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Today there are two operating GNSS (Global Navigation Satellite
System) systems - the United States' Global Positioning System (GPS),
and the Russian Federation's Global Orbiting Navigation Satellite
System (GLONASS). A third one is about to become operetional in
2008 - Europe's Galileo.
A GNSS is a satellite system that is used to pinpoint the geographic
location of a user's receiver anywhere in the world. Each of the GNSS
systems employs a constellation of orbiting satellites working in
conjunction with a network of ground stations. Satellite-based
navigation systems use a version of triangulation to locate the user,
through calculations involving information from a number of satellites.
Each satellite transmits coded signals at precise intervals. The
receiver converts signal information into position, velocity, and time
estimates. Using this information, any receiver on or near the earth's
surface can calculate the exact position of the transmitting satellite
and the distance (from the transmission time delay) between it and
the receiver. Coordinating current signal data from four or more
satellites enables the receiver to determine its position.
NAVSTAR GPS
The Global Positioning System (GPS) satellite network is operated by
the U.S. Air Force to provide highly accurate navigation information to
military forces around the world. The network is also being used by a
growing number of commercial products. The GPS space segment
consists of into six orbital planes, requiring a minimum of four
satellites in each, to operate. The GPS control segment consists of
five monitoring stations (Hawaii, Kwajalein, Ascension Island, Diego
Garcia, Colorado Springs), three ground antennas, (Ascension Island,
Diego Garcia, Kwajalein), and a Master Control station located at
Schriever AFB in Colorado. The NAVSTAR Global Positioning System is
managed by the NAVSTAR GPS Joint Program Office at the Space and
Missile Systems Center, Los Angeles Air Force Base, California.
GLONASS
The Global Navigation Satellite System (GLONASS) is based on a
constellation of active satellites which continuously transmit coded
signals in two frequency bands, which can be received by users
anywhere on the Earth's surface to identify their position and velocity
in real time based on ranging measurements. The system is a
counterpart to the United States Global Positioning System (GPS) and
both systems share the same principles in the data transmission and
positioning methods. GLONASS is managed for the Russian Federation
Government by the Russian Space Forces and the system is operated
by the Coordination Scientific Information Center (KNITs) of the
Ministry of Defense of the Russian Federation.
The operational space segment of GLONASS consists of 21 satellites
in 3 orbital planes, with 3 on-orbit spares. The three orbital planes
are separated 120 degrees, and the satellites within the same orbit
plane by 45 degrees. Each satellite operates in circular 19,100 km
orbits at an inclination angle of 64.8 degrees and each satellite
completes an orbit in approximately 11 hours 15 minutes. The ground
control segment of GLONASS is entirely located within former Soviet
Union territory. The Ground Control Center and Time Standards is
located in Moscow and the telemetry and tracking stations are in St.
Petersburg, Ternopol, Eniseisk, Komsomolsk-na-Amure.
The first GLONASS satellites were launched into orbit in 1982 and
work is underway to modernize the system. The new GLONASS-M
satellite will have better signal characteristics as well as a longer
design life (7-8 years instead of the current 3 years). In the future,
plans are being developed to transition to a low mass third
generation GLONASS-K satellites with a guaranteed lifespan of 10
years.
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 system. 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. 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.
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 plane. 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.
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