Satellite navigation system

These positioning systems broadcast a radio pulse from a known master location, followed by repeated pulses from a number of slave stations. In particular, the ability to reduce unintended casualties has particular advantages for wars where public relations is an important aspect of warfare.

However, in the case of fast-moving receivers, the position of the signal moves as signals are received from several satellites. 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 basic computation thus attempts to find the shortest directed line tangent to four oblate spherical shells centered on four satellites. In order to ensure accuracy, the US Naval Observatory (USNO) continuously observed the precise orbits of these satellites.

The orbital data is transmitted in a data message that is superimposed on a code that serves as a timing reference. This can also be used by the gateway to enforce restrictions on geographically bound calling plans. DCF77 HBG JJY RJH66 Time from NPL TDF WWVB BPM CHU HD2IOA HLA RWM WWV WWVH YVTO GPS Beidou Galileo GLONASS IRNSS OMA OLB5 VNG Y3S .

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 receivers will be able to combine the signals from both Galileo and GPS satellites to greatly increase the accuracy. 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 European Union s Galileo positioning system is a GNSS in initial deployment phase, scheduled to be operational in 2013. 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 Russian GLONASS is a GNSS in the process of being restored to full operation. 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.

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. 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.

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. As a satellite s orbit deviated, the USNO would send the updated information to the satellite.

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 Medium Earth Orbit (MEO) satellites in different orbital planes. Subsequent broadcasts from an updated satellite would contain the most recent accurate information about its orbit. Modern systems are more direct.

The operator of a satellite navigation system potentially has the ability to degrade or eliminate satellite navigation services over any territory it desires. The United States Global Positioning System (GPS), which as of 2007 is the only fully functional, fully available global navigation satellite system. The actual systems use orbit inclinations of >50° and orbital periods of 11 hours 58 minutes (height 20,200 km). GNSS that provide enhanced accuracy and integrity monitoring usable for civil navigation are classified as follows: Early predecessors were the ground based DECCA, LORAN and Omega systems, which used terrestrial longwave radio transmitters instead of satellites.

The received frequency will differ slightly from the broadcast frequency because of the movement of the satellite with respect to the receiver. 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.

The satellite uses an atomic clock to maintain synchronization of all the satellites in the constellation. Global Navigation Satellite Systems (GNSS) is the standard generic term for satellite navigation systems that provide autonomous geo-spatial positioning with global coverage.

A goal of complete Indian control has been stated, with the space segment, ground segment and user receivers all being built in India. The Quasi-Zenith Satellite System (QZSS), is a proposed three-satellite regional time transfer system and enhancement for GPS covering Japan. The government approved the project in May 2006, with the intention of the system to be completed and implemented by 2012.

Galileo is expected to be compatible with the modernized GPS system. 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. 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 Russian Federation has pledged to restore it to full global availability by 2010 with the help of India, who is participating in the restoration project. The European Union and European Space Agency agreed on March 2002 to introduce their own alternative to GPS, called the Galileo positioning system. It is intended to provide an absolute position accuracy of better than 20 meters throughout India and within a region extending approximately 2,000 km around it.

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. 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. Chinese regional network to be expanded into the global COMPASS Navigation System Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS) is a French precision navigation system. The Indian Regional Navigational Satellite System (IRNSS) is an autonomous regional satellite navigation system being developed by Indian Space Research Organisation which would be under the total control of Indian government.

There are many such systems in place and they are generally named or described based on how the GNSS sensor receives the information. 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.

For these reasons, a satellite navigation system is an essential asset for any aspiring military power. GNSS systems have a wide variety of uses: Note that the ability to supply satellite navigation signals is also the ability to deny their availability. The coordinates are sent back to the transceiver unit where they can be read using AT commands or a graphical user interface.

Operational since 1978 and globally available since 1994, GPS is currently the world s most utilized satellite navigation system. The formerly Soviet, and now Russian, GLObal naya NAvigatsionnaya Sputnikovaya Sistema(Global navigation satellite system), or GLONASS, was a fully functional navigation constellation but since the collapse of the Soviet Union has fallen into disrepair, leading to gaps in coverage and only partial availability. 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.

By taking several such measurements and then looking for a point where they meet, a fix is generated. At a cost of about GBP £2.4 billion, the system is scheduled to be working from 2012.

The first experimental satellite was launched on 28 December 2005. It will consist of a constellation of 7 navigational satellites by 2012.

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. All the 7 satellites will be placed in the Geostationary orbit (GEO) to have a larger signal footprint and lower number of satellites to map the region.

The Compass system is proposed to utilize 30 medium Earth orbit satellites and five geostationary 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. The original motivation for satellite navigation was for military applications.

Satellite navigation also allows forces to be directed and to locate themselves more easily, reducing the fog of war. In these ways, satellite navigation can be regarded as a force multiplier. The first demonstration satellite is scheduled to be launched in 2009. GNSS Augmentation involves using external information, often integrated into the calculation process, to improve the accuracy, availability, or reliability of the satellite navigation signal.

(See smart bomb).