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Roles of ITRF in the Establishment and Maintenance of a Regional Geodetic
Coordinate System
A geocentric coordinate system, by definition, is unique. However, different
methods and data used in the realization will result in different geocentric coordi-
nate systems. Since the geocenter of ITRF is highly precise, globally distributed,
and considerably authoritative, other geocentric systems are also aligned to it. For
instance, the WGS84 has been refined many times and the European reference
frame has been incorporated into ITRF. ITRF station coordinates are strongly
constrained by countries worldwide while processing their own GPS data, so as
to make the national coordinate system approximate or pertain to the ITRF.
At present, the accuracy of ITRF station positions is at a level better than 1 cm
and that of the station velocities is better than 3 mm per annum. We take these
points as the initial stations and use GPS relative positioning to carry out GPS
observations in the area of interest. After data processing, we will obtain the high-
precision station coordinates in this area, which means that the geocentric coordi-
nate system based on the technique of GPS has been established in this region of
interest.
ITRF is crucially important in the establishment and maintenance of regional
coordinate systems.
First, the precise IGS ephemeris and Earth orientation parameters (EOP) are
used in establishing regional coordinate systems, and the reference frame of the IGS
precise ephemeris belongs to the ITRF.
Second, the initial stations used in the establishment of regional coordinate
systems are ITRF stations. In calculation, these stations are mostly tightly
constrained. In this sense, the established coordinate system will be made well
aligned to the ITRF. Therefore, the regional coordinate systems established in
recent years have all made clear that their coordinate systems agree with ITRF.
For instance, the European reference frame is fixed to a stable part of the Eurasian
Plate and is coincident with ITRF at the reference epoch of 1989.0.
Although ITRF stations are chosen as the initial stations in the establishment of
all regional geocentric coordinate systems, the approaches for selection are dra-
matically different, either by strongly constraining ITRF stations within and sur-
rounding the area of interest (like the South American reference frame SIRGAS), or
by constraining ITRF stations fixed to the stable part of different plates (such as the
European reference frame). If different initial stations are chosen, the reference
systems established will be different and such differences are often systematic.
WGS84
Since the 1960s, in order to establish an internationally unified geodetic coordinate
system, the former American Defense Mapping Agency (DMA) established the
WGS60, followed by the updated WGS66 and WGS72. In the mid-1980s the
WGS84 coordinate system was established, which was a CTRS. Meanwhile,
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