Satellite navigation

Global Navigation Satellite Systems (GNSS) is the standard generic term for satellite navigation systems ("sat nav") 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 calculate the precise time as well as position, which can be used as a reference for scientific experiments.

As of 2010, the United States NAVSTAR Global Positioning System (GPS) and the Russian GLONASS are the only two fully operational GNSS. The European Union's Galileo positioning system is a GNSS in initial deployment phase, scheduled to be operational in 2014. The People's Republic of China has indicated it will expand its regional Beidou navigation system into the global Compass navigation system by 2020.

The global coverage for each system is generally achieved by a 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).

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 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

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.

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

Main article: 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 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.

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

Main article: Global Positioning System

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.

In development

Galileo

Main article: Galileo (satellite navigation)

The European Union and European Space Agency agreed in March 2002 to introduce their own alternative to GPS, called the Galileo positioning system. At a estimated price of GBP £2.4 billion, the system of 30 MEO satellites was originally scheduled to be operational in 2010. The estimated year to become operational is 2012 . 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.

GLONASS

Main article: GLONASS

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 had fallen into disrepair, leading to gaps in coverage and only partial availability. As of September 2010 GLONASS is fully restored (24 of 24 satellites are operational).

COMPASS

Main article: COMPASS navigation system

China has indicated they intend to expand their regional navigation system, called Beidou or Big Dipper, into a global navigation system by 2020 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.

Comparison of GNSS systems

System	Country	Coding	Orbital height & period	Number of satellites	Frequency	Status
GPS	United States	CDMA	20,200 km, 12.0h	≥ 24	1.57542 GHz (L1 signal) 1.2276 GHz (L2 signal)	operational
GLONASS	Russia	FDMA/CDMA	19,100 km, 11.3h	24 (30 when CDMA signal launches)	Around 1.602 GHz (SP) Around 1.246 GHz (SP)	operational with restrictions, CDMA in preparation
Galileo	European Union	CDMA	23,222 km, 14.1h	2 test bed satellites in orbit 22 operational satellites budgeted	1.164-1.215 GHz (E5a and E5b) 1.215-1.300 GHz (E6) 1.559-1.592 GHz (E2-L1-E11)	in preparation
COMPASS	China	CDMA	21,150 km, 12.6h	35	B1: 1,561098 GHz B1-2: 1.589742 GHz B2: 1.207.14 GHz B3: 1.26852 GHz	5 satellites operational, additional 30 satellites planned

Regional navigation systems

Beidou 1

Main article: Beidou navigation system

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

DORIS

Main article: DORIS (geodesy)

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

IRNSS

Main article: Indian Regional Navigational Satellite 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. The government approved the project in May 2006, with the intention of the system to be completed and implemented by 2014. It will consist of a constellation of 7 navigational satellites. 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. A goal of complete Indian control has been stated, with the space segment, ground segment and user receivers all being built in India.

QZSS

Main article: 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 demonstration satellite is scheduled to be launched in 2009.

GNSS Augmentation

Main article: GNSS 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.

GNSS positioning calculation

Main article: GNSS positioning calculation

Topics to be covered

• Differential satellite navigation
• GNSS reflectometry
• Phase-counting differential satellite navigation
• Trends within GNSS

See also

• Differential GPS
• Global Positioning System
• Galileo (satellite navigation)
• GLONASS
• GPS-Aided Geo-Augmented Navigation
• Wide Area Augmentation System
• European Geostationary Navigation Overlay Service
• GNSS reflectometry
• Space-Integrated GPS/INS
• Receiver Autonomous Integrity Monitoring
• Geoinformatics
• List of emerging technologies

External links

Information on specific GNSS systems

• ESA information on EGNOS
• Information on the Beidou system
• German Federal Waterways Administration Traffic Technologies Centre Information on GPS / DGPS

Organizations related to GNSS

• United Nations International Committee on Global Navigation Satellite Systems (ICG)
• Korean GNSS Technology Council (GTC)
• Institute of Navigation (ION) GNSS Meetings
• The International GNSS Service (IGS), formerly the International GPS Service
• International Global Navigation Satellite Systems Society Inc (IGNSS)
• International Earth Rotation and Reference Systems Service (IERS) International GNSS Service (IGS)
• National Geodetic Survey Orbits for the Global Positioning System satellites in the Global Navigation Satellite System
• UNAVCO GNSS Modernization
• Asia-Pacific Economic Cooperation (APEC) GNSS Implementation Team
• Fédération Aéronautique Internationale (FAI) GNSS Flight Recorder Approval Committee (GFAC)

Sat Nav Manufacturers

• Leica Geosystems
• Sapper Oy
• Trimble
• Garmin
• Magellan
• TomTom
• NDrive

Supportive or illustrative sites

• GPS and GLONASS Simulation (Java applet) Simulation and graphical depiction of the motion of space vehicles, including DOP computation.
• GPS Forums Discussion forums relating to GPS.
• GPS Sat Nav Buying Guide