DGPS(Differential Global Positioning System)
DGPS:
Differential Global Positioning System (DGPS) is an enhancement to Global Positioning System which provides improved location accuracy, from the 15-meter nominal GPS accuracy to about 10 cm in case of the best implementations.
DGPS uses a network of fixed ground-based reference stations to broadcast the difference between the positions indicated by the GPS 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. The digital correction signal is typically broadcast locally over ground-based transmitters of shorter range.
The term refers to a general technique of augmentation. The United States Coast Guard (USCG) and Canadian Coast Guard (CCG) each run such systems in the U.S. and Canada on the longwave radio frequencies between 285 kHz and 325 kHz near major waterways and harbors.
The USCG's DGPS system has been named NDGPS (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 including Alaska, Hawaii and Puerto Rico.
A similar system which transmits corrections from orbiting satellites instead of ground-based transmitters is called a Wide-Area DGPS (WADGPS)or Satellite Based Augmentation System.
History of DGPS:
When GPS was first being put into service, the US military was concerned about the possibility of enemy forces using the globally available 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.Starting in March 1990,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 signal for non-military users.
More accurate guidance was possible for users of dual-frequency 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 encryption 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 (USCG) and 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".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.
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.
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 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 DGPS 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.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 (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 DGPS 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. 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 U.S. forces meant that leaving SA turned on was thought to harm the U.S. more than if it were turned After many years of pressure, it took an executive order by President Bill Clinton to get SA turned off permanently in 2000.
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 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 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 who 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 of DGPS:
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.
Accuracy of DGPS:
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[8] but measurements of accuracy across the Atlantic, in Portugal, suggest a degradation of just 0.22 m per 100 km.
Post processing:
Post-processing is used in Differential GPS to obtain precise positions of unknown points by relating them to known points such as survey markers.
The GPS measurements are usually stored in computer memory in the GPS receivers, and are subsequently transferred to a computer running the GPS post-processing software. The software computes baselines using simultaneous measurement data from two or more GPS receivers.
The baselines represent a three-dimensional line drawn between the two points occupied by each pair of GPS antennas. The post-processed measurements allow more precise positioning, because most GPS errors affect each receiver nearly equally, and therefore can be cancelled out in the calculations.
Differential GPS measurements can also be computed in real time by some GPS receivers if they receive a correction signal using a separate radio receiver, for example in Real Time Kinematic (RTK) surveying or navigation.
The improvement of 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 were developed, using the receiver by quick turns of positions or loops of 3-10 survey points.
D G P S
The PENTAX Positioning System G3100-R1 is a high precision satellite receiver and communication unit specifically designed for the surveying market. Integrated with state-of-the-art technology, the G3100-R1 provides surveyors high productivity, performance and flexibility.State of the Art receiver:
The G3100-R1 uses the AsteRx2e GNSS engine which measures both GPS and GLONA SS constellations for robust and accurate satellite positioning.
The advanced receiver technology includes Receiver Autonomous Integrity Monitoring Multipath Estimation and a standard output rate up to 25 Hz. The G3100-R1 combination of a GNSS receiver with a matched internal antenna provides an integrated product with optimal performance that is ready for use at turn-on.
Base or Rover Configuration:
With the internal radio designed into each G3100-R1, any unit may be configured as a local base station to transmit corrections for RTK surveys without any change in hardware. For extended transmission range, external radios may be interfaced through a serial port.
Multiple Communication Choices:
Surveyors have a choice of communication options that are all integrated into the single rugged housing. The communication options include a GMS/GPRS modem for connecting to Real Time Reference Station Networks, a choice of either digital Spread Spectrum (900 MHz) or digital UHh a serial port.F (406-470 MHz) radios for local data transmissions, or the option to use an external radio throug
A rugged, lightweight single housing, mounted on a pole or tripod, the wireless G3100-R1 receiver works seamlessly and is recognized as the most powerful and easy-to-use field data collection software on the market. Complete with a “Ready To Go” equipment package.
Hot Swap Batteries with Fuel Gauges:
The G3100-R1 houses two batteries that may be hot swapped for continuous operation. The efficient G3100-R1 provides a full day’s operation from the two internal rechargeable Li-Ion batteries (7.2V, 5000 mAh). Re-charging is done within a few hours with the included charger. All PENTAX batteries integrate fuel gauge technology to display current battery status. The unit may also be powered from an external battery for extended operation.
Easily Removable SD Card for Data Logging:
For ultra portability and data management, the G3100-R1 logs raw data onto a removable SD card that is accessed easily through a convenient door. With the G3100-R1, getting data to the PC for post processing is simply a matter of inserting the SD card into the office PC, eliminating the need for cable download and additional software.
Bluetooth Controller – No Cables Integrated:
Bluetooth provides cable free operation for use with a pole mounted data collection system with the ease of use and portability required for survey/GIS applications. Real time records are also logged on the controller and the user can do wireless transfer to a PC easily.
Open Architecture:
PENTAX believes in Open Architecture and the advantages that this brings to the market including the ability for users to “plug and play” and swap equipment when required, to create easy upgrade paths, and not to be “locked in” to any one supplier on the market.
Differential Global Positioning System (DGPS) is an enhancement to Global Positioning System which provides improved location accuracy, from the 15-meter nominal GPS accuracy to about 10 cm in case of the best implementations.
DGPS uses a network of fixed ground-based reference stations to broadcast the difference between the positions indicated by the GPS 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. The digital correction signal is typically broadcast locally over ground-based transmitters of shorter range.
The term refers to a general technique of augmentation. The United States Coast Guard (USCG) and Canadian Coast Guard (CCG) each run such systems in the U.S. and Canada on the longwave radio frequencies between 285 kHz and 325 kHz near major waterways and harbors.
The USCG's DGPS system has been named NDGPS (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 including Alaska, Hawaii and Puerto Rico.
A similar system which transmits corrections from orbiting satellites instead of ground-based transmitters is called a Wide-Area DGPS (WADGPS)or Satellite Based Augmentation System.
History of DGPS:
When GPS was first being put into service, the US military was concerned about the possibility of enemy forces using the globally available 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.Starting in March 1990,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 signal for non-military users.
More accurate guidance was possible for users of dual-frequency 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 encryption 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 (USCG) and 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".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.
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.
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 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 DGPS 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.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 (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 DGPS 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. 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 U.S. forces meant that leaving SA turned on was thought to harm the U.S. more than if it were turned After many years of pressure, it took an executive order by President Bill Clinton to get SA turned off permanently in 2000.
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 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 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 who 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 of DGPS:
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.
Accuracy of DGPS:
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[8] but measurements of accuracy across the Atlantic, in Portugal, suggest a degradation of just 0.22 m per 100 km.
Post processing:
Post-processing is used in Differential GPS to obtain precise positions of unknown points by relating them to known points such as survey markers.
The GPS measurements are usually stored in computer memory in the GPS receivers, and are subsequently transferred to a computer running the GPS post-processing software. The software computes baselines using simultaneous measurement data from two or more GPS receivers.
The baselines represent a three-dimensional line drawn between the two points occupied by each pair of GPS antennas. The post-processed measurements allow more precise positioning, because most GPS errors affect each receiver nearly equally, and therefore can be cancelled out in the calculations.
Differential GPS measurements can also be computed in real time by some GPS receivers if they receive a correction signal using a separate radio receiver, for example in Real Time Kinematic (RTK) surveying or navigation.
The improvement of 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 were developed, using the receiver by quick turns of positions or loops of 3-10 survey points.
D G P S
The PENTAX Positioning System G3100-R1 is a high precision satellite receiver and communication unit specifically designed for the surveying market. Integrated with state-of-the-art technology, the G3100-R1 provides surveyors high productivity, performance and flexibility.State of the Art receiver:
The G3100-R1 uses the AsteRx2e GNSS engine which measures both GPS and GLONA SS constellations for robust and accurate satellite positioning.
The advanced receiver technology includes Receiver Autonomous Integrity Monitoring Multipath Estimation and a standard output rate up to 25 Hz. The G3100-R1 combination of a GNSS receiver with a matched internal antenna provides an integrated product with optimal performance that is ready for use at turn-on.
Base or Rover Configuration:
With the internal radio designed into each G3100-R1, any unit may be configured as a local base station to transmit corrections for RTK surveys without any change in hardware. For extended transmission range, external radios may be interfaced through a serial port.
Multiple Communication Choices:
Surveyors have a choice of communication options that are all integrated into the single rugged housing. The communication options include a GMS/GPRS modem for connecting to Real Time Reference Station Networks, a choice of either digital Spread Spectrum (900 MHz) or digital UHh a serial port.F (406-470 MHz) radios for local data transmissions, or the option to use an external radio throug
A rugged, lightweight single housing, mounted on a pole or tripod, the wireless G3100-R1 receiver works seamlessly and is recognized as the most powerful and easy-to-use field data collection software on the market. Complete with a “Ready To Go” equipment package.
Hot Swap Batteries with Fuel Gauges:
The G3100-R1 houses two batteries that may be hot swapped for continuous operation. The efficient G3100-R1 provides a full day’s operation from the two internal rechargeable Li-Ion batteries (7.2V, 5000 mAh). Re-charging is done within a few hours with the included charger. All PENTAX batteries integrate fuel gauge technology to display current battery status. The unit may also be powered from an external battery for extended operation.
Easily Removable SD Card for Data Logging:
For ultra portability and data management, the G3100-R1 logs raw data onto a removable SD card that is accessed easily through a convenient door. With the G3100-R1, getting data to the PC for post processing is simply a matter of inserting the SD card into the office PC, eliminating the need for cable download and additional software.
Bluetooth Controller – No Cables Integrated:
Bluetooth provides cable free operation for use with a pole mounted data collection system with the ease of use and portability required for survey/GIS applications. Real time records are also logged on the controller and the user can do wireless transfer to a PC easily.
Open Architecture:
PENTAX believes in Open Architecture and the advantages that this brings to the market including the ability for users to “plug and play” and swap equipment when required, to create easy upgrade paths, and not to be “locked in” to any one supplier on the market.
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