differential global positioning system

8/8/2019 Differential Global Positioning System

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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. The

correction signal is typically broadcast over UHF radio modem.


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Transportable DGPS reference station Baseline HD by CLAAS for use in satellite-assisted

steering systems in modern agriculture

The term can refer both to the generalized technique as well as specific

implementations using it. It is 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 and Canadian Coast Guard run 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 two DGPS systems: one is mainly for marine navigation,

broadcasting its signal on the longwave band; the other is used for land surveys

and land navigation, and has corrections broadcast on the Commercial FM radio

band.

Two systems for air navigation and precision landing of aircraft, in Australia, will

eventually replace the Instrument Landing System. Both utilise DGPS techniquesand are called the Ground Based Augmentation System and Ground based

Regional Augmentation Systems. Both of these systems broadcast corrections via

the aviation VHF band.

A similar system that transmits range corrections from orbiting satellites instead of

ground-based transmitters is called a Satellite Based Augmentation System.


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Different versions of this system include the Wide Area Augmentation System,

European Geostationary Navigation Overlay Service, Japan's Multi-Functional

Satellite Augmentation System, Canada's CDGPS and the commercial VERIPOS,

StarFire and OmniSTAR.

History

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. To avoid this, the main "coarse acquisition" signal

(C/A) 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 that also

received the L2 frequency (1227.6 MHz), but the L2 transmission, intended for

military use, was encrypted and was only available to authorised 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


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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 asthe 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.

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.

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 SA was thought to harm the U.S. more than if it were turned off.[citation needed]

After many years of pressure, it took an executive order by President Bill Clinton

to get SA turned off permanently in 2000.[2]

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


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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 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

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

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 across the Atlantic, in Portugal suggest a degradation of just 0.22 m per

100 km.

Most GPS manufacturers quote 17 meters (49 feet) as the accuracy of horizontal

positions anywhere on earth. However, independent testing has shown that modern

receivers can achieve 10 meters fairly reliably with a clear sky view.

A GPS can only estimate the accuracy and many manufacturers are a bit optimistic

in the numbers they quote. To be more precise the Accuracy number presented is


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often based on a 50% to 60% probability rather than the 95% probability that is

usually considered in a scientific evaluation. Note that no receiver can guarantee a

particular level of accuracy without stating a probability and one of the features of

some of the systems described below is to help identify when the data in the GPS

might have a higher than average amount of error.

They are well understood and the dominant contributors are listed in this table

along with the likely amount that they contribute (assuming a good sky view and

reasonable satellite geometry). Note that in real life the errors may be higher than

those listed in the table.

Error Value

Ionosphere 4.0 meters

Clock 2.1 meters

Ephemeris 2.1 meters

Troposphere 0.7 meters

Receiver 0.5 meters

Multipath 1.0 meter

Total 10.4 meters

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

(Nationwide Differential Global Positioning System

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.


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In the UK and Ireland, the system was implemented as a maritime navigational 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 covering England, Wales and the Channel Islands, the

Northern Lighthouse Board covering 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.

United States NDGPS

The United States Department of Transportation, in conjunction with the FederalHighway Administration, the Federal Railroad Administration and the National

Geodetic Survey appointed the Coast Guard as the maintaining agency for the U.S.

Nationwide DGPS network. The system is an expansion of the previous Maritime

Differential GPS (MDGPS) which the Coast Guard began in the late 1980s and

completed in March 1999. MDGPS only covered coastal waters, the Great Lakes,

and the Mississippi River inland waterways, while NDGPS expands this to include

complete coverage of the continental United States.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 ofTransportation to the Department of Homeland Security. There are 82 currently

broadcasting NDGPS sites in the US network, with plans for up to 128 total sites to

be online within the next 15 years.

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.

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.


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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 measurementsof 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. At the TU Vienna the

method was named qGPSand adequate post processing software was developed.


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DGPS mode using a beacon receiver

All Garmin receivers support DGPS except the Geko 101. All current Magellan

receivers support DGPS except the GPS Companion as do all current Lowrance

units. Many other models and manufacturers support DGPS as well.

In DGPS operation a station, often called a beacon, transmits correction data in real

time that is received by a separate box, called a beacon receiver, which sends the

correction information to the GPS receiver. In principle this is quite simple. A GPS

receiver normally calculates it position by measuring the time it takes for a signalfrom a satellite to reach its position. By knowing where the satellite is, how long it

takes to send the signal, and knowing the speed of the signal it can compute what is

called a pseudo range (distance) to the satellite. This range must be corrected

before it is used to compute the final position. Corrections such as compensation

for ionospheric errors due to the fact that the ionosphere slows down the speed of

travel of the radio wave is one form of correction that can be applied. A DGPS


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beacon transmitter site has already calculated all of the pseudo range correction

data based on the fact that it already knows exactly where it is and can compute the

errors in the satellite computed position from its known location. Once the pseudo

range correction data is computed it is sent to the GPS and used to compute a more

accurate fix. The data is sent at either 100 baud or 200 baud depending on the

station and this can result in a typical delay of 2 to 5 seconds between the

computation of the correction and the application of the correction. However, since

most errors are slow moving this time delay is not usually a problem.

Each beacon transmitter is autonomous and computes its own corrections based on

its reception of GPS signals. It then packages the correction data in groups of 3

satellites and sends the data to the GPS receiver. Note that the design of a beacon

DGPS transmitter will send corrections for up to 9 satellites and these are only

those at least 7.5 degrees above the horizon. The assumption is that the GPS

receiver will be close enough to have the same sort of errors that the beacon stationsaw and they can be applied without modification to any SV's that they share a

view of. This works well in practice since most of the error sources would be

common between the two locations. Beacon sites have some ability to improve the

system integrity as well, however there is no standard that is defined as to exactly

what they can identify. They can easily identify a satellite where the required

corrections exceed a prescribed value and should not be used.

It seems that Garmin will favor differentially corrected satellites when at least four

exist to the exclusion of regular satellites. If the four are in a poor geometric

relationship the epe number, and possibly the accuracy of the solution, can beworse than it was with a regular GPS solution.

While the major source of DGPS corrections are done via beacon transmitters

operating in the 300KHz band this is not the only source of correction data. It is

possible to get data from any source that can be received at your location. Some

sources include FM radios using the subcarrier capability of these transmitters, the

internet, and even satellites. In all cases a custom receiver (or software) is used to

assemble the data in a form that is acceptable to the GPS receiver, which by

standard is RTCM-104. Data conforming to this standard is then sent via the serialport to the GPS receiver on a cable. Even WAAS/EGNOS data could be massaged

and delivered via standard DGPS techniques. One such system is SisNet which

supplies corections via the internet.


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DGPS Receivers

DGPS beacon receivers capable of receiving the government corretion signal can

be obtained from Garmin , from StarLink, or from Lowrance. There are probably

others as well.

Cable for DGPS

Most folks fabricate a custom cable to work with the beacon receiver. Here is a

diagram for a fairly complicated version, but you may not need a setup that is this

complicated depending on what else you may be doing.

For simple DGPS connections you can just wire a beacon receiver output signal

along with its ground to the data input terminals of the GPS. If you need to be able

to control the beacon receiver from the GPS receiver then you will also need to

send the output for the GPS receiver to the beacon receiver. A standard computer


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interface cable can usually be used for this connection. If you also need to talk to a

pc at the same time things start to get a little more complicated. To talk to a pc in

NMEA mode you can simply send the output of the GPS to both units. Wire the

output signal to the input on the computer and the input on the beacon receiver.

There is sufficient power in the signal from the Garmin to drive both units and

even a third item. Note if the beacon receiver doesn't need to receive commands

from the GPS then there is no reason to send the signal both places but the ground

wire is still needed. Finally if the GPS needs to talk to the pc in Garmin mode or

other proprietary mode and also to the beacon receiver you will need a switch to

permit the beacon receiver to transmit difference signals or the pc to interact with

the GPS. You won't be able to do both at the same time. This should not present

any real problems since the bi-directional proprietary mode is used to upload and

download waypoint, route, and track data which does not need the beacon receiver

to be operational.

WAAS (WADGPS)

WAAS, Wide Area Augmentation System, is the latest method of providing better

accuracy from the GPS constellation. It is similar in principle the the DGPS

capability that is built into all Garmin and many other units except that a second

receiver is not required. Instead of a beacon receiver the correction data is sent via

a geo-stationary satellite and is decoded by one of the regular channels already

present in the GPS receiver. Thus one of the 12 channels can be designated to

decode regular GPS signals or can be used to decode the WAAS data. Actually, as

currently implemented, when WAAS is enabled two channels will be dedicated to

WAAS. While WAAS is the name of the implementation of this technology in the

US the system is intended for worldwide use. The generic name for WAAS is

SBAS (Space Based Augmentation System) or WADGPS (Wide Area Differential

GPS). Since most receivers call the correction signal WAAS as a generic term this

article will use WAAS in this way.

The way this works is that a set of ground stations all over the US (as shownbelow) collect correction data relative to the area of the country they are located in.

The entire data is then packaged together, analyzed, converted to a set of correction

data by a master station and then uploaded to the geo-stationary satellite, which in

turn transmits the data down to the local GPS receiver. The GPS receiver then

figures out which data is applicable to its current location and then applies the


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appropriate corrections to the receiver. Similar systems are being set up in other

areas of the world but they are not yet operational.

In addition to correction information the ground stations can also identify a GPS

satellite that is not working within specification thereby improving the integrity of

the system for aviation use.


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User Segment

Military. Search and rescue. Disaster relief. Surveying. Marine, aeronautical and terrestrial navigation. Remote controlled vehicle and robot guidance. Satellite positioning and tracking. Shipping. Geographic Information Systems (GIS). Recreation.


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differential global positioning system

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