Satellite navigation


Satellite navigation
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v · 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 calculate the precise time as well as position, which can be used as a reference for scientific experiments. A satellite navigation system with global coverage may be termed a global navigation satellite system or GNSS.

As of October 2011, the United States NAVSTAR Global Positioning System (GPS) and the Russian GLONASS are fully globally operational GNSSs. The People's Republic of China is in the process of expanding its regional Beidou navigation system into the global Compass navigation system by 2020.[1] The European Union's Galileo positioning system is a GNSS in initial deployment phase, scheduled to be fully operational by 2020 at the earliest.[2] The Indian Regional Navigational Satellite System (IRNSS) is an autonomous regional satellite navigation system being developed by Indian Space Research Organisation.[3]

Global coverage for each system is generally achieved by a satellite constellation of 20–30 Medium Earth Orbit (MEO) satellites spread between several orbital planes. The actual systems vary, but use orbit inclinations of >50° and orbital periods of roughly twelve hours (height 20,000 km / 12,500 miles).

Contents

Classification

Satellite navigation systems that provide enhanced accuracy and integrity monitoring usable for civil navigation are classified as follows:[4]

  • GNSS-2[citation needed] 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[citation needed]
  • 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 as China's Beidou, India's yet-to-be-operational IRNSS, and Japan's proposed QZSS.
  • 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

Accuracy of Navigation Systems.svg

Early predecessors were the ground based DECCA, LORAN 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 (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 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. (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.[citation needed]

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

Comparison of GPS, GLONASS, Galileo and Compass (medium earth orbit) satellite navigation system orbits with the International Space Station, Hubble Space Telescope and Iridium constellation orbits, Geostationary Earth Orbit, and the nominal size of the Earth. The Moon's orbit is 9.1 times larger (in radius and length) than geostationary orbit.[5]

Operational

GPS

The United States' Global Positioning System (GPS) 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 (Global Navigation Satellite System), 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 restored in 2011.

In development

Compass

China has indicated they intend to expand their regional navigation system, called Beidou or Big Dipper, into a global navigation system by 2020[6] 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. A 12-satellite regional version is expected to be completed by 2012.

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,[7] the system of 30 MEO satellites was originally scheduled to be operational in 2010. The estimated year to become operational is 2014.[8] The first experimental satellite was launched on 28 December 2005[citation needed]. 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.[9]

Comparison of systems

System GPS GLONASS Galileo COMPASS
Political entity United States Russia European Union China
Coding CDMA FDMA/CDMA CDMA CDMA
Orbital height 20,200 km (12,600 mi) 19,100 km (11,900 mi) 23,222 km (14,429 mi) 21,150 km (13,140 mi)
Period 12.0 hours 11.3 hours (11 h 18 m) 14.1 hours (14 h 6 m) 12.6 hours (12 h 36 m)
Number of
satellites
At least 24 30, including
23 operational
4 in preparation
2 on maintenance
1 reserve[10]
(30 when CDMA signal launches)
2 test bed satellites in orbit,
22 operational satellites budgeted
35[11]
Frequency 1.57542 GHz (L1 signal)
1.2276 GHz (L2 signal)
Around 1.602 GHz (SP)
Around 1.246 GHz (SP)
1.164–1.215 GHz (E5a and E5b)
1.215–1.300 GHz (E6)
1.559–1.592 GHz (E2-L1-E11)
1.561098 GHz (B1)
1.589742 GHz (B1-2)
1.20714 GHz (B2)
1.26852 GHz (B3)
Status Operational Operational,
CDMA in preparation
In preparation 5 satellites operational,
30 additional satellites planned

Regional navigation systems

Beidou 1

Chinese regional network to be expanded into the global COMPASS Navigation System.

DORIS

Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS) is a French precision navigation system.[12]

IRNSS

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. The government approved the project in May 2006, with the intention of the system to be completed and implemented by 2014.[13] It will consist of a constellation of 7 navigational satellites.[14] 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. 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.[15] A goal of complete Indian control has been stated, with the space segment, ground segment and user receivers all being built in India.[16]

QZSS

The Quasi-Zenith Satellite System (QZSS), is a proposed three-satellite regional time transfer system and enhancement for GPS covering Japan. The first demonstration satellite was launched in September 2010.[17]

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.[18][19] This can also be used by the gateway to enforce restrictions on geographically bound calling plans.

Positioning calculation


See also

References

  1. ^ Beidou satellite navigation system to cover whole world in 2020
  2. ^ http://www.spacenews.com/civil/110120-galileo-assessment-punches.html
  3. ^ http://www.livemint.com/2007/09/05002237/India-to-build-a-constellation.html
  4. ^ "A Beginner’s Guide to GNSS in Europe" (PDF). IFATCA. http://www.ifatca.org/docs/gnss.pdf. 
  5. ^ Orbital periods and speeds are calculated using the relations 4π²R³ = T²GM and V²R = GM, where R = radius of orbit in metres, T = orbital period in seconds, V = orbital speed in m/s, G = gravitational constant ≈ 6.673×10−11 Nm²/kg², M = mass of Earth ≈ 5.98×1024 kg.
  6. ^ Beidou satellite navigation system to cover whole world in 2020
  7. ^ "Boost to Galileo sat-nav system". BBC News. 25 August 2006. http://news.bbc.co.uk/1/hi/sci/tech/5286200.stm. Retrieved 2008-06-10. 
  8. ^ "Commission awards major contracts to make Galileo operational early 2014". 2010-01-07. http://europa.eu/rapid/pressReleasesAction.do?reference=IP/10/7&language=en. Retrieved 2010-04-19. 
  9. ^ http://www.spacenews.com/civil/110120-galileo-assessment-punches.html
  10. ^ GLONASS status
  11. ^ China to send third navigation satellite into orbit
  12. ^ DORIS information page
  13. ^ April 15 launch to give India its own GPS
  14. ^ India to develop its own version of GPS
  15. ^ Launch of first satellite for Indian Regional Navigation Satellite system next year
  16. ^ India to build a constellation of 7 navigation satellites by 2012
  17. ^ "JAXA Quasi-Zenith Satellite System". JAXA. http://qzss.jaxa.jp/is-qzss/qzss_e.html. Retrieved 2009-02-22. 
  18. ^ Globalstar GSP-1700 manual
  19. ^ http://www.skyhelp.net/acrobat/jan_05/Iridium%20SBD-FAQ%201-05.pdf

External links

Information on specific GNSS systems

Organizations related to GNSS

Sat Nav Manufacturers

Supportive or illustrative sites


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