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Differential Global Positioning Systems (DGPS) are enhancements to the Global Positioning System
Global Positioning System
(GPS) which provide improved location accuracy, in the range of operations of each system, from the 15-meter nominal GPS
GPS
accuracy to about 10 cm in case of the best implementations. Each D GPS
GPS
uses a network of fixed ground-based reference stations to broadcast the difference between the positions indicated by the GPS satellite systems and 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. The digital correction signal is typically broadcast locally over ground-based transmitters of shorter range. The United States Coast Guard
United States Coast Guard
(USCG) and the Canadian Coast Guard (CCG) each run D GPS
GPS
systems in the United States
United States
and Canada on longwave radio frequencies between 285 kHz and 325 kHz near major waterways and harbors. The USCG's D GPS
GPS
system has been named ND GPS
GPS
(Nationwide DGPS) and is now jointly administered by the Coast Guard and the U.S. Department of Transportation’s Federal Highway Administration. It consists of broadcast sites located throughout the inland and coastal portions of the United States
United States
including Alaska, Hawaii and Puerto Rico.[1] Other countries have their own D GPS
GPS
system. A similar system which transmits corrections from orbiting satellites instead of ground-based transmitters is called a Wide-Area DGPS (WADGPS)[2] or Satellite Based Augmentation System.

Contents

1 History 2 Operation 3 Accuracy 4 Variations

4.1 European D GPS
GPS
Network 4.2 United States
United States
NDGPS 4.3 Canadian DGPS 4.4 Australia

5 Post processing 6 See also 7 References 8 External links

History[edit] When GPS
GPS
was first being put into service, the US military was concerned about the possibility of enemy forces using the globally available GPS
GPS
signals to guide their own weapon systems. Originally, the government thought the "coarse acquisition" (C/A) signal would give only about 100-meter accuracy, but with improved receiver designs, the actual accuracy was 20 to 30 meters.[3] Starting in March 1990,[4] to avoid providing such unexpected accuracy, the C/A signal transmitted on the L1 frequency (1575.42 MHz) was deliberately degraded by offsetting its clock signal by a random amount, equivalent to about 100 meters of distance. This technique, known as "Selective Availability", or SA for short, seriously degraded the usefulness of the GPS
GPS
signal for non-military users. More accurate guidance was possible for users of dual-frequency GPS
GPS
receivers which also received the L2 frequency (1227.6 MHz), but the L2 transmission, intended for military use, was encrypted and was available only to authorized users with the decryption keys. This presented a problem for civilian users who relied upon ground-based radio navigation systems such as LORAN, VOR and NDB systems costing millions of dollars each year to maintain. The advent of a global navigation satellite system (GNSS) could provide greatly improved accuracy and performance at a fraction of the cost. The accuracy inherent in the S/A signal was however too poor to make this realistic. The military received multiple requests from the Federal Aviation Administration (FAA), United States Coast Guard
United States Coast Guard
(USCG) and United States Department of Transportation
United States Department of Transportation
(DOT) to set S/A aside to enable civilian use of GNSS, but remained steadfast in its objection on grounds of security. Through the early to mid 1980s, a number of agencies developed a solution to the SA "problem".[dubious – discuss] 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
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
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] The US Coast Guard was one of the more aggressive proponents of the D GPS
GPS
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
GPS
receivers. Almost all major GPS
GPS
vendors offered units with D GPS
GPS
inputs, not only for the USCG signals, but also aviation units on either VHF
VHF
or commercial AM radio bands. They started sending out "production quality" D GPS
GPS
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
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 D GPS
GPS
corrections generally fell with distance, and 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. As of November 2013 the USCG's national D GPS
GPS
system comprises 85 broadcast sites which provide dual coverage to almost the entire US coastline and inland navigable waterways including Alaska, Hawaii, and Puerto Rico. In addition the system provides single or dual coverage to a majority of the inland portion of United States.[5] Instead, the FAA (and others) started studying broadcasting the signals across the entire hemisphere from communications satellites in geostationary orbit. This led to the Wide Area Augmentation System
Wide Area Augmentation System
(WAAS) and similar systems, although these are generally not referred to as DGPS, or alternatively, "wide-area DGPS". WAAS offers accuracy similar to the USCG's ground-based D GPS
GPS
networks, and there has been some argument that the latter will be turned off as WAAS becomes fully operational. By the mid-1990s it was clear that the SA system was no longer useful in its intended role. D GPS
GPS
would render it ineffective over the US, precisely where it was considered most needed. Additionally, experience during the Gulf War
Gulf War
demonstrated that the widespread use of civilian receivers by U.S. forces meant that leaving SA turned on was thought to harm the U.S. more than if it were turned off.[6][citation needed] After many years of pressure, it took an executive order by President Bill Clinton
Bill Clinton
to get SA turned off permanently in 2000.[7] Nevertheless, by this point D GPS
GPS
had evolved into a system for providing more accuracy than even a non-SA GPS
GPS
signal could provide on its own. There are several other sources of error which 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 D GPS
GPS
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 D GPS
GPS
services, selling their signal (or receivers for it) to users who require better accuracy than the nominal 15 meters GPS
GPS
offers. Almost all commercial GPS
GPS
units, even hand-held units, now offer D GPS
GPS
data inputs, and many also support WAAS directly. To some degree, a form of D GPS
GPS
is now a natural part of most GPS
GPS
operations. Operation[edit]

D GPS
GPS
Reference Station (choke ring antenna)

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 D GPS
GPS
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. Accuracy[edit] The United States
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
United States
Department of Transportation's 1993 estimated error growth of 0.67 m per 100 km from the broadcast site[8] but measurements of accuracy across the Atlantic, in Portugal, suggest a degradation of just 0.22 m per 100 km.[9] Variations[edit] D GPS
GPS
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 ND GPS
GPS
(Nationwide Differential Global Positioning System). A list can be found at World D GPS
GPS
Database for Dxers European D GPS
GPS
Network[edit] The European D GPS
GPS
network has been developed mainly by the Finnish and Swedish maritime administrations in order to improve safety in the archipelago between the two countries. In the UK and Ireland, the system was implemented as a maritime navigation aid to fill the gap left by the demise of the Decca Navigator System in 2000. With a network of 12 transmitters sited around the coastline and three control stations, it was set up in 1998 by the countries' respective General Lighthouse Authorities (GLA) — Trinity House
Trinity House
covering England, Wales
Wales
and the Channel Islands, the Northern Lighthouse Board
Northern Lighthouse Board
covering Scotland
Scotland
and the Isle of Man and the Commissioners of Irish Lights, covering the whole of Ireland. Transmitting on the 300-kHz band, the system underwent testing and two additional transmitters were added before the system was declared operational in 2002.[10][11] Trinity House
Trinity House
- DGNSS Stations: UK and Ireland Effective Solutions (Data Products) - European Differential Beacon Transmitters - Details and map United States
United States
NDGPS[edit] The United States
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 U.S. Nationwide D GPS
GPS
network (NDGPS). The system is an expansion of the previous Maritime Differential GPS
GPS
(MDGPS), which the Coast Guard began in the late 1980s and completed in March 1999. MD GPS
GPS
covered only coastal waters, the Great Lakes, and the Mississippi River inland waterways, while ND GPS
GPS
expands this to include complete coverage of the continental United States.[12] The centralized Command and Control unit is the USCG Navigation
Navigation
Center, based in Alexandria, VA.[13] There are currently 85 ND GPS
GPS
sites in the US network, administered by the U.S. Department of Homeland Security Navigation
Navigation
Center. Canadian DGPS[edit] 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
Great Lakes
and Saint Lawrence Seaway. Australia[edit] Australia runs three D GPS
GPS
systems: one is mainly for marine navigation, broadcasting its signal on the long-wave band;[14] another is used for land surveys and land navigation, and has corrections broadcast on the Commercial FM radio band. The third at Sydney airport is currently undergoing testing for precision landing of aircraft (2011), as a backup to the Instrument Landing System
Instrument Landing System
at least until 2015. It is called the Ground Based Augmentation System. Corrections to aircraft position are broadcast via the aviation VHF
VHF
band. Post processing[edit] Post-processing is used in Differential GPS
GPS
to obtain precise positions of unknown points by relating them to known points such as survey markers. The GPS
GPS
measurements are usually stored in computer memory in the GPS receivers, and are subsequently transferred to a computer running the GPS
GPS
post-processing software. The software computes baselines using simultaneous measurement data from two or more GPS
GPS
receivers. The baselines represent a three-dimensional line drawn between the two points occupied by each pair of GPS
GPS
antennas. The post-processed measurements allow more precise positioning, because most GPS
GPS
errors affect each receiver nearly equally, and therefore can be cancelled out in the calculations. Differential GPS
GPS
measurements can also be computed in real time by some GPS
GPS
receivers if they receive a correction signal using a separate radio receiver, for example in Real Time Kinematic
Real Time Kinematic
(RTK) surveying or navigation. The improvement of GPS
GPS
positioning doesn't require simultaneous measurements of two or more receivers in any case, but can also be done by special use of a single device. In the 1990s when even handheld receivers were quite expensive, some methods of quasi-differential GPS
GPS
were developed, using the receiver by quick turns of positions or loops of 3-10 survey points. See also[edit]

Assisted GPS
GPS
(A-GPS) - System used primarily in GPS-equipped cellular devices to improve start-up performance GNSS augmentation GNSS enhancement List of DGPS-Stations

References[edit]

^ "US Government page on GPS
GPS
augmentation systems". Gps.gov. 2012-03-14. Retrieved 2013-07-07.  ^ Kee, C., Parkinson, B. W., and Axelrad, P. (1991), "Wide area differential GPS", Navigation, Journal of the Institute of Navigation, 38, 2 (Summer, 1991), <https://www.ion.org/publications/abstract.cfm?articleID=100207> ^ McNamara, Joel (2008), GPS
GPS
for Dummies (2nd ed.), ISBN 978-0-470-15623-0  ^ Ho, Angela; Mozdzanowski, Alex; Ng, Christine (2005), GPS
GPS
Case (PDF), Open Courseware, MIT , page 11. ^ "USCG D GPS
GPS
coverage plot via USCG Navigation
Navigation
Center". Retrieved 2013-07-07.  ^ GPS
GPS
for Dummies, stating that there weren't enough military GPS receivers, so " Selective Availability
Selective Availability
was temporarily turned off in 1990 during the Persian Gulf War" so that Coalition troops could use civilian GPS
GPS
receivers. ^ "Statement by the President regarding the United States' Decision to Stop Degrading Global Positioning System
Global Positioning System
Accuracy". Office of Science and Technology Policy. May 1, 2000. Retrieved 2007-12-17.  ^ Department of Transportation and Department of Defense (March 25, 2002). "2001 Federal Radionavigation Plan" (PDF). Retrieved November 27, 2005.  ^ Monteiro, Luís Sardinha; Moore, Terry and Hill, Chris. 'What is the accuracy of DGPS?', The Journal of Navigation
Navigation
(2005) 58, 207-225. ^ "Marine Differential GPS". Satellite Navigation. Trinity House. Archived from the original on 2008-01-20.  ^ "UK & Republic of Ireland
Ireland
General Lighthouse Authorities Turn to Trimble GPS
GPS
For Future Navigation". Trimble Navigation
Navigation
Limited (Press release). PRNewsire. 22 January 1998.  ^ "2005 FEDERAL RADIONAVIGATION PLAN" (PDF). Retrieved 2013-07-07.  ^ United States Coast Guard
United States Coast Guard
Navigation
Navigation
Center, Alexandria, VA; Standard Operating Procedures (2002) ^ "AMSA's D GPS
GPS
Service - Status". Australian Maritime Safety Authority. Retrieved 2017-03-29. 

External links[edit]

Wikimedia Commons has media related to Differential Global Positioning System.

SiReNT information page US ND GPS
GPS
fact sheet USCG Navigation
Navigation
Center National D GPS
GPS
system USCG coverage maps Canadian Coast Guard
Canadian Coast Guard
D GPS
GPS
information (English) Canadian Coast Guard
Canadian Coast Guard
D GPS
GPS
information (French) Product Survey on RTK D GPS
GPS
receivers for (mainly) hydrographic use D GPS
GPS
Decoding Software Useful D GPS
GPS
Links, Databases and Resources Worldwide database of IALA D GPS
GPS
Reference stations on an interactive map

v t e

Satellite navigation
Satellite navigation
systems

Operational

BDS / BeiDou-2 / COMPASS DORIS Galileo GLONASS GPS
GPS
/ NavStar IRNSS / NAVIC

Historical

BDS / BeiDou-1 Transit Timation Tsiklon

GNSS augmentation

EGNOS GAGAN GPS·C (retired) JPALS LAAS MSAS NTRIP QZSS / Michibiki StarFire WAAS

Related topics

GNSS reflectometry Kalman fi

.