A satellite navigation or sat nav system is a system of satellites that provide autonomous
geo-spatial positioning with global coverage. It allows small electronic receivers to determine
their location to high precision using time signals transmitted along a line of sight
by radio from satellites. The signals also allow the electronic receivers to calculate
the current local time to high precision, which allows time synchronisation. A satellite
navigation system with global coverage may be termed a global navigation satellite system
or GNSS. As of April 2013, only the United States NAVSTAR
Global Positioning System and the Russian GLONASS are global operational GNSSs. China
is in the process of expanding its regional Beidou navigation system into the global Compass
navigation system by 2020. The European Union's Galileo positioning system is a GNSS in initial
deployment phase, scheduled to be fully operational by 2020 at the earliest. France, India and
Japan are in the process of developing regional navigation systems.
Global coverage for each system is generally achieved by a satellite constellation of 20–30
medium Earth orbit satellites spread between several orbital planes. The actual systems
vary, but use orbital inclinations of >50° and orbital periods of roughly twelve hours).
Classification Satellite navigation systems 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, with Satellite Based Augmentation
Systems or Ground Based Augmentation Systems. In the United States, the satellite based
component is the Wide Area Augmentation System, in Europe it is the European Geostationary
Navigation Overlay Service, and in Japan it is the Multi-Functional Satellite Augmentation
System. Ground based augmentation is provided by systems like the Local Area Augmentation
System. 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; including aircraft. 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, GLONASS, Galileo and Compass. Global Satellite Based Augmentation Systems
such as Omnistar and StarFire. Regional SBAS including WAAS, EGNOS, MSAS
and GAGAN. Regional Satellite Navigation Systems such
as China's Beidou, India's yet-to-be-operational IRNSS, and Japan's proposed QZSS.
Continental scale Ground Based Augmentation Systems for example the Australian GRAS and
the US Department of Transportation National Differential GPS service.
Regional scale GBAS such as CORS networks. Local GBAS typified by a single GPS reference
station operating Real Time Kinematic corrections. History and theory
Early predecessors were the ground based DECCA, LORAN, GEE and Omega radio navigation systems,
which used terrestrial longwave radio transmitters instead of satellites. These positioning 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 continuously observed the precise 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 orbital data and the precise time the signal was transmitted.
The orbital data 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 using an adapted version of trilateration: see GNSS positioning calculation for details.
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
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..
Satellite navigation also allows forces to be directed and to locate themselves more
easily, reducing the fog of war. 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. Global navigation systems
Operational GPS
The United States' Global Positioning System 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
The formerly Soviet, and now Russian, Global'naya Navigatsionnaya Sputnikovaya Sistema, or GLONASS,
was a fully functional navigation constellation in 1995. After the collapse of the Soviet
Union, it fell into disrepair, leading to gaps in coverage and only partial availability.
It was recovered and fully restored in 2011. BeiDou
China has indicated they plan to complete the entire second generation Beidou Navigation
Satellite System, by expanding current regional service into global coverage by 2020. The
BeiDou-2 system is proposed to consists of 30 medium Earth orbit satellites and five
geostationary satellites. A 16-satellite regional version was completed by December 2012.
In development DORIS
Doppler Orbitography and Radio-positioning Integrated by Satellite is a French precision
navigation system. Galileo
The European Union and European Space Agency agreed in March 2002 to introduce their own
alternative to GPS, called the Galileo positioning system. At an estimated cost of EUR 3.0 billion,
the system of 30 MEO satellites was originally scheduled to be operational in 2010. The original
year to become operational was 2014. The first experimental satellite was launched on 28
December 2005. Galileo is expected to be compatible with the modernized GPS system. The receivers
will be able to combine the signals from both Galileo and GPS satellites to greatly increase
the accuracy. Galileo is now not expected to be in full service until 2020 at the earliest
and at a substantially higher cost. Comparison of systems
Regional navigation systems BeiDou-2
Chinese regional network to be expanded into the whole global system which consists of
all 35 satellites by 2020. IRNSS
The Indian Regional Navigational Satellite System is an autonomous regional satellite
navigation system being developed by Indian Space Research Organisation which would be
under the total control of Indian government. The government approved the project in May
2006, with the intention of the system to be completed and implemented by 2015. It will
consist of a constellation of 7 navigational satellites. All the 7 satellites will be placed
in the Geostationary orbit to have a larger signal footprint and lower number of satellites
to map the region. It is intended to provide an all-weather absolute position accuracy
of better than 7.6 meters throughout India and within a region extending approximately
1,500 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 first two satellites IRNSS-1A
and IRNSS-1B of the proposed constellation were precisely launched on 1 July 2013 and
4 April 2014 respectively from Satish Dhawan Space Centre. The next two satellites IRNSS-1C
and IRNSS-1D of the proposed constellation are planned to be launched by end of 2014
and the remaining three satellites IRNSS-1E, IRNSS-1F and IRNSS-1G are planned to be launched
by middle of 2015. QZSS
The Quasi-Zenith Satellite System, is a proposed three-satellite regional time transfer system
and enhancement for GPS covering Japan. The first demonstration satellite was launched
in September 2010. Augmentation
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.
Low Earth orbit satellite phone networks The two current operational low Earth orbit
satellite phone networks are able to track transceiver units with accuracy of a few kilometers
using doppler shift calculations from the satellite. The coordinates are sent back to
the transceiver unit where they can be read using AT commands or a graphical user interface.
This can also be used by the gateway to enforce restrictions on geographically bound calling
plans. Positioning calculation
See also
Notes
References
Further reading Office for Outer Space Affairs of the United
Nations, Report on Current and Planned Global and Regional Navigation Satellite Systems
and Satellite-based Augmentation Systems. [2]
External links Information on specific GNSS systems
ESA information on EGNOS Information on the Beidou system
Organizations related to GNSS United Nations International Committee on
Global Navigation Satellite Systems Institute of Navigation GNSS Meetings
The International GNSS Service, formerly the International GPS Service
International Global Navigation Satellite Systems Society Inc
International Earth Rotation and Reference Systems Service International GNSS Service
US National Executive Committee for Space-Based Positioning, Navigation, and Timing
US National Geodetic Survey Orbits for the Global Positioning System satellites in the
Global Navigation Satellite System UNAVCO GNSS Modernization
Asia-Pacific Economic Cooperation GNSS Implementation Team
Supportive or illustrative sites GPS and GLONASS Simulation Simulation and
graphical depiction of the motion of space vehicles, including DOP computation.
GPS, GNSS, Geodesy and Navigation Concepts in depth