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|Category=Fundamentals
|Category=Fundamentals
|Title={{PAGENAME}}
|Editors=GMV
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|YearOfPublication=2011
|YearOfPublication=2011
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The classical [[DGNSS Fundamentals|DGNSS technique]] is an enhancement to a primary GNSS system that consists of the determination of the GNSS position for an accurately-surveyed position known as reference station. DGNSS accuracy is in the order of 1 m (1 sigma) for users in the range of few tens of km from the reference station.
==The classical DGNSS technique==


The classical [[Work in Progress:DGNSS Fundamentals|DGNSS technique]] technique is an enhancement to a primary GNSS system, that consists of the determination of the GNSS position for an accurately-surveyed position known as reference station. DGNSS accuracy is in the order of 1 m (1 sigma) for users in the range of few tens of km from the reference station.
The standard [[DGNSS Fundamentals|DGNSS technique]] consists of the determination of the GNSS position from an accurately-surveyed position known as reference station. The method takes advantage of the slow variation with time and user position of the errors due to ephemeris prediction, residual satellite [[Clock Modelling|clocks]], [[Ionospheric Delay|ionospheric]] and [[Tropospheric Delay|tropospheric]] delays. Starting from the reference station, the system computes and broadcasts either corrections to the GNSS position or to the pseudorange measurements to the DGNSS users. In order to be able to apply these corrections, the receiver has to be enabled for DGNSS and stay in the vicinity of the reference station to ensure that the two receivers (station and rover) observe the same GNSS satellite. Other uncorrelated errors (e.g.  [[Multipath|multipath]]) cannot be corrected by this method and specific techniques have to be applied to mitigate them.


==The classical DGNSS technique==
DGNSS accuracy is in the order of 1 m (1 sigma) for users in the range of few tens of kilometers from the reference station, growing at the rate of 1 m per 150 km of separation. The United States Federal Radionavigation Plan and the [http://www.iala-aism.org IALA] Recommendation on the Performance and Monitoring of DGNSS Services in the Band 283.5–325 kHz cite the [http://www.dot.gov/ 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.<ref name=DGPS_Wiki>[http://en.wikipedia.org/wiki/Differential_GPS Differential GPS page in Wikipedia]</ref>
 
Variations of the method using corrections from multiple reference stations exist, leading to higher levels of accuracy.
 
==DGNSS Algorithm==
 
The classical DGNSS algorithm is based on single differences of pseudorange observables. At a given epoch, and for a given satellite, the simplified pseudorange observation equation is the following:
 
<math>\qquad  P =\rho+I+Tr+c(b_{Rx}-b_{Sat} )+\varepsilon_P \qquad \mbox{(1)}</math>
 
Where:
 
<math>I</math> is the signal path delay due to the ionosphere;
 
<math>Tr</math> is the signal path delay due to the troposphere;
 
<math>b_{Rx}</math> is the receiver clock offset from the reference (GPS) time;


The classical [[Work in Progress:DGNSS Fundamentals|DGNSS technique]] technique  is an enhancement to a primary GNSS system, that consists of the determination of the GNSS position for an accurately-surveyed position known as reference station. Given that the position of the reference station is accurately known, the deviation of the measured position to the actual position and more importantly the corrections to the measured pseudoranges to each of the individual satellites can be calculated. These corrections can thereby be used for the correction of the measured positions of other GNSS user receivers.
<math>b_{Sat}</math> is the satellite clock offset from the reference (GPS) time;


wiki:
<math>c</math> is the vacuum speed of light;
*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
<math>\varepsilon_P</math> are the measurement noise components, including multipath and other effects;
DGNSS accuracy is in the order of 1 m (1 sigma) for users in the range of few tens of km from the reference station, growing at the rate of 1 m per 150 km of separation
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[3][4] but measurements of accuracy across the Atlantic, in Portugal suggest a degradation of just 0.22 m per 100 km.[5]


*Post processing
<math>\rho</math> is the geometrical range between the satellite and the receiver, computed as a function of the satellite <math>(x_{Sat}, y_{Sat},z_{Sat})</math> and receiver <math>(x_{Rx}, y_{Rx},z_{Rx})</math> coordinates as:
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.
<math>\qquad  \rho=\sqrt{〖(x_{Sat}-x_{Rx})〗^2+〖(y_{Sat}-y_{Rx})〗^2+〖(z_{Sat}-z_{Rx})〗^2 }  \qquad \mbox{(2)}</math>.


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, this is the case in Real Time Kinematic (RTK) surveying or navigation.
The next step is using a reference station at an accurately calibrated location <math>(x_o, y_o,z_o)</math>, the reference-to-satellite range can be calculated as:


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. At the TU Vienna the method was named qGPS and adequate post processing software was developed.
<math>\qquad  R_o=\sqrt{〖(x_{Sat}-x_o)〗^2+〖(y_{Sat}-y_o)〗^2+〖(z_{Sat}-z_0)〗^2 }  \qquad \mbox{(3)}</math>.


When single differencing of both pseudoranges, <math> \Delta P = P_o - P </math>, the ionospheric and tropospheric delays cancell out and also the satellite clock offset. The basic equation per satellite to solve is: 


<math>\qquad  \Delta P=\mathbf{{u}_o^{sat} } \Delta \mathbf{{x}} +c \Delta b_{Rx} +\Delta \varepsilon  \qquad  \mbox{(4)}</math>


==DGNSS funcionalities and performances==
where <math>\mathbf{{u}_o^{sat} }</math> is the unitary vector between the reference station and the satellite. As there are not only one satellite in view, the estimation process is in fact a linear equation system, easily to solve. Depending on the distance between the rover and the reference stations, it could be needed to perform an iterative computation.


==Notes==
==Notes==

Latest revision as of 13:33, 20 May 2020


FundamentalsFundamentals
Title DGNSS Fundamentals
Edited by GMV
Level Basic
Year of Publication 2011
Logo GMV.png

The classical DGNSS technique is an enhancement to a primary GNSS system that consists of the determination of the GNSS position for an accurately-surveyed position known as reference station. DGNSS accuracy is in the order of 1 m (1 sigma) for users in the range of few tens of km from the reference station.

The classical DGNSS technique

The standard DGNSS technique consists of the determination of the GNSS position from an accurately-surveyed position known as reference station. The method takes advantage of the slow variation with time and user position of the errors due to ephemeris prediction, residual satellite clocks, ionospheric and tropospheric delays. Starting from the reference station, the system computes and broadcasts either corrections to the GNSS position or to the pseudorange measurements to the DGNSS users. In order to be able to apply these corrections, the receiver has to be enabled for DGNSS and stay in the vicinity of the reference station to ensure that the two receivers (station and rover) observe the same GNSS satellite. Other uncorrelated errors (e.g. multipath) cannot be corrected by this method and specific techniques have to be applied to mitigate them.

DGNSS accuracy is in the order of 1 m (1 sigma) for users in the range of few tens of kilometers from the reference station, growing at the rate of 1 m per 150 km of separation. 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.[1]

Variations of the method using corrections from multiple reference stations exist, leading to higher levels of accuracy.

DGNSS Algorithm

The classical DGNSS algorithm is based on single differences of pseudorange observables. At a given epoch, and for a given satellite, the simplified pseudorange observation equation is the following:

[math]\displaystyle{ \qquad P =\rho+I+Tr+c(b_{Rx}-b_{Sat} )+\varepsilon_P \qquad \mbox{(1)} }[/math]

Where:

[math]\displaystyle{ I }[/math] is the signal path delay due to the ionosphere;

[math]\displaystyle{ Tr }[/math] is the signal path delay due to the troposphere;

[math]\displaystyle{ b_{Rx} }[/math] is the receiver clock offset from the reference (GPS) time;

[math]\displaystyle{ b_{Sat} }[/math] is the satellite clock offset from the reference (GPS) time;

[math]\displaystyle{ c }[/math] is the vacuum speed of light;

[math]\displaystyle{ \varepsilon_P }[/math] are the measurement noise components, including multipath and other effects;

[math]\displaystyle{ \rho }[/math] is the geometrical range between the satellite and the receiver, computed as a function of the satellite [math]\displaystyle{ (x_{Sat}, y_{Sat},z_{Sat}) }[/math] and receiver [math]\displaystyle{ (x_{Rx}, y_{Rx},z_{Rx}) }[/math] coordinates as:

[math]\displaystyle{ \qquad \rho=\sqrt{〖(x_{Sat}-x_{Rx})〗^2+〖(y_{Sat}-y_{Rx})〗^2+〖(z_{Sat}-z_{Rx})〗^2 } \qquad \mbox{(2)} }[/math].

The next step is using a reference station at an accurately calibrated location [math]\displaystyle{ (x_o, y_o,z_o) }[/math], the reference-to-satellite range can be calculated as:

[math]\displaystyle{ \qquad R_o=\sqrt{〖(x_{Sat}-x_o)〗^2+〖(y_{Sat}-y_o)〗^2+〖(z_{Sat}-z_0)〗^2 } \qquad \mbox{(3)} }[/math].

When single differencing of both pseudoranges, [math]\displaystyle{ \Delta P = P_o - P }[/math], the ionospheric and tropospheric delays cancell out and also the satellite clock offset. The basic equation per satellite to solve is:

[math]\displaystyle{ \qquad \Delta P=\mathbf{{u}_o^{sat} } \Delta \mathbf{{x}} +c \Delta b_{Rx} +\Delta \varepsilon \qquad \mbox{(4)} }[/math]

where [math]\displaystyle{ \mathbf{{u}_o^{sat} } }[/math] is the unitary vector between the reference station and the satellite. As there are not only one satellite in view, the estimation process is in fact a linear equation system, easily to solve. Depending on the distance between the rover and the reference stations, it could be needed to perform an iterative computation.

Notes


References