Global Navigation Satellite Systems
Global Navigation Satellite Systems (GNSS) is the standard generic term for satellite navigation systems that provide autonomous geo-spatial positioning with global coverage. 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 2009, 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 GNSS in initial deployment phase, scheduled to be operational in 2013. The People's Republic of China has indicated it will expand its regional Beidou navigation system into the global Compass navigation system by 2015.
The global coverage is achieved by constellations of about 30 MEO satellites in different orbital planes. The actual systems use orbit inclinations of >50Â° and orbital periods of 12 hours (height 20,200 km).
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.
* Core Satellite navigation systems, currently GPS, Galileo and GLONASS.
* Global Satellite Based Augmentation Systems (SBAS) such as Omnistar and StarFire.
* 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 Augmentation 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 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 (USNO) 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 (from which the position of the satellite can be calculated) 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.
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. (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 receiver
In these ways, satellite navigation can be regarded as a force multiplier. In particular, the ability to reduce unintended casualties has particular advantages for wars 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 hiking, to devices fitted to cars, trucks, ships and aircraft
* Time transfer and synchronization
* Location-based services such as enhanced 911
* Entering data into a geographic information system
* Search and rescue
* Geophysical Sciences
* Tracking devices used in wildlife management
* Asset Tracking, as in trucking fleet management
* Road Pricing
* Location-based media
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.
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.ion Satellite System
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