Differential Global Positioning System (DGPS) is an enhancement to Global Positioning System that uses a network of fixed ground based reference stations to broadcast the difference between the positions indicated by the satellite systems and the known fixed positions. These stations broadcast the difference between the measured satellite pseudoranges and actual (internally computed) pseudoranges, and receiver stations may correct their pseudoranges by the same amount. GPS satellite in orbit, image courtesy NASA GPS redirects here. ... A satellite is any object that orbits another object (which is known as its primary). ... The pseudorange (from pseudo and range) is a first-approximation measurement for the distance between a satellite and a navigation satellite receiver - for instance Global Positioning System (GPS) receivers. ...

The term can refer both to the generalized technique, as well as specific implementations using it. For instance, the FAA's WAAS system uses differential techniques to improve accuracy, broadcasting the corrections via L band satellite transmissions. Other examples of similar systems include the European Geostationary Navigation Overlay System, Japan's Multi-Functional Satellite Augmentation System, Canada's CDGPS and the commercial StarFire and OmniSTAR systems. The Federal Aviation Administration is the entity of the United States government which regulates and oversees all aspects of civil aviation in the U.S. // Activities Along with the European Joint Aviation Authorities, the FAA is one of the two main agencies worldwide responsible for the certification of new aircraft. ... WAAS Operation The Wide Area Augmentation System (WAAS) is a system that improves the precision and accuracy of global positioning system (GPS) signals. ... L band (20-cm radar long-band) is a portion of the microwave band of the electromagnetic spectrum ranging roughly from 0. ... The European geostationary navigation overlay system (EGNOS) is a satellite navigation system under development by the European Space Agency, the European Commission and EUROCONTROL. It is intended to supplement the GPS and GLONASS systems by reporting on the reliability and accuracy of the signals. ... MTSAT are a series of weather and aviation control satellites. ... GPS·C, short for GPS Correction, is a set of Differential GPS data for most of Canada maintained by the Canadian Active Control System, part of Natural Resources Canada. ... StarFire is a wide-area Differential GPS system developed by John Deeres NavCom and Precision Farming groups. ...

The term is also often used to refer specifically to systems that re-broadcast the corrections from ground-based transmitters of shorter range. For instance, the United States Coast Guard runs one such system in the US and Canada on the longwave radio frequencies between 285 kHz and 325 kHz. These frequencies are commonly used for marine radio, and are broadcast near major waterways and harbors. Australia runs a similar service for land and air navigation, broadcasting their signal on commercial AM radio stations. Coast Guard shield The United States Coast Guard (USCG) is a military branch of the United States involved in maritime law, mariner assistance and search and rescue, among other duties of any coast guard. ... Longwave radio frequencies are those below 500 kHz, which correspond to wavelengths longer than 600 meters. ... Mediumwave radio transmissions (sometimes called Medium frequency or MF) are those between the frequencies of 300 kHz and 3000 kHz. ...

History

When GPS was first being put into service, the military was concerned about the possibility of enemy forces using the globally-available GPS signals to guide their own weapon systems. To avoid this, the main "coarse acquisition" signal (C/A) was deliberately degraded by offsetting its clock signal by a random amount, equivalent to about 100 metres of distance. More accurate guidance was possible, but only for users that had the proper decryption keys. Known as "Selective Availability", or SA for short, the system seriously degraded the usefulness of the GPS signal for non-military users. GPS satellite in orbit, image courtesy NASA GPS redirects here. ...

This presented a problem for civilian agencies who currently operated various radio navigation systems such as LORAN for maritime use, or the VOR and NDB systems used for air navigation. These systems cost millions of dollars a year to operate, and could be replaced for little cost (to them) with GPS-based systems. However, the accuracy offered with SA turned on was simply too poor to make this realistic. The simplest solution to the problem would be to simply turn SA off, but requests from various agencies (notably the FAA, USCG and DOT) were rejected by the military for security reasons. Radio navigation is the application of radio frequencies to determining a position on the earth. ... LORAN (LOng RAnge Navigation) is a terrestrial navigation system using low frequency radio transmitters that use the time interval between radio signals received from three or more stations to determine the position of a ship or aircraft. ... D-VOR (Doppler VOR) ground station, co-located with DME. VOR, short for VHF Omni-directional Radio Range, is a type of radio navigation system for aircraft. ... A non-directional beacon (NDB) is a radio broadcast station in a known location, used as an aviation or marine navigational aid. ... The Federal Aviation Administration is the entity of the United States government which regulates and oversees all aspects of civil aviation in the U.S. // Activities Along with the European Joint Aviation Authorities, the FAA is one of the two main agencies worldwide responsible for the certification of new aircraft. ... Coast Guard shield The United States Coast Guard (USCG) is a military branch of the United States involved in maritime law, mariner assistance and search and rescue, among other duties of any coast guard. ... Established October 15, 1966 Activated April 1, 1967 Secretary Norman Mineta Deputy Secretary Maria Cino Budget $58 billion (2004 estimate) Employees 58,622 (2004 estimate) The United States Department of Transportation (DOT) is a Cabinet department of the United States government concerned with transport. ...

Through the early to mid 1980s, a number of agencies developed a solution to the SA "problem". Since the SA signal was changed slowly, the effect of its offset on positioning was relatively fixed – that is, if the offset was "100 meters to the east", that offset would be true over a relatively wide area. This suggested that broadcasting this offset to local GPS receivers could eliminate the effects of SA, resulting in measurements closer to GPS's theoretical performance, around 15 meters. Additionally, another major source of errors in a GPS fix is due to transmission delays in the ionosphere, which could also be measured and corrected for in the broadcast. This offered an improvement to about 5 meters accuracy, more than enough for most civilian needs.^[1] Relationship of the atmosphere and ionosphere The ionosphere is the part of the atmosphere that is ionized by solar radiation. ...

The US Coast Guard was one of the more aggressive proponents of the DGPS system, experimenting with the system on an ever-wider basis through the late 1980s and early 1990s. These signals are broadcast on marine longwave frequencies, which could be received on existing radiotelephones and fed into suitably equipped GPS receivers. Almost all major GPS vendors offered units with DGPS inputs, not only for the USCG signals, but also aviation units on either VHF or commercial AM radio bands. Longwave radio frequencies are those below 500 kHz, which correspond to wavelengths longer than 600 meters. ... A radiotelephone is a communications device that allows two or more people to talk using radio. ... Very high frequency (VHF) is the radio frequency range from 30 MHz (wavelength 10 m) to 300 MHz (wavelength 1 m). ... Mediumwave radio transmissions (sometimes called Medium frequency or MF) are those between the frequencies of 300 kHz and 3000 kHz. ...

They started sending out "production quality" DGPS signals on a limited basis in 1996, and rapidly expanded the network to cover most US ports of call, as well as the Saint Lawrence Seaway in partnership with the Canadian Coast Guard. Plans were put into place to expand the system across the US, but this would not be easy. The quality of the DGPS corrections generally fell with distance, and most large transmitters capable of covering large areas tend to cluster near cities. This meant that lower-population areas, notably in the midwest and Alaska, would have little coverage by ground-based GPS. These are the Eisenhower Locks, located in Massena, NY. The Saint Lawrence Seaway in its broadest sense is the system of canals that permits ocean-going vessels to travel from the Atlantic Ocean to the Great Lakes as far as Lake Superior. ...

Instead, the FAA (and others) started studies for broadcasting the signals across the entire hemisphere from communications satellites in geostationary orbit. This has led to the Wide Area Augmentation System (WAAS) and similar systems, although these are generally not referred to as DGPS, or alternately, "wide-area DGPS". WAAS offers accuracy similar to the USCG's ground-based DGPS networks, and there has been some argument that the latter will be turned off as WAAS becomes fully operational. WAAS Operation The Wide Area Augmentation System (WAAS) is a system that improves the precision and accuracy of global positioning system (GPS) signals. ...

By the mid-1990s it was clear that the SA system was no longer useful in its intended role. DGPS would render it ineffective over the US, precisely where it was considered most needed. Additionally, experience during the Gulf War demonstrated that the widespread use of civilian receivers by military users meant that SA was ostensibly "hurting" its own troops more than if it were turned off. After many years of pressure, the GPS operators agreed to turn off SA forever in 2000. Combatants U.S.-led coalition Iraq Commanders George H. W. Bush Norman Schwarzkopf Colin Powell Saddam Hussein Ali Hassan al-Majid Hussein Kamel Strength 660,000 545,000 Casualties 345 dead, 1,000 wounded 25,000 - 100,000 dead, 100,000 - 300,000 wounded The 1991 Gulf War (also called...

Nevertheless, by this point DGPS had evolved into a system for providing more accuracy than even a non-SA GPS signal could provide on its own. There are several other sources of error that share the same characteristics as SA in that they are the same over large areas and for "reasonable" amounts of time. These include the ionospheric effects mentioned earlier, as well as errors in the satellite position ephemeris data and clock drift on the satellites. Depending on the amount of data being sent in the DGPS correction signal, correcting for these effects can reduce the error significantly, the best implementations offering accuracies of under 10 cm.

In addition to continued deployments of the USCG and FAA sponsored systems, a number of vendors have created commercial DGPS services, selling their signal (or receivers for it) to users that require better accuracy than the nominal 15 meters GPS offers. Almost all commercial GPS units, even hand-held units, now offer DGPS data inputs, and many also support WAAS directly. To some degree, a form of DGPS is now a natural part of most GPS operations.

Operation

A reference station calculates differential corrections for its own location and time. Users may be up to 200 nautical miles (370 km) from the station, however, and some of the compensated errors vary with space: specifically, satellite ephemeris errors and those introduced by ionospheric and tropospheric distortions. For this reason, the accuracy of DGPS decreases with distance from the reference station. The problem can be aggravated if the user and the station lack "inter visibility"—when they are unable to see the same satellites. Wikisource has ephemerides of several planets: Ephemeris of Sun Ephemeris of Moon Ephemeris of Mercury Ephemeris of Venus Ephemeris of Mars Ephemeris of Jupiter Ephemeris of Saturn Ephemeris of Uranus Ephemeris of Neptune Ephemeris of Pluto Ephemeris of Ceres Ephemeris of Pallas Ephemeris of Juno Ephemeris of Vesta An ephemeris... Relationship of the atmosphere and ionosphere The ionosphere is the part of the atmosphere that is ionized by solar radiation. ... The Troposphere is the lowermost portion of Earths atmosphere. ...

Accuracy

The United States Federal Radionavigation Plan and the IALA Recommendation on the Performance and Monitoring of DGNSS Services in the Band 283.5–325 kHz cite the United States Department of Transportation's 1993 estimated error growth of 0.67 m per 100 km from the broadcast site but measurements of accuracy in Portugal suggest a degradation of just 0.22 m per 100 km.^[2] International Association of Lighthouse Authorities (or IALA for short) is a non-profit organization founded 1957 to collect and provide nautical expertise and advise. ... Established October 15, 1966 Activated April 1, 1967 Secretary Norman Mineta Deputy Secretary Maria Cino Budget $58 billion (2004 estimate) Employees 58,622 (2004 estimate) The United States Department of Transportation (DOT) is a Cabinet department of the United States government concerned with transport. ...

Variations

DGPS can refer to any type of Ground Based Augmentation System (GBAS). There are many operational systems in use throughout the world, according to the US Coast Guard, 47 countries operate systems similar to the US NDGPS.

European DGPS Network

The European DGPS network has been mainly developed by the Finnish and Swedish maritime administrations in order to improve safety in the archipelago between the two countries.

United States NDGPS

The United States Department of Transportation, in conjunction with the Federal Highway Administration, the Federal Railroad Administration and the National Geodetic Survey appointed the Coast Guard as the maintaining agency for the US Nationwide DGPS network. The centralized Command and Control unit is USCG Navigation Center, based in Alexandria, VA. The USCG has carried over its NDGPS duties after the transition from the Department of Transportation to the Department of Homeland Security. There are 84 currently broadcasting NDGPS sites in the US network, with plans for up to 128 total sites to be online within the next 15 years. Established October 15, 1966 Activated April 1, 1967 Secretary Norman Mineta Deputy Secretary Maria Cino Budget $58 billion (2004 estimate) Employees 58,622 (2004 estimate) The United States Department of Transportation (DOT) is a Cabinet department of the United States government concerned with transport. ... The Federal Highway Administration (FHWA) is a division of the United States Department of Transportation that specializes in highway transportation. ... The Federal Railroad Administration (FRA) was created in 1966 as a division of the U.S. Department of Transportation to promote safe, environmentally sound, successful rail transportation. ... The National Geodetic Survey is the successor agency in the United States to the U.S. Coast and Geodetic Survey. ... Coast Guard shield The United States Coast Guard (USCG) is a military branch of the United States involved in maritime law, mariner assistance and search and rescue, among other duties of any coast guard. ... The United States Department of Homeland Security (DHS) is a Cabinet department of the Federal Government of the United States with the responsibility of protecting the territory of the United States from terrorist attack and responding to natural disasters. ...

Canadian DGPS

The Canadian system is similar to the US system and is primarily for maritime usage covering the Atlantic and Pacific coast as well as the Great Lakes and Saint Lawrence Seaway. The Great Lakes from space The Great Lakes are a group of five large lakes on or near the United States-Canadian border. ... These are the Eisenhower Locks, located in Massena, NY. The Saint Lawrence Seaway in its broadest sense is the system of canals that permits ocean-going vessels to travel from the Atlantic Ocean to the Great Lakes as far as Lake Superior. ...

See also

• Global Positioning System
• Wide Area Augmentation System - A Space Based Augmentation System (SBAS) primarily for aviation usage
• Local Area Augmentation System - Another type of Ground Based Augmentation System (GBAS), but designed primarily for aviation usage

GPS satellite in orbit, image courtesy NASA GPS redirects here. ... WAAS Operation The Wide Area Augmentation System (WAAS) is a system that improves the precision and accuracy of global positioning system (GPS) signals. ... LAAS Architecture The Local Area Augmentation System (LAAS) is an all-weather landing system based on real-time differential correction of the GPS signal. ...

References

• ↑  Department of Transportation and Department of Defense (March 25, 2002). 2001 Federal Radionavigation Plan. (PDF) Retrieved on November 27, 2005.
• ↑  Department of Transportation and Department of Defense (March 25, 2002). 2001 Federal Radionavigation Systems. (PDF) Retrieved on November 27, 2005.
• Monteiro, Luís Sardinha; Moore, Terry and Hill, Chris. 'What is the accuracy of DGPS?', The Journal of Navigation (2005) 58, 207-225.
• United States Coast Guard Navigation Center, Alexandria, VA; Standard Operating Procedures (2002)

November 27 is the 331st day (332nd on leap years) of the year in the Gregorian Calendar. ... 2005 (MMV) was a common year starting on Saturday of the Gregorian calendar. ... November 27 is the 331st day (332nd on leap years) of the year in the Gregorian Calendar. ... 2005 (MMV) was a common year starting on Saturday of the Gregorian calendar. ...

External links

• Performance Monitoring of Libby Dam with a Differential Global Positioning System
• US NDGPS fact sheet
• US coverage maps
• Canadian Coast Guard DGPS information (English)
• Canadian Coast Guard DGPS information (French)