ZED-F9P-Integration Manual
UBX-18010802 - R02
10 Appendix Page 110 of 114
Advance Information
For example viewing RTK accurate positions that could be in any ITRF transform reference frame
(based on reference receiver (or base) reference frame) on Google Earth: The heights on Google
Earth refer to EGM96 and are, therefore, Geoidal heights.
The lat/long are referred to the WGS 84 (ITRF2008) ellipsoid. The result is a visible inaccuracy, the
RTK receivers position output will need to be transformed to the Google Earth transform system
before it can be realistically used. This is ignoring any local map tile inaccuracies.
Geodetic Coordinate Systems and Ellipsoids:
In practice, the relevant organizations choose to keep their respective frames very close to the
International Terrestrial Reference Frame (ITRF), defined and managed by the International Earth
Rotation and Reference Systems Service (IERS).
However, because the Earth's tectonic plates and even parts of the Earth's core move, new versions
of ITRF are defined every few years, generally with changes of the order of a few millimeters.
Consequently, the major GNSS occasionally decide that they need to update their reference frames
to be better aligned to the latest ITRF.
So, for example, GPS switched to WGS84 (G1150) in GPS week 1150 (early 2002) based on
ITRF2000, while GLONASS switched from PZ90.02 to PZ90.11 at the end of 2013, based on
ITRF2008. The net effect of this, is that all the major GNSS use almost the same reference frame,
but there are some small (generally sub-cm) differences between them and these differences
occasionally change.
In order to produce positions that can be shown on a map, it is necessary to translate between raw
coordinates (e.g. x, y, z) and a position relative to the Earth's surface (e.g. latitude, longitude and
altitude) and that requires defining the form of ellipsoid that best matches the shape of the Earth.
Historically many different ellipsoid definitions have been used for maps, many of which predate the
existence of GNSS and show quite significant differences, leading to discrepancies of as much as
100m in places. Fortunately, most digital maps now use the WGS84 ellipsoid, which is distinct from
the WGS84 coordinate system, but defined by the same body.
However for RTK position accuracies now in the cm level the ITRF year for the WGS84 datum used
by the mapping system must be known in order to transform the RTK rover position into the correct
reference frame.
10.2 Virtual Reference Station
Reaching centimeter-level accuracy of positioning typically requires use of precise dual-frequency
carrier phase observations. Furthermore, these observations are usually processed using a
differential GNSS (DGNSS) algorithm, such as real time kinematic (RTK) or post-processing (PP).
Regardless of the specific differential algorithm, however, implicit in the process is an assumption
that the quality of the reference station data is consistent with the desired level of positioning
accuracy. The virtual reference station (VRS) concept can help to satisfy this requirement using a
network of reference stations.
As a quick review, a simple DGNSS setup consists of a single reference station from which the
raw data (or corrections) are sent to the rover receiver (i.e. the user). The user then forms the
carrier phase differences (or corrects their raw data) and performs the data processing using the
differential corrections.
In contrast, GNSS reference network architectures often make use of multiple reference stations.
This approach allows a more precise modeling of distance-dependent systematic errors principally
caused by ionospheric and tropospheric refractions, and satellite orbit errors. More specifically, a
GNSS reference network decreases the dependence of the error budget on the distance of nearest
antenna.
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