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To compute his own position a Global Positioning System (GPS) receiver needs to use the ephemerides broadcasted by each satellite, each one of those ephemeris will allow the receiver to calculate the position in space of the satellite from which the ephemeris was downloaded, those satellite positions are one of the key elements that will enable the GPS receiver to compute his own coordinates, because the broadcasted ephemerides by GPS are expressed in WGS84 (G1150) datum the computed receiver position coordinates will also be in this same datum.
To calculate the WGS84 (G1150) datum the United States Department of Defense (US DoD), owner of the GPS system, used a subset of Global Navigation Satellite System (GNSS) ground reference stations part of the network used by the International Earth Rotation and Reference System Service (IERS) to compute the International Terrestrial Reference System (ITRS). This reference system imposes a no net rotation (NNR) condition for horizontal motions which means that the datum is not tied to any specific tectonic plate. In practice this means that the coordinates of any point located in the surface of the planet Earth will slowly change over time because tectonic plates slowly drift over time and the reference frame is not tied to any of them.
Because the Earth orbit and rotation is affected by external factors like other planets gravity the ITRS datum has to be updated regularly, each update is named International Terrestrial Reference Frame (ITRF) plus the year in which the recalculus is done, for instance ITRF2008 or alternatively ITRF08.
To avoid the ITRS slow temporal drift the European Comission mandated the creation of a reference frame consistent with the Euroasiatic plate motion named European Terrestrial Reference System 1989 (ETRS89). This datum has many realizations but the EUREF Technical Working Group (TWG) has recommended to adopt the European Terrestrial Reference Frame (ETRF) made in year 2000. When the ETRS89 datum was first defined in 1989 it was coincident to the ITRS, however because the European continent displaces at 2.5 centimetres per year when seen from the ITRS datum the positional difference between those two datums has progressively grown to the point that today, more than 20 years after the creation of the ETRS89 datum the coordinate difference becomes obvious even in mid accuracy GIS applications.
It is a common practice for the European reference network operators to configure the coordinates their ground reference stations expressed in ETRS89 datum instead of WGS84 (G1150), unfortunately very rarely is clearly stated what exact datum and realization is in being used. This is not a problem when a survey is performed using only a single reference network but when a survey should be performed with additional Satellite Based Augmentation System (SBAS) corrected coordinates, like EGNOS or OmniSTAR it becomes a problem because when using SBAS positions the output of the receiver will be expressed in ITRS reference frame being noticeable offseted from the coordinates that would be obtained if the GNSS rover would be using the local reference network.
The following NovAtel commands will store in the receiver Non Volatile Memory (NVM) a 14 parameter datum transformation that will convert the receiver position output from one ITRS realization to ETRS89:
ITRF year 1989 to ETRF year 2000:
ITRF year 1990 to ETRF year 2000:
ITRF year 1991 to ETRF year 2000:
ITRF year 1992 to ETRF year 2000:
ITRF year 1993 to ETRF year 2000:
ITRF year 1994, 1996 and 1997 to ETRF year 2000:
ITRF year 2000 to ETRF year 2000:
ITRF year 2005 to ETRF year 2000:
ITRF year 2008 to ETRF year 2000:
To enable the datum transformation it is necessary to input the command DATUM USER, this command should always be issued after inputting the datum transformation parameters, never before, otherwise the transformation will not be applied.
The NovAtel logs that will be affected by the datum transformation are: BESTPOS, BESTUTM, MASTERPOS, MATCHEDPOS, OMNIHPPOS, PDPPOS, PSRPOS, ROVERPOS, RTKPOS. Note that some of those logs may or may not be available deepening on the firmware and model of the NovAtel receiver.
The logs that provide Earth Centered Earth Fixed (ECEF) coordinates like: BESTXYZ, MATCHEDXYZ, PDPXYZ, PSRXYZ, RTKXYZ are not affected by datum transformations and will always provide the position in the same datum the augmentation system is broadcasting the corrections.
The datum transformation commands mentioned here can be used in NovAtel OEM4, OEMV, OEMStar as well as OEM6 receivers, for extended operation insight consult the corresponding firmware reference guide.
The after mentioned datum transformations are based on table 5 of the following memo: http://etrs89.ensg.ign.fr/memo-V8.pdf and are applicable for all Europe up to the Ural Mountains.
Current OmniSTAR datum can be consulted at OmniSTAR webpage, public SBAS systems like EGNOS broadcast corrections in ITRF 2000 datum, IGS precise orbits are expressed in the latest ITRF realization.
Measurements performed on a single GPS-702-GGL antenna for the same day and location:
2 hours of converged OmniSTAR HP (GPS only) averaged position expressed in ITRF 2005 datum:
1 hour GPS only network RTK (UK OS Net)14,2 km baseline averaged position, in theory expressed in ETRF 2000 datum:
The difference in between the coordinates of the same location expressed in ITRF 2005 datum (OmniSTAR) and expressed in ETRS 2000 datum (network RTK) is UTM X= 0.470m, UTM Y=0.312m (0.564m 2D)Ellipsoidal height= -0.003m
After being applied the previous ITRF year 2005 to ETRF year 2000 datum transformation to the OmniSTAR HP position the result becomes:
The difference in between the coordinates from the network RTK solution and the transformed OmniSTAR solution now is: UTM X= -0.026m, UTM Y= 0.015m (0.030m 2D) Ellipsoidal height= 0.003m
It is out of the scope of the present document to discuss why there is a 3 centimetre residual, however it is worth noting that the printed cartography scale at which that error would be noticeable to the human eye would be 1:150 while a difference of 0.564m would be noticeable on a 1:2730 scale, in other words, the OmniSTAR transformed result 18.2 times closer to the desired result in ETRF 2000 than the untransformed ITRF 2005.
NovAtel does not grantee in any way the accuracy of the after-mentioned datum transformation parameters.
NovAtel can not asses on what datum a particular augmentation system is using, it is responsibility of the system operators to inform the users.