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Global Navigation Satellite System (GNSS) is the standard generic term for
satellite navigation systems that provide autonomous geo-spatial positioning
with global coverage. A GNSS allows small electronic receivers to determine
their location (longitude, latitude, and altitude) to within a few metres using
time signals transmitted along a line of sight by radio from satellites.
Receivers on the ground with a fixed position can also be used to calculate the
precise time as a reference for scientific experiments.
As of 2007, the United States NAVSTAR Global Positioning System (GPS) is the
only fully operational GNSS. The Russian GLONASS is a GNSS in the process of
being restored to full operation. The European Union's Galileo positioning
system is a next generation GNSS in the initial deployment phase, scheduled to
be operational in 2010. China has indicated it may expand its regional Beidou
navigation system into a global system. India's IRNSS, a next generation GNSS is
in developmental phase and is scheduled to be operational around 2012.
GNSS classification
GNSS that provide enhanced accuracy and integrity monitoring usable for civil
navigation are classified as follows:
GNSS-1 is the first generation system and is the combination of existing
satellite navigation systems (GPS and GLONASS), with Satellite Based
Augmentation Systems (SBAS) or Ground Based Augmentation Systems (GBAS). In the
United States, the satellite based component is the Wide Area Augmentation
System (WAAS), in Europe it is the European Geostationary Navigation Overlay
Service (EGNOS), and in Japan it is the Multi-Functional Satellite Augmentation
System (MSAS). Ground based augmentation is provided by systems like the Local
Area Augmentation System (LAAS).
GNSS-2 is the second generation of systems that independently provides a full
civilian satellite navigation system, exemplified by the European Galileo
positioning system. These systems will provide the accuracy and integrity
monitoring necessary for civil navigation. This system consists of L1 and L2
frequencies for civil use and L5 for system integrity. Development is also in
progress to provide GPS with civil use L2 and L5 frequencies, making it a GNSS-2
system.1
Core Satellite navigation systems, currently GPS, Galileo and GLONASS.
Global Satellite Based Augmnetation Systems (SBAS) such as Omnistar and Stafire.
Regional SBAS including WAAS(US), EGNOS (EU), MSAS (Japan) and GAGAN (India).
Regional Satellite Navigation Systems such a QZSS (Japan), IRNSS (India) and
Beidou (China).
Continental scale Ground Based Augmentaion Systems (GBAS) for example the
Australian GRAS and the US Department of Transportation National Differential
GPS (DGPS) service.
Regional scale GBAS such as CORS networks.
Local GBAS typified by a single GPS reference station operating Real Time
Kinematic (RTK) corrections.
History and theory
Early predecessors were the ground based DECCA, LORAN and Omega systems, which
used terrestrial longwave radio transmitters instead of satellites. These
systems broadcast a radio pulse from a known "master" location, followed by
repeated pulses from a number of "slave" stations. The delay between the
reception and sending of the signal at the slaves was carefully controlled,
allowing the receivers to compare the delay between reception and the delay
between sending. From this the distance to each of the slaves could be
determined, providing a fix.
The first satellite navigation system was Transit, a system deployed by the US
military in the 1960s. Transit's operation was based on the Doppler effect: the
satellites traveled on well-known paths and broadcast their signals on a well
known frequency. The received frequency will differ slightly from the broadcast
frequency because of the movement of the satellite with respect to the receiver.
By monitoring this frequency shift over a short time interval, the receiver can
determine its location to one side or the other of the satellite, and several
such measurements combined with a precise knowledge of the satellite's orbit can
fix a particular position.
Part of an orbiting satellite's broadcast included its precise orbital data. In
order to ensure accuracy, the US Naval Observatory (USNO) continuously observed
precisely the orbits of these satellites. As a satellite's orbit deviated, the
USNO would send the updated information to the satellite. Subsequent broadcasts
from an updated satellite would contain the most recent accurate information
about its orbit.
Modern systems are more direct. The satellite broadcasts a signal that contains
the position of the satellite and the precise time the signal was transmitted.
The position of the satellite is transmitted in a data message that is
superimposed on a code that serves as a timing reference. The satellite uses an
atomic clock to maintain synchronization of all the satellites in the
constellation. The receiver compares the time of broadcast encoded in the
transmission with the time of reception measured by an internal clock, thereby
measuring the time-of-flight to the satellite. Several such measurements can be
made at the same time to different satellites, allowing a continual fix to be
generated in real time.
Each distance measurement, regardless of the system being used, places the
receiver on a spherical shell at the measured distance from the broadcaster. By
taking several such measurements and then looking for a point where they meet, a
fix is generated. However, in the case of fast-moving receivers, the position of
the signal moves as signals are received from several satellites. In addition,
the radio signals slow slightly as they pass through the ionosphere, and this
slowing varies with the receiver's angle to the satellite, because that changes
the distance through the ionosphere. The basic computation thus attempts to find
the shortest directed line tangent to four oblate spherical shells centered on
four satellites. Satellite navigation receivers reduce errors by using
combinations of signals from multiple satellites and multiple correlators, and
then using techniques such as Kalman filtering to combine the noisy, partial,
and constantly changing data into a single estimate for position, time, and
velocity.
Civil and military uses
GNSS applications
The original motivation for satellite navigation was for military applications.
Satellite navigation allows for hitherto impossible precision in the delivery of
weapons to targets, greatly increasing their lethality whilst reducing
inadvertent casualties from mis-directed weapons. (See smart bomb). Satellite
navigation also allows forces to be directed and to locate themselves more
easily, reducing the fog of war.
Satellite navigation using a laptop and a GPS receiverIn these ways, satellite
navigation can be regarded as a force multiplier. In particular, the ability to
reduce unintended casualties has particular advantages for wars being fought by
democracies, where public relations is an important aspect of warfare. For these
reasons, a satellite navigation system is an essential asset for any aspiring
military power.
GNSS systems have a wide variety of uses:
Navigation, ranging from personal hand-held devices for trekking, to devices
fitted to cars, trucks, ships and aircraft
Time transfer and synchronization
Location-based services such as enhanced 911
Surveying
Entering data into a geographic information system
Search and rescue
Geophysical Sciences
Tracking devices used in wildlife management
Note that the ability to supply satellite navigation signals is also the ability
to deny their availability. The operator of a satellite navigation system
potentially has the ability to degrade or eliminate satellite navigation
services over any territory it desires. Thus, as satellite navigation becomes an
essential service, countries without their own satellite navigation systems
effectively become client states of those which supply these services.
The same applies to the use of smart bombs: the operator of a satellite
navigation system can effectively degrade the performance of smart bombs being
used by other states using its satellite navigation system to that of gravity
bombs, or even offset them from their targets in such a way as to render them
useless.
Current global navigation systems
GPS
Global Positioning System
The United States' Global Positioning System (GPS), which as of 2007 is the only
fully functional, fully available global navigation satellite system. It
consists of up to 32 medium Earth orbit satellites in six different orbital
planes, with the exact number of satellites varying as older satellites are
retired and replaced. Operational since 1978 and globally available since 1994,
GPS is currently the world's most utilized satellite navigation system.
GLONASS
GLONASS
The formerly Soviet, and now Russian, global navigation system is called
GLONASS. It was a fully functional navigation constellation but since the
collapse of the Soviet Union has fallen into disrepair, leading to moving gaps
in coverage and only partial availability. It is expected to return to full
global availability by 2010 with the help of India, who is participating in the
restoration project.
Proposed global navigation systems
Compass
COMPASS navigation system
China has indicated they intend to expand their regional navigation system,
called Beidou or Big Dipper, into a global navigation system; a program that has
been called Compass in China's official news agency Xinhua. The Compass system
is proposed to utilize 30 medium Earth orbit satellites and five geostationary
satellites. Having announced they are willing to cooperate with other countries
in Compass's creation, it is unclear how this proposed program impacts China's
commitment to the international Galileo position system.
DORIS
DORIS
Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS) is a
French precision system.
Galileo
Galileo Positioning System
The European Union and European Space Agency agreed on March 2002 to introduce
their own alternative to GPS, called the Galileo positioning system. At a cost
of about GBP ￡2.4 billion, the required satellites will be launched between 2006
and 2008 and the system will be working, under civilian control, from 2010. The
first experimental satellite was launched on 28 December 2005. Galileo is
expected to be compatible with the modernized GPS system that will be
operational by after 2012. The receivers will be able to combine the signals
from both Galileo and GPS satellites to greatly increase the accuracy.
IRNSS
Indian Regional Navigational Satellite System
The Indian Regional Navigational Satellite System (IRNSS) is a proposed
autonomous regional satellite navigation system to be constructed and controlled
by the Indian government. It is intended to provide an absolute position
accuracy of better than 20 meters throughout India and within a region extending
approximately 1,500 to 2,000 km around it. A goal of complete Indian control has
been stated, with the space segment, ground segment and user receivers all being
built in India. The government approved the project in May 2006, with the
intention it be implemented within six to seven years.
QZSS
Quasi-Zenith Satellite System
The Quasi-Zenith Satellite System (QZSS), is a proposed three-satellite regional
time transfer system and enhancement for GPS covering Japan. The first satellite
is scheduled to be launched in 2008.
GNSS Augmentation
GNSS Augmentation
GNSS Augmentation involves using external information, often integrated into the
calculation process, to improve the accuracy, availability, or reliability of
the satellite navigation signal. There are many such systems in place and they
are generally named or described based on how the GNSS sensor receives the
information. Some systems transmit additional information about sources of error
(such as clock drift, ephemeris, or ionospheric delay), others provide direct
measurements of how much the signal was off in the past, while a third group
provide additional navigational or vehicle information to be integrated in the
calculation process.
Examples of augmentation systems include the Wide Area Augmentation System, the
European Geostationary Navigation Overlay Service, the Multi-functional
Satellite Augmentation System, Differential GPS, and Inertial Navigation
Systems.
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