|
 On
the Way to a
New GNSS
BERNHARD HOFMANN-WELLENHOF
Editor's
Note:
This
article represents the first installment in what will be an ongoing
column in our magazine, entitled "Understanding Galileo." The column
will explore the science and technology behind satellite-based positioning,
navigation, and timing systems in general and the proposed Galileo
system in particular. These may include systemwide issues, such
as orbits, signal structure, radio frequencies, or timing standards.
They may also address more general, but related topics, such as
geodetic coordinate systems, international timing systems (for instance,
Coordinated Universal Time or UTC), signal interference and multipath,
and user equipment design and operation. As the planning for Galileo
proceeds and decisions are reached on particular system design elements,
the column will explore these in greater detail.
The
coordinating editor for this column will be Prof.-Dr. Bernhard Hofmann-Wellenhof.
Since 1986, he has been professor of Positioning and Navigation
in the Institute of Applied Geodesy at Graz University of Technology,
Austria. Hofmann-Wellenhof is author of some 90 publications, among
them coauthor of four books. Well known is the textbook "GPS - Theory
and Practice," now with the fifth printing being prepared by Springer
Verlag. Hofmann-Wellenhof has spent numerous research and teaching
stays abroad. The most important awards he has received include
Senior Scientist Grant in Geodesy of the National Research Council,
Washington D.C. (1987), Fulbright Research Award of the Council
for International Exchange of Scholars (1989), Vice-President of
the Austrian Institute of Navigation (1998), and Honorary Member
of the Hungarian Society of Photogrammetry and Remote Sensing (1999).
Readers
who would like to propose possible column topics or authors may
reach him at <>
or by way of fax +43 316 873 8888.
Our
first presentation of "Understanding Galileo" is written by Prof.-Dr.
Hofmann-Wellenhof and presents an overview of the nominal Galileo
system design and sketches its politico-historical background.
Glen
Gibbons, editor
Bernhard Hofmann-Wellenhof
Positioning and Navigation, Graz
University of Technology, Austria
Galileo
is a space-based, all-weather navigation system under consideration
by the European Union (EU) and the European Space Agency (ESA) that
would enable users to determine their three- dimensional position,
velocity, and time with a high degree of performance in terms of
accuracy, availability, integrity, and continuity.
The recommended approach is to develop
Galileo as a global system independent from GPS, but fully compatible
and interoperable with it. By compatible and interoperable, we mean
that a Galileo receiver could exploit simultaneously the signals
received from Galileo and GPS. To some extent, Europe could be open
to participation in the Galileo program by international partners
outside the EU, although the full nature of this participation has
yet to be announced.
Galileo will offer new capabilities
that can support new applications and make the overall Global Navigation
Satellite System (GNSS) more robust. Furthermore, Galileo is expected
to eliminate certain shortcomings of the present means of satellite-based
positioning and navigation -- Russia's GLONASS and the United States'
Global Positioning System (GPS) -- such as poor signal availability
in urban areas and the northernmost latitudes, temporary gaps in
coverage, and single points of failure.
A Little History
In February 1999, the European Commission
(EC) released a communication proposing the creation of a European
strategy to develop the next generation of a European contribution
to GNSS. The main point of this communication asserted that Europe
should develop Galileo, a new satellite navigation system combined
with terrestrial infrastructure.
In June 1999, the EU Council of Transport
Ministers invited the EC to set up a management organisation for
the Galileo definition phase. On 17 June 1999, the Galileo Steering
Committee (GSC) was established. The GSC is chaired by the EC and
composed of representatives from the 15 EU Member States, with ESA
exercising an observer status. The GSC represents the policy level
and covering recommendations on strategic issues and supporting
the international negotiations and the establishment of a common
position on frequency requirements.
In parallel to the GSC, ESA has founded
a Navigation Programme Board with the EC acting as an observer.
ESA is responsible, directly in support of the Galileo initiative,
for the space segment and the related ground infrastructure. Its
main contributions will be part of the GalileoSat programme specifically
developed for that purpose.
The "driving force" of the Galileo
initiative is the Programme Management Board (PMB) that coordinates
the Galileo definition phase. It will be supported by a Galileo
Programme Office (GPO) that mainly concentrates on technical issues,
for example, mission requirements or overall system architecture.
Politics and Economics
Several factors contributed to the
emergence of the Galileo initiative. Europe is interested in achieving
its own state-of-the-art GNSS that should play a leading role in
the twenty-first century. An independent European system could avoid
problems that might arise from the current dependence on GPS and
GLONASS. In the current situation, Europe cannot control any of
the critical satellite-navigation operations that are playing an
ever-increasing role in its transport networks, as well as other
nontransport-related commercial and professional activities.
Apart from this problem of political
dependence, Europe recognises the growing market potential for numerous
GNSS applications, for example, reducing traffic congestion, car
and truck routing and tracking, farming, fishing, infrastructure
planning, mineral exploration, and mapping and surveying.
In comments on the Galileo proposal
as presented in the 10 February 1999 EC communication "Galileo:
Involving Europe in a New Generation of Satellite Navigation Services,"
Neil Kinnock, the commissioner responsible for European transport
policy, observed "As the Internet has revolutionised electronic
communication, so global satellite systems are revolutionising navigation.
Europe must put itself in a position to capture a fair share of
world market that could be worth an estimated e40 billion within
a few years and would offer the potential to generate quality jobs.
"Europe must also have control of
the systems on which safe movement by air, land and water transport
will increasingly depend. But that cannot happen without combined
and consistent political action. Failure to act now would mean missing
a huge and probably unrepeatable opportunity. The investment needed
is less than e3 billion is not unbearable since it would be spread
between 15 Member States over 10 years. The returns would be immense.
"The Galileo project has to ensure
that the major political and economic European interests are guarded.
Strategically, the long-term aspect of GNSS development must also
be considered. Neither GPS nor GLONASS expressly incorporates formal
GNSS service guarantees, and the international partners appear unwilling
or unable to offer a full European participation in future system
design, development, and operation. Moreover, in the absence of
a fair European share in all industrial segments of the GNSS market,
the proposal of the EC and ESA to develop their own system becomes
easily understandable."
Requirements and Capabilities
The basic civil Galileo navigation
service should at least offer the same performance as the future
civil GPS service based on the next generation of satellites and
after the GPS modernisation. The preliminary navigation performance
signal-in-space (SIS) requirements of a standalone Galileo are summarised
in Table 1 as defined in the Draft Programme proposal for the Qualification
of the Global Navigation Satellite System Galileo released by the
Joint Board on Communication Satellite Programmes of the European
Space Agency on 16 February 1999.
Furthermore, Galileo will possibly
offer an additional communication service greatly improving its
reliability and acceptance for safety-critical applications and
information related services like such as transport telematics.
System Design
The proposal for Galileo is based
on MEO (medium Earth orbit) and, possibly, GEO (geostationary Earth
orbit) satellites combined with appropriate terrestrial infrastructure
and supporting systems. As described in several key documents published
last year (see the "Readings and Resources" sidebar), the system
architecture may be described by four segments (see also Figure
1).
The space segment includes the satellite
constellation providing the navigation signals and the corresponding
control stations. Thus, the space segment comprises the Galileo
satellites and the Ground Control System consisting of the Telemetry
and Control (T&C) stations required to uplink data to and receive
data from the Galileo spacecraft and the Satellite Control Centre
(SCC) responsible for monitoring and controlling the satellites.
The mission segment includes the various
applications and systems necessary to manage and control the system.
The mission segment comprises Mission Control Centres (MCC), Integrity
Control Centres (ICC), Orbitography and Synchronisation Stations
(OSS), and Ranging and Integrity Monitoring Stations (RIMS).
The MCC perform the following tasks:
generate all the reference parameters to be used by the system (time,
synchronisation parameters, almanac); monitor, validate and control
the other parts of the segment; archive data; assess and monitor
the system performance; manage the Galileo navigation system.
Beyond the MCC, the ICC monitor and
validate the Galileo satellites signal-in-space performance using
data from the control and measurement stations (such as RIMS and
OSS). The OSS form a global network of stations providing data to
allow an onboard-the-satellites computation of the ephemeris (satellite
orbital positions) and parameters to synchronise the Galileo satellite
clocks with Galileo Time. The RIMS are remote stations acting as
signal-in-space data collection sites.
The user segment includes the different
types of receivers for processing the navigation signals from the
Galileosats, the European Geostationary Navigation Overlay Service
(EGNOS), GPS, GLONASS, and possible Local Area Augmentation Systems
(LAAS). And finally, the communication segment includes the communication
links among the Galileo assets and is based on the Galileo Wide
Area Network (GWAN) connecting the elements of the space and the
mission segment.
Service Categories. The system's
sponsors propose that Galileo provide three levels of service: Open
Access Service (OAS), similar to the present GPS Standard Positioning
Service (SPS), which would be employed mainly for mass-market application;
Controlled Access Service number 1 (CAS 1), dedicated to commercial
and professional applications with enhanced service features (better
performance, liability of system operators in case of service interruption);
and Controlled Access Service number 2 (CAS 2), available to those
safety-of-life and other critical applications that cannot tolerate
any interruption or disturbance of the system performance.
Frequency Plans
Three options for Galileo frequency
plans are being investigated. The first one requires negotiations
with the United States because of band sharing with GPS; the second
one, negotiations with the Russian Federation in order to use the
bands currently assigned to GLONASS; and the third one being spectrum
self-sufficient.
This third option, the spectrum self-sufficient
Galileo frequency plan, is partly based on frequencies already reserved
by ESA and partly on access to other portions of the radio spectrum.
Its nominal design includes the following:
e Reusing the E1 (15871591 MHz)
and E2 (15591563 MHz) narrow subbands. E1 would be used to
emit a narrow-band signal as part of the CAS 2 service; E2, to emit
a narrow band as part of the OAS and CAS 1 services.
e Gaining access to 11511215
MHz and E6 (12601300 MHz).
e Using a 2024-MHz wide band
in the 11511215 MHz range (but distinct from the future GPS
L5 in 11641188 MHz) to provide an encrypted signal as part
of the CAS 2 service.
e In a 2024 MHz wide band in
E6 (12601300 MHz), Galileo would emit signal(s) as part of
the CAS 1 (and possibly CAS 2) service(s). This E6 could even be
extended to E4 (12541260 MHz), already assigned to Galileo.
However, the final frequency plan
depends on decisions taken at forthcoming conferences to be held
by the International Telecommunication Union (ITU).
A Preliminary Schedule
The definition phase began in June
1999 and is scheduled for completion by the end of this year. A
decision regarding implementation of Galileo will be made at the
beginning of 2001. In the case of a positive decision, Galileo will
be developed and implemented from 2001 to 2008. An initial test
constellation to validate the design assumptions and to obtain the
necessary experience for the operations will be in orbit at the
end of 2003. From 2005 onwards, the initial constellation will be
complemented leading to full operational capability (FOC) by 2008
at the latest (see Figure 2).
Cost Estimates and Financing. The
cost of the system is strongly related to the number of satellites
planned for the constellation. Two primary versions have been proposed:
either 21 MEO plus 3 GEO or 36 MEO plus 9 GEO, with the corresponding
costs projected as e2.2 and e2.9 billion, respectively.
Galileo's sponsors are proposing a
three-part strategy for financing the system. First, substantial
financing will occur at the European level through the EU (mainly
funds allocated for the trans-European transport networks) and ESA
budgets. Second, various revenue streams will be established. Third,
the identification of potential revenue streams could attract the
involvement of public private partnerships (PPP).
International Aspects
Even without direct U.S. involvement
in Galileo, a strong need exists for cooperation because Galileo
is desired to be fully interoperable and compatible with GPS. From
the user's point of view, the possibility of combined applications
will yield improved service, for instance, in urban areas where
obstruction problems frequently occur.
At the next ITU World Radiocommunication
Conference (WRC) this year in Istanbul, Turkey, from May 8 through
June 2, the United States is expected to seek a realignment of EU
positions for the withdrawal of WRC Resolution 220. This resolution
was adopted during the last WRC in 1998, calling for technical studies
on reusing -- or sharing -- satellite navigation frequency bands
with mobile satellite services (MSS).
According to reports on the ITU studies,
it would be "unlikely" MSS could share frequencies successfully
(see the "Readings and Resources" sidebar for further details).
However, there is a great demand for MSS spectrum, thus the issue
is pending. On the one hand, Europe supported the resolution at
a meeting of the Conference of European Ministers of Post and Telecommunications
(CEPT) in January 2000 at Stockholm, Sweden. On the other hand,
the European Commission could accept the withdrawal of Resolution
220 provided that additional frequencies for Galileo will be put
on the agenda of the next WRC in 2003.
A second EU/ESA priority at the WRC
2000 is to obtain the E5/L5 frequency band for Galileo, in which
the United States is also interested. As a result of a 10 November
1999 negotiation session in Washington, D.C., the United States
appears willing to consider the possibility of sharing an uncoded
L5 signal with Galileo. Furthermore, the United States will "study"
the proposal of the European Community to share frequencies in the
L1 and L2 portions of the spectrum. However, U.S. concerns remain
about opening frequency bands because of the difficulties in controlling
a shared, uncoded L1/L2 signal in critical situations. Thus, one
of the most urgent questions remains whether the U.S. government
is ready to share frequency bands with Europe.
Russia represents an important cooperation
partner because of its rich experience gained with GLONASS, especially
in deploying and operating a space segment and in all frequency
aspects. Russia has access to a number of frequencies that might
be very helpful in the process of selecting appropriate Galileo
frequencies. In 1999 and also in 2000, several negotiation meetings
were held with the Russian Federation. For future meetings, the
main points of discussion will be the present status of Galileo
activities, the possibility to reuse GLONASS frequencies, and the
definition of the Galileo system architecture. Furthermore, of particular
interest is how to respond to the Russian proposal for a joint GNSS
satellite system.
Recently, the EC worked out a draft
plan for technical subgroups to support international negotiations.
The five subgroups operate as a group of experts for frequency,
security, management interface, legal issues and certification,
and interoperability.
With the Galileo Programme well underway
and key decisions on the horizon, Europe has positioned itself to
move rapidly toward the next-generation GNSS. The future will undoubtedly
bring changes to the currently proposed system components and configuration,
with new technological innovations further broadening today's concept
of satellite-based positioning. And as the world of Galileo continues
to evolve ,this column will keep you up to date on the technology
behind the system. 
Readings and Resources
e Divis, DA (2000):
"Spectrum Allocation -- Frequency Decisions Pending." GPS
World 11(3): 1820.
e Fromm H-H (1999):
"Galileo: The Performance Characteristics and Technical Implementation."
In: German Institute of Navigation and Nederlands Instituut voor
Navigatie (eds): Proceedings of the 1st European
Symposium on Transport Telematics, Potsdam,
812 November 1999, pp. 7782.
e Lebée JP,
Faurois JY (1999): Galileo System Specific
Security Requirements Statement (SSRS),
Draft Version 0.4, 16 November 1999.
e Lucas R, Ludwig
D (1999): "Requirements and Architectures for Galileo." In: GNSS´99,
Proceedings of the 3rd International Symposium on Global Navigation
Satellite Systems, Genoa, 58 October
1999, pp. 409415.
e Wolfrum J, Healy
M, Provenzano J-P, Sassorossi T (1999): "Galileo Europe´s
Space-based Navigation Infrastructure." In: GNSS´99,
Proceedings of the 3rd International Symposium on Global Navigation
Satellite Systems, Genoa, 58 October
1999, pp. 5964.
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