What is Galileo - DOC by franklinr


									             European Space Agency (ESA)
ЕSA          The future “GALILEO”

                                What is Galileo?

Galileo will be Europe’s own global navigation satellite system, providing a highly accurate,
guaranteed global positioning service under civilian control. It will be inter-operable with GPS and
GLONASS, the two other global satellite navigation systems.

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, however, Galileo will deliver real-time
positioning accuracy down to the metre range, which is unprecedented for a publicly available

It will guarantee availability of the service under all but the most extreme circumstances and will
inform users within seconds of a failure of any satellite. This will make it suitable for applications
where safety is crucial, such as running trains, guiding cars and landing aircraft.

The first experimental satellite, GIOVE-A, was launched on 28 December 2005. The objective of
this satellite is to characterize the critical technologies, which have already been developed under
ESA contracts.

Two further experimental satellites are planned: GIOVE-B, scheduled for launch end of 2007 and
GIOVE-A2, to be ready for launch in the second half of 2008. The actual launch date of this satellite
will be decided later, taking into account the situation of GIOVE-A and GIOVE-B.

Thereafter, four operational satellites - the basic minimum for satellite navigation in principle - will
be launched by end 2008 / 2009 to validate the Galileo concept with both segments: space and
related ground infrastructure . Once this In-Orbit Validation (IOV) phase has been completed, the
remaining satellites will be installed to reach the Full Operational Capability (FOC).

The fully deployed Galileo system consists of 30 satellites (27 operational + 3 active spares),
positioned in three circular Medium Earth Orbit (MEO) planes at 23 222 km altitude above the
Earth, and at an inclination of the orbital planes of 56 degrees with reference to the equatorial

Once this is achieved, the Galileo navigation signals will provide good coverage even at latitudes up
to 75 degrees north, which corresponds to the North Cape, and beyond. The large number of
satellites together with the optimisation of the constellation, and the availability of the three active
spare satellites, will ensure that the loss of one satellite has no discernible effect on the user.

Two Galileo Control Centres (GCCs) will be implemented on European ground to provide for the
control of the satellites and to perform the navigation mission management. The data provided by
a global network of twenty Galileo Sensor Stations (GSSs) will be sent to the Galileo Control
Centres through a redundant communications network. The GCC’s will use the data from the
Sensor Stations to compute the integrity information and to synchronise the time signal of all
satellites with the ground station clocks. The exchange of the data between the Control Centres
and the satellites will be performed through up-link stations. Five S-band up-link stations and 10 C-
band up-link stations will be installed around the globe for this purpose.

As a further feature, Galileo will provide a global Search and Rescue (SAR) function, based on the
operational COSPAS-SARSAT system. To do so, each satellite will be equipped with a transponder,
which is able to transfer the distress signals from the user transmitters to the Rescue Co-ordination
Centre, which will then initiate the rescue operation.
At the same time, the system will provide a signal to the user, informing him that his situation has
been detected and that help is under way. This latter feature is new and is considered a major
upgrade compared to the existing system, which does not provide feedback to the user.

Altogether Galileo will provide five levels of services with guaranteed quality which marks the
difference from this first complete civil positioning system.

                        Why Europe needs Galileo

Satellite navigation users in Europe today have no alternative other than to take their positions
from US GPS or Russian GLONASS satellites. Yet the military operators of both systems give no
guarantee to maintain an uninterrupted service.

Satellite positioning has already become the standard way of navigating. If the signals were
switched off tomorrow, many ship and aircraft crews would find it inconvenient and difficult to
revert to traditional navigation methods. As the use of satellite navigation spreads, the implications
of a signal failure will be even greater, jeopardising not only the efficient running of transport
systems, but also human safety.

As far back as the early 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 system under civilian control which will be guaranteed to operate at all times, bar
the direst emergency.

European independence is the chief reason for taking this major step. However, other subsidiary
reasons include:

       By being inter-operable with GPS and GLONASS, Galileo will be a cornerstone of the Global
        Navigation Satellite System (GNSS). This system will be under civilian control and will allow
        positions to be determined accurately for most places on Earth, even in high rise cities
        where buildings obscure signals from satellites low on the horizon. This is because the
        number of satellites available from which to take a position is more than doubled
       By placing satellites in orbits at a greater inclination to the equatorial plane than GPS,
        Galileo will achieve better coverage at high latitudes. This will make it particularly suitable
        for operation over northern Europe, an area not well covered by GPS
       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

               Who's involved in Galileo?

Galileo is a joint initiative of the European Commission (EC) and the European Space Agency

The EC is responsible for the political dimension and the high-level mission requirements. The EC
initiated in particular studies on the overall architecture, the economic benefits and the user needs.
These include the GALILEI studies that address the local architectures, interoperability and signals
and frequencies. Moreover, they provide a market observatory and cater for investigations into
legal, institutional, standardisation, certification and regulatory issues.

ESA’s responsibility covers the definition, development, and in-orbit validation of the space
segment and related ground element. Work on the new technologies needed for the satellite
constellation and the ground segment has been continuing at ESA's European Space Research and
Technology Centre (ESTEC), at Noordwijk, in the Netherlands for a number of years. These critical
technologies include the high precision clocks to be carried on-board the satellites (rubidium and
passive hydrogen maser frequency standards), on-board timing units for synchronising the
individual clocks to a common Galileo system time, signal generators to produce the positioning
signals that the Galileo spacecraft will broadcast, power amplifiers, radio-frequency multiplexers &
antennas and telecommand & telemetry transponders.

In parallel, the Galileo System Simulation Facility (GSSF) has been built to test strategies for
coping with contingencies when the full system is up and running. In addition, the Galileo Signal
Simulation Facility helps with the fine-tuning of the Galileo signal design. ESA has also supported
work on technologies needed for Galileo receivers.

The Galileo System Test Bed Version 1 (GSTB V1) has allowed engineers to validate Galileo-specific
control algorithms, such as clock adjustments, and procedures for predicting individual satellite
orbits, before the full system goes into operation.

The second phase of the GSTB, now named Galileo In-Orbit Validation Element (GIOVE), comprises
test satellites whose missions are primarily to check the critical technologies needed for the Galileo
system. They will also characterise the medium Earth orbit chosen for the Galileo constellation.
Europe has no experience of this environment, since there has never been a European satellite
orbiting in this region of near-Earth space.

The Galileo partners include the GNSS Supervisory Authority, which replaced the Galileo Joint
Undertaking (GJU) on 1 January 2007. The GJU initiated the development of a full set of
applications through calls for ideas in the framework of the European Commission’s research and
development programmes. The GJU was also intended to select a Galileo concessionaire within a
private-public partnership.

                Galileo technology developments

Galileo will facilitate European industry’s participation in technologies related to very accurate
position and time determination and in their applications. It will also encourage synergy with other
technologies being developed for the information society such as mobile telecommunications and
interactive services.

Critical technology

Two on board atomic clocks have been developed for Galileo:

       a Rubidium Atomic Frequency Standard
       a Passive Hydrogen Maser

In parallel, a system for generating navigation signals has been developed, with a navigation signal
generator, a navigation antenna and associated equipment.

Galileo satellite clocks

The clocks that will mark time for the next generation of navigation satellites are based on
oscillations at the atomic level. They will keep time to within a few hundred-millionths of a second
per day.

Why would anyone want to keep time so closely? The answer has to do with the speed of light.
Nowadays, you can determine your position on the Earth's surface by measuring the time taken for
a signal broadcast by a navigation satellite to reach you. As signals travel at the speed of light, this
means measuring tiny fractions of a second very accurately. And to do that, you need to know
precisely when the signal left the satellite and precisely when it arrived at your receiver. “In
navigation, clocks are the driving factor for accurately determining positions. With an accuracy of
better than one billionth second in one hour, the clocks on the Galileo satellites will allow you to
resolve your position anywhere on the Earth's surface to within 45 cm”, according to Franco Emma,
the clock expert and navigation engineer at ESTEC, ESA's technical centre in the Netherlands.

Each of the 30 satellites in the Galileo system will have two of each type of clock on board. The
clocks use different technologies, but make use of the same principle - if you force atoms to jump
from one particular energy state to another, it will radiate the associated microwave signal at an
extremely stable frequency.
This frequency is at around 6 GHz for the rubidium clock and at around 1.4 GHz for the hydrogen
clock. “We will use the clock frequency as a very stable reference by which other units can
generate the accurate signals that the Galileo satellites will broadcast”, says Franco Emma. The
broadcast signals will also provide a reference by which the less stable user receiver clocks can
continuously reset their time.

ESA has chosen the rubidium and hydrogen maser clocks as they are very stable over a few hours
and because their technology can fly onboard the Galileo satellites. If they were left to run
indefinitely, though, their timekeeping would drift, so they need to be synchronised regularly with a
network of even more stable ground-based reference clocks. These will include clocks based on the
caesium frequency standard, which show a far better long-term stability than rubidium or hydrogen
maser clocks. “The clocks on the ground will also generate what we are calling Galileo System
Time”, says Franco Emma.

The clocks that will fly on the satellites are the first of their type to be developed and built in
Europe. “Similar clocks are available in the US and in Russia (for example. those flown on the GPS
and GLONASS satellites), but we believe that we need to have an independent capability”, states
Franco Emma.

The passive hydrogen maser clock will actually be the first one of its type ever to fly. It has been
built by the Observatoire de Neuchatel in co-operation with Officine Galileo of Italy, the former
being responsible for the overall development and in particular for the so-called physics package,
the latter being in charge of the electronics. A similar arrangement applies for the rubidium clock
for which Temex Neuchâtel Time assumes overall responsibility and Astrium Germany contributes
the electronics.

The first two flight models of the rubidium clock are in orbit on board GIOVE-A, the first Galileo
satellite. The passive hydrogen maser is scheduled to launch on GIOVE-B at the end of 2007.

How to build up a constellation of 30 navigation satellites

When Galileo, Europe's own global satellite navigation system, is fully operational, there will be 30
satellites in Medium Earth Orbit (MEO) at an altitude of 23 222 kilometres. Ten 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.

Planners and engineers at ESA had good reasons for choosing such a structure for the Galileo
constellation. With 30 satellites at such an altitude, there is a very high probability (more than
90%) that anyone anywhere in the world will always be in sight of at least four satellites and hence
will be able to determine their position from the ranging signals broadcast by the satellites. The
inclination of the orbits was chosen to ensure good coverage of polar latitudes, which are poorly
served by the US GPS system.

From most locations, six to eight satellites will always be visible, allowing positions to be
determined very accurately – to within a few centimetres. Even in high rise cities, there will be a
good chance that a road user will have sufficient satellites overhead for taking a position, especially
as the Galileo system will be interoperable with the US system of 24 GPS satellites.

So how do you build up such a constellation of satellites and ensure that each one is in precisely
the correct position at any time? This delicate operation will take place in stages.

ESA launched an experimental satellite at the end of 2005, on board a Soyuz launcher. Galileo
satellites have magneto-torquers and reaction wheels to help maintain them in the correct orbit,
but they do not have engines to manoeuvre themselves into the right orbit in the first place.
Therefore, it is essential for the launcher to place the satellite directly into the correct position.

The first Galileo satellite, GIOVE-A, has been placed in the first orbital plane from where it is being
used to test the equipment on board and the functioning of ground station equipment. It has also
permitted the securing of the Galileo frequencies within the International Telecommunications
Union. The test campaign is planned to last two-and-a-half years.

Initially, the performance of the two atomic clocks on-board was characterised. Then the signal
generator was turned on to provide experimental signals with various modulation characteristics.
Over the course of the test period, scientific instruments on board are measuring various aspects of
the space environment around the orbital plane, in particular the level of radiation, which is greater
than in low Earth or geostationary orbits.

A second experimental satellite (GIOVE-B) is scheduled to be launched in late 2007. GIOVE-B will
continue the testing begun by its older sister craft, but with the addition of a passive hydrogen
maser and with a mechanical design more representative of the operational satellites.

Long lead items for a third experimental satellite (GIOVE-A2) have been ordered so as to be ready
for launch, if needed, in the second half of 2008. GIOVE-A2 is meant to maintain the International
Telecommunications Union (ITU) frequency filing that was secured by its predecessor and facilitate
further development of ground equipment, should anything happen to GIOVE-A or B since ITU
regulations do not allow a broadcast gap of more than two years.

The new satellite will incorporate some enhancements over GIOVE-A, allowing additional signals to
be generated and received on the ground. The aim is to keep on providing early in-orbit
experimentation with the common baseline L1 open service signals recommended by the European
Union and the United States. In the future, these open service signals will provide free of charge
position and timing competitive with other GNSS systems.

Next, ESA will launch the first four operational satellites using two separate launchers. The first two
satellites will be placed in the first orbital plane and the second in the second orbital plane. These
four satellites, plus part of the ground segment, will then be used to validate the Galileo system as
a whole, using advanced system simulators. Then, the next two satellites will be launched into the
third orbital plane.

Once the Galileo system has been validated, the final stage will be to build up the rest of the
constellation by completing it on all its three orbital planes. This will then require several launches
with Ariane-5 or Soyuz from the Europe’s Space Port in French Guyana. Galileo will then be fully
operational, providing its services to a wide variety of users throughout the world.

    Related links:

    Galileo Joint Undertaking (GJU) (http://www.GalileoJU.com)
     Galileo website (European Commission)
       CENT (http://www.cenc.org.cn.en)
       GLONASS (http://www.glonass-center.ru/)
    Galileo videos:

     Europe shows the way - Windows media player
     Europe shows the way - Windows media player

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