 W 1900 W
Size 1.3 ×1.8 ×1.65 m 0.95 ×0.95 ×2.4 m 2.7 ×1.2 ×1.1 m 2.5 ×1.2 ×1.1 m
Design
lifetime
27 months 27 months 12 years 12 years
Launch dates 28 Dec 2005
(decommissioned
26 Apr 2008
(decommissioned
PFM/FM2: 21 October
2011
FM5/FM6: Q3 2014
(scheduled)
5 Jun 2012) 23 Jul 2012) FM3/FM4: 12 October
2012
1 launch every
3/4 months (plan)
2 Overview of Galileo System 19

Galileo Ground Mission Segment
The main roles of the Galileo Ground Mission Segment (GMS) are the generation
of the Galileo C-Band uplink signals including the data required in the Galileo
navigation downlink signals and the online monitoring of the downlink navigation
signals in closed-loop.
The main GMS functional chains are the mission data generation chain,
responsible for the generation of the OS and PRS navigation products, and the
mission data distribution chain responsible for the timely distribution of the nav-
igation products to the Galileo satellites. Moreover, the GMS interfaces with the
Galileo Service Facilities briefly introduced previously in this Chapter and it also
exchanges mission data required to deliver the Galileo services.
More specifically, the GMS is responsible for the following main functions:
•Generation and distribution of Galileo System Time (GST) to all elements
within the Galileo Core Infrastructure perimeter, including the satellites, and
support the overall system time steering.
•Generation and distribution to Galileo spacecrafts of mission products for
generation of Galileo OS and PRS navigation messages.
Fig. 2.4 Galileo satellite flight configuration (courtesy of OHB)
20 J.P. Bartolomé et al.

•Distribution to Galileo spacecrafts of mission data forwarded by Galileo Service
Facilities supporting provision of Galileo CS and SAR services.
•Provision of the physical interface for the exchange of data products with
Galileo Service Facilities to support smooth provision of Galileo services.
•Provision of the physical interface for the exchange of data products with the US
Naval Observatory (USNO) for coordinating the generation of GPS Galileo
interoperability mission products defined in the OS navigation message (i.e.
GPS To Galileo Time Offset)
•Galileo mission monitoring and control and archiving
Besides the main service related functions listed above, the GMS supports other
internal support functions related to the management and operations of the ground
infrastructure. The coordination of the system operations both in ground and in orbit
is paramount to ensure the continuity of the Galileo services. In order to support this
coordination as efficiently as possible, the GMS design includes physical interfaces
with the Galileo Ground Control Segment (GCS) at each Galileo Control Centre.
GMS Architecture
The GMS comprises a worldwide network of facilities that include the following
three types of ground elements:
1. Ground Sensor Stations (GSS)
2. Mission Ground Control Centres (GCC)
3. Mission Up-link Local Station (ULS),
A dedicated low latency global telecommunication network ensures the permanent
connectivity of the remote sites (i.e. GSSs and ULSs) with the mission ground control
center for the routing of mission data and system monitoring and control signals.
Figure 2.5 shows the geographical location of the GMS facilities already
deployed or under procurement. It shows also the GCS facilities that are described
later in the chapter.
The Ground Sensor Stations
The GSS is an unmanned GMS facility whose main role is the collection of L-Band
sensor data from all Galileo satellites in view in all frequencies and forward those data
to the GCC for navigation processing and mission monitoring. It is essentially a high-
quality Galileo receiver with some additional elements. The collected sensor data
includes carrier phase and code phase measurements based on the processing of the
pilot components, navigation message symbols demodulated from the data compo-
nents and some signal quality indicators. The GSS can support SIS measurement rates
up to 1 Hz (one measurement every second) in order to ensure continuous SIS
monitoring and, in case of anomaly, reduce the notification time to Galileo users.
2 Overview of Galileo System 21

The GSS are equipped with rubidium atomic frequency standards and high
performance Galileo receivers, which are the core equipment of the Galileo Receiver
Chain (GRC). The GSS design allows hosting two GRC types, the first one sup-
porting the GMS PRS mission data generation chain and the second one supporting
the OS and CS mission data generation chains. In addition, each GSS is equipped
with Dual-Frequency COTS GPS receivers to support site synchronization with
Galileo System Time and realization of the Galileo Terrestrial Reference Frame
during the early system operations phase until the number of Galileo satellites
deployed ensures the system standalone synchronization worldwide.
The Mission Ground Control Centres
The GMS operations are managed from two fully redundant Galileo Ground
Control Centres located in Oberpfaffenhofen (Germany) and Fucino (Italy).
The GCCs centralise several GMS key functions critical for the provision of the
Galileo services such as GST generation, navigation data generation and distribution,
management of the Galileo system interfaces with external entities, and mission
monitoring and control. Outside the perimeter of the GMS, the GCCs play also a
central role in the monitoring and control operations of the Galileo satellite constel-
lation, as explained later in the section devoted to the Ground Control Segment (GCS) .
Fig. 2.5 Galileo GMS and GCS sites
22 J.P. Bartolomé et al.

The Mission Up-Link Local Stations
The continuous routing of Galileo ground mission data from the GCC to each
operational spacecraft is required for the onboard generation of Galileo Signal In
Space with meaningful service information (i.e. navigation messages). The ULSs
are unmanned facilities which realize the physical ground to space interface sup-
porting the mission data distribution functional chain.
Each operational ULS receives mission data from the GCC for uploading to the
Galileo constellation according to a satellite contact plan received also from the
GCC. In order to perform this function, each ULS can host up to four dish steerable
antennas of 3.5-m diameter to upload mission data in the C-Band part of the RF
spectrum (around 5 GHz). Each ULS antenna can track a single Galileo spacecraft
at a time.
The accuracy of the Galileo Positioning Velocity Timing (PVT) services
depends directly on the accuracy of the mission data uploaded to the Galileo
satellites. It shall be noted that a part of the mission data are predictions based on
dynamic or empirical models and therefore their accuracy degrades rapidly with
time (“ageing”effect). This applies in particular to the satellite GST clock offset
prediction broadcast model. In order to meet the minimum Galileo PVT accuracy
performance requirements continuously worldwide, it is essential that the design of
the ULS network allows for refreshing the satellite onboard mission information of
the whole constellation at the required latency on a continuous basis. When the
Galileo ULS network is fully deployed, the maximum time between navigation data
uploads to any Galileo spacecraft should not exceed 100 min under nominal
operational conditions.
Galileo Ground Control Segment
The Galileo GCS is responsible for the management of the Galileo constellation
during the normal operation of the system. For achieving this goal, the GCS can
exchange monitoring and control signals with individual Galileo spacecraft at
scheduled contacts. Further to the monitoring and control data, the GCS can also
upload Galileo mission data through the telecommand uplink channel to ensure the
continuity of Galileo navigation services under degraded GMS operation modes.
GCS Architecture
The GCS elements are located worldwide and comprise two main facility types:
1. Constellation Galileo Control Centres (GCC)
2. Telemetry, Tracking and Commanding (TT&C) Stations
2 Overview of Galileo System 23

The Constellation Ground Control Centres
The GCCs host all centralised functions of the GCS, including spacecraft Con-
stellation Monitoring & Control, spacecraft Constellation Planning, Flight
Dynamics and Operations Preparation. The GCC implements as well the GCS
network interface with external Galileo entities involved in the system operations
such as the External Satellite Control Centers (ESCCs) and the In Orbit Test (IOT)
station located at Redu, Belgium.
The Telemetry, Tracking and Commanding Facility
The GCS architecture comprises a worldwide network of Telemetry, Tracking and
Control ground stations or facilities connected to the Galileo Control Centres. At
the time of writing the Telemetry, Tracking and Commanding Facility (TTCF)
network includes stations in 5 locations. Additional stations might be added to the
TTCF network in the future.
Each TTCF supports telemetry (TM) downlink, telecommand (TC) uplink and
can also support collection of satellite tracking data for the management of the
constellation. The transmitted TC signal data, and received TM data, together with
the TT&C monitoring and control data is exchanged between the TT&C stations
and the GCC via a dedicated communication network. Under nominal operations
conditions, the TTCFs are autonomous and manned intervention is only required
for the purpose of either anomaly investigation or maintenance purposes.
In routine operations, the TT&C stations are utilised to upload TCs to and to
receive TM from the Galileo spacecraft through an RF data channel. Besides
supporting routine satellite housekeeping tasks, the TTCFs have also the capability
to collect satellite tracking data (i.e. two-way ranging measurements) intended for
off-line satellite orbit determination under special spacecraft operation scenarios.
The link between the TTCF ground station and the Galileo satellites is estab-
lished through a 11 m dish antenna. The TT&C facility operates over specifically
allocated S-Band RF ranges, between 2 and 2.2 GHz approximately (Fig. 2.6).
Cospas-Sarsat and Galileo
The LEOSAR system developed by the International Cospas-Sarsat Program cur-
rently provides accurate and reliable distress alert and location data to help search
and rescue (SAR) authorities to assist persons in distress. In 2000, consultations
started between the Cospas-Sarsat Program and the European Commission on the
feasibility to install 406 MHz SAR instruments on the Medium Orbit navigation
satellites systems in order to develop a 406 MHz MEOSAR component to the
Cospas-Sarsat system. The main benefits of the MEOSAR system will be the
near instantaneous global coverage with accurate independent location capability
24 J.P. Bartolomé et al.

(in opposition with the current LEO system which has a higher latency to provide
location information). The USA MEOSAR program based on GPSIII is called the
Distress Alerting Satellite System, (DASS), the European System based on Galileo
is called SAR/Galileo, and the Russian program based on GLONASS is referred to
as SAR/GLONASS. This has a direct impact on the probability of survival of the
person in distress at sea or on land.
The Galileo Program involvement into Cospas-Sarsat goes beyond the space
component of the MEOSAR system. Indeed, the European Union has deployed a
significant Ground Segment infrastructure, which provides localization services for
distress alerts transmitted by SAR beacons over a wide area comprising continental
Europe, and vast oceanic areas around the continent (see Fig. 2.7 below). The SAR/
Galileo Ground Segment can receive and process SAR distress signals relayed by
any operational Galileo spacecraft or other satellite of the COSPAS/SARSAT
MEOSAR constellation and determine thereby the location of the beacon within the
coverage area.
The ground segment of the Search and Rescue Service of Galileo consists of 3
receiving ground stations, called Medium Earth Orbit Local User Terminal
(MEOLUTs), which receive the distress signals relayed by the Galileo Search and
Rescue repeater in the 1544 MHz band. Each MEOLUT includes a minimum of 4
antennas tracking different Galileo satellites.
Receiving the signal relayed from four different satellites makes it possible to
determine the distressed beacon position by triangulation using Time of Arrival
(TOA) and Frequency of Arrival (FOA) techniques. The MEOLUT then decodes
the distress signal message, determines the beacon localization and provides this
information to the Cospas-Sarsat Mission Control Center (MCCs).
Fig. 2.6 TTCF S-band 11 m
dish antenna (courtesy of
ESA)
2 Overview of Galileo System 25

The 3 European MEOLUTs are located in Svalbard (Norway), Makarios
(Cyprus) and Maspalomas (Spain) and provide the SAR/Galileo service over the
European Coverage Area (ECA) as shown in Fig. 2.7. Each MEOLUT is connected
to a central facility, the MEOLUT Tracking Coordination Facility (MTCF) located
at the SAR/Galileo control centre in Toulouse, (France) and which optimizes the
MEOLUT tracking plan of the 3 European MEOLUT in order to achieve the best
location accuracy and availability over the European Coverage Area.
As a component of the Cospas-Sarsat MEOSAR system (Cospas-Sarsat 2012),
the SAR/Galileo ground segment is also capable of receiving the distress signal
relayed by the MEOSAR payloads embarked on the Glonass and GPS satellites
(SAR/Glonass and GPS/DASS payloads).
The performances achieved by the SAR/Galileo Service when the full Galileo
constellation is operational are indicated in Table 2.3
The SAR/Galileo ground segment also includes the Return Link Service Pro-
vider (RLSP), which is responsible for providing Return Link Acknowledgment
Fig. 2.7 SAR/Galileo European coverage area and ground facilities
26 J.P. Bartolomé et al.

Messages to the COSPAS-SARSAT distress beacons equipped with a Galileo
receiver. The Return Link Messages are embedded within the navigation message
of the E1 signal.
Galileo User Segment
Definition
The Galileo user segment (GUS) is the third and largest segment that forms the
Galileo system. Contrary to the other two segments, namely, the Galileo space
segment and the Galileo ground segment, the GUS is certainly the one with the
closest connection to the end user. In particular, the GUS is targeted at providing
the means for allowing end users to fully exploit all the capabilities of signals being
transmitted by the Galileo satellites. It is therefore, the segment encompassing user
applications and services, as well as user receiver technologies.
Galileo Receivers, the Key to Galileo Success
The Galileo user receiver is the key technological equipment for translating the
Galileo signals received from space into practical applications and services. The
goal of Galileo receivers, as it happens with many other GNSS receivers, is to
provide the end user with an accurate estimate of its current position and time. This
is typically achieved in three steps:
1. The signals from space impinging on the user’s receiver antenna are amplified
and filtered by the receiver RF front-end in order to compensate for the huge
propagation losses undergone in their way from the satellites to the Earth. This
signal conditioning is typically followed by an analog-to-digital conversion,
which allows processing the received signal in a reliable manner by means of
digital signal processing (DSP) techniques.
2. The user receiver generates a local replica of the Galileo signal of interest, which
is gradually aligned to the actual signal received from space. As a result of this
alignment process, the receiver is able to coarsely determine the actual time-
delay and carrier parameters of the received signal (i.e. this is the so-called
Table 2.3 SAR/Galileo service performance recorded at ground segment
Performance parameter Value
Detection probability 99.5 %
Localization probability 98.0 %
Localization accuracy <5 km (2σ) within 10 min
Worst case service availability 95.0%
2 Overview of Galileo System 27

“acquisition”stage). Later on, these parameters are further refined and moni-
tored as a function of time (i.e. the so-called “tracking”stage).
3. Once the receiver is precisely synchronized to the actual received signal, and
more than four satellites are in view, the refined and accurate signal parameters
are used to solve the navigation equations, which lead to the user position,
velocity and time information (i.e. the so-called “navigation”or “PVT”solution).
The above three steps are rather standard in most GNSS receivers, but for the
particular case of a Galileo receiver, the key point is being able to fully exploit the
specific features that Galileo signals bring to end users:
From a physical-layer perspective, this involves receivers taking advantage of
the Binary-Offset Carrier (BOC) modulation format adopted in Galileo signals. Due
to this format, which can be understood as a one-bit quantization of a sinusoidal
signal, and for a given total bandwidth, it is possible to achieve a larger mean
square bandwidth (also known as Gabor bandwidth) than with a traditional binary
phase shift keying (BPSK) modulated signal. This involves that Galileo receivers
may provide an improved accuracy in the determination of the user position, par-
ticularly when compared to traditional mass-market receivers that make use of the
traditional BPSK-modulated GPS C/A signal.
From a frequency planning perspective, this involves paving the way for the
development of multi-frequency receivers, which take advantage of processing
different frequency bands with the goal of providing additional diversity and
compensating ionospheric effects. These bands correspond to the E1 band com-
patible with current GPS mass-market receivers and centered at 1.575,42 MHz; the
E6 band, centered at 1.278,75 MHz; and the E5 band, which is further split into two
adjacent bands, namely the E5a and E5b bands, with center frequencies at 1.176.45
and 1.207,14 MHz, respectively. The availability of these frequency bands is cer-
tainly a key feature to be exploited by advanced Galileo receivers. Moreover, the
coexistence with GPS within the E1 band opens the door for the use of the so-called
multi-constellation GNSS receivers, capable of processing both GPS and Galileo
signals with the same front-end, and thus solving the problems of limited visibility
of satellites that are suffered in certain working scenarios (e.g. in urban canyons).
From a data perspective, Galileo signals allow the emergence of a myriad of
new applications and services due to the definition of different types of data
channels, such as the case of open and publicly available data channels, authenti-
cated data channel and even possibly encrypted data channels.
Paving the Way Forward for Galileo User Receivers
The development and widespread deployment of Galileo receivers is certainly the
necessary piece to guarantee the success of Galileo. In that sense, the validation of
the above-mentioned Galileo features, and their proof-of-concept, has been under-
taken in the framework of several R + D initiatives carried out in the recent years.
28 J.P. Bartolomé et al.

Most of these efforts have been supported by public funding, either through the 6th
and 7th Framework Programs funded by the European Commission, through
research projects funded by the European Space Agency (ESA), or through different
initiatives funded by national authorities. The goal of all these efforts has been to
foster the development of Galileo user receiver technologies and to achieve the
deepest and broadest development of Galileo applications. This is particularly
important when taking into account the wide range of fields and applications where
the potential benefits of Galileo have already been recognized. They span across
several domains such as location-based services (LBS) powered by smartphones or
portable devices, road and maritime transportation, aviation, precision agriculture,
safety or environment protection, just to mention a few. In general terms, GNSS
positioning and timing information is expected to have a critical impact in economic
terms of more than 6 % of the whole GDP of the European Union (European
Commission 2010).
From this perspective, it becomes clear that the expected economical impact of
Galileo in a commercially competitive environment is certainly one of the main
differences with respect to previous GNSS systems. GPS and GLONASS were
mainly developed for military applications with military funding, without a pre-
determined mass-market orientation, although in the end GPS in particular has had
a huge commercial impact. This is inline with the fact that barely 10 % of the US
total public spending in R + D has a market or application-driven orientation,
whereas for the EU, this amount exceeds 40 % (Fernández-Hernández 2011). This
example shows the actual importance of the GUS, and the determined efforts that
are being taken in order to make it successful. With Galileo receivers at the fore-
front of the GUS, most of these efforts have concentrated on the development of
Galileo user receivers and technologies. This is the case of the three calls (in 2002,
2003 and 2005) launched by the former Galileo Joint Undertaking (GJU) under the
Galileo work program, where nearly 90MEur were dedicated to promote the
development of Galileo receiver technologies, applications as well as the stan-
dardization of different aspects of Galileo and its mission implementation.
One of the key initiatives being funded under the first GJU call was the “Galileo
receiver development activities”(GARDA) project, whose goal was to develop a
Galileo receiver analysis and design application (GRANADA) in order to serve as a
test-bench for integration and evaluation of technologies, on one side, and as a
software receiver tool for Galileo application developers, on the other side (Marradi
et al. 2006). The validation of this software tool was carried out by the development
of a Galileo simulator according to the Galileo signal-in-space (SIS) interface
control document (ICD) available at that time. For the subsequent calls, GJU
funding on the GUS targeted the introduction of mass-market Galileo receivers,
such as in the GAMMA project (Abwerzger et al. 2006) (which was followed by
the GAMMA-A project), where low-cost and low-power receiver solutions were
developed for specific market applications. Two main applications were identified:
location-based services (LBS) and the automotive area, with the latter involving
route guidance, fleet management or road tolling. Complementarily, the GRAIL
project focused on the development of Galileo receivers for rail applications, where
2 Overview of Galileo System 29

the emphasis was placed on safety-critical applications related to the European
Train Control System (ETCS) (Albanese et al. 2006). Finally, the HIMALAYA
activity focused on the design and development of a “ready-to-market”single chip
GNSS mass-market receiver for GPS, EGNOS and GALILEO signals. Many other
projects have been funded in the last years, and being exhaustive exceeds the goals
of this introductory chapter.
Security, environment protection and SoL applications are also key research lines
were efforts are being devoted for the development of advanced Galileo receivers.
Integrity (not necessarily provided by the system, but by the receiver) is currently
emerging as a critical requirement for high-end Galileo receivers, due to the
increasing concerns on potential threats to GNSS users caused by the presence of
interference, spoofing and other disturbing effects (e.g. multipath, non-line-of-sight).
The study of this problem is indeed the goal of the iGNSSrx project funded by the
European Commission and initiated in 2012, whose objective is to improve posi-
tioning services to terrestrial users by developing user receiver technologies for
integrity monitoring, at signal and observable level.
Finally, the integration of Galileo receivers with external technologies is also one
of the main topics that are expected to pave the way for the success of Galileo
receivers. Such integration can be used to further improve the overall performance of
Galileo, particularly in the presence of harsh working scenarios with signal block-
ages and limited sky visibility. For instance, the work carried out within the IADIRA
project has focused on the tight integration of Galileo with external information
coming from inertial sensors (INS) (Silva et al. 2006). A step forward was addressed
in the DINGPOS project funded by ESA, where apart from the use of INS, wireless
communication technologies such as WiFi were also considered for developing a
fully integrated platform capable to provide seamless location in indoor environ-
ments (López-Salcedo et al. 2008). Indeed, the advent of new applications and
services are gradually pushing the use of GNSS in working conditions different from
the ones for which the system was designed to operate (Seco-Granados et al. 2012).
This is the case of LBS in handheld devices (e.g. location-aware marketing), social
networking applications, indoor navigation, etc. In that sense, the integration of the
GUS with communication technologies (e.g. WLAN or cellular networks such as
Long Term Evolution—LTE Del Peral-Rosado et al. 2012) is certainly one of the
most promising future research lines for which significant efforts will be devoted in
the coming years.
Galileo Interoperability and Compatibility
While the independence of systems may be a way to promote competition among
systems and could virtually provide greater reliability and integrity, the advantages
of interoperability and compatibility are widely recognized by all actors. An early
(February 1999) Communication of the European Commission (EC) stated:
“Galileo must be an open, global system, fully compatible to GPS, but independent
30 J.P. Bartolomé et al.

of it…” Later on, the EU-US Agreement on the Promotion, Provision and Use of
Galileo and GPS Satellite-Based Navigation Systems and Related Applications
signed June 26, 2004, in Dublin, Ireland, set up the models and methodology for the
radio frequency compatibility of satellite navigation systems, in particular between
GPS and Galileo. Further, Global and regional system providers at the third meeting
of the Providers’Forum of the International Committee on Global Navigation
Satellite Systems (ICG) in 2008 agreed that at a minimum, all global navigation
satellite systems (GNSS) signals and services must be compatible. To the maximum
extent possible, open signals and services should also be interoperable, in order
to maximize benefit to all GNSS users. For many applications, common carrier
frequencies are essential to interoperability and commonality of other signal
characteristics is desirable. In some cases, carrier frequency diversity may be
preferable to improve performance. The definitions of compatibility and interop-
erability according to the ICG are as follows (International Committee on Global
Navigation Satellite Systems 2008).
Interoperability refers to the ability of global and regional navigation satellite
systems and augmentations and the services they provide to be used together to
provide better capabilities at the user level than would be achieved by relying solely
on the open signals of one system:
i. Interoperability allows navigation with signals from different systems with
minimal additional receiver cost or complexity;
ii. Multiple constellations broadcasting interoperable open signals will result in
improved observed geometry, increasing end-user accuracy everywhere and
improving service availability in environments where satellite visibility is
often obscured;
iii. Geodetic reference frames realization and system time steerage standards
should adhere to existing international standards to the maximum extent
practical;
iv. Any additional solutions to improve interoperability should be encouraged.
Compatibility refers to the ability of global and regional navigation satellite
systems and augmentations to be used separately or together without causing
unacceptable interference and/or other harm to an individual system and/or service:
i. The International Telecommunication Union provides a framework for dis-
cussions on radiofrequency compatibility. Radiofrequency compatibility
should involve thorough consideration of detailed technical factors, including
effects on receiver noise floor and cross-correlation between interfering and
desired signals;
ii. Compatibility should also respect spectral separation between each system’s
authorized service signals and other systems’signals. Recognizing that some
signal overlap may be unavoidable, discussions among providers concerned
will establish the framework for determining a mutually acceptable solution;
iii. Any additional solutions to improve compatibility should be encouraged.
2 Overview of Galileo System 31

In the particular case of Galileo, the application of the interoperability principle
with GPS was guided by the following considerations (Hein 2006).
•Signals-in-Space. The driver was the selection of common center frequencies (at
least for some signals such as L1/E1 and L5/E5a) since this aspect has a clear
impact of the cost of multi-frequency receivers and it is necessary to make
combined processing of phase observation possible. The choice of CDMA
instead of FDMA was also a definite step in favor of the interoperability
between Galileo and GPS. GLONASS, which uses FDMA, can be considered to
be “system interoperable”but not “signal interoperable”with Galileo. Although
the use of the same modulations, data messages, etc. is not considered as an
essential feature to facilitate interoperability because digital receivers can easily
encompass a variety of formats, the military GPS-M code and the Galileo Public
Regulated Service (PRS) have signal interoperability on L1 band.
•Coordinate Reference Frame. Although the Galileo Terrestrial Reference Frame
(GTRF) is different to the GPS coordinate reference frame (WGS84), both of
them differ less than very few centimeters with respect to the International
Terrestrial Reference Frame (ITRF), hence guaranteeing interoperability for
most applications.
•Time Reference. Galileo System Time (GST) as well as GPS Time are different
real-time realizations of UTC (Universal Time Coordinated)/TAI (Atomic
Time), which is the international civilian time standard. Both times are expected
to be within the nanoseconds order of magnitude. The service providers have
agreed to broadcast the GPS/Galileo time offset, and alternatively the offset can
be determined in a combined receiver with very high accuracy at the very small
cost of using one satellite observation.
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