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Therefore, under the constraint of the Tisser and condition, the transformation
parameters calculated and published by the international service agency define
the frame of the realized system.
7.4.3 Establishment and Maintenance of the CTRF
To go from definition to realization of a TRS, the following need to be attained:
1. Give the theoretical definition and define the conventions of
the TRS
(as described in Sect.
7.4.2
).
2. Establish observation sites on the Earth's surface and carry out space geodetic
measurements.
3. Process the observed data using the internationally recommended set of models
and constants according to the conventions of the CTRS and solve the station
coordinates of each observation site at a certain epoch; namely, establish
the CTRF.
4. Analyze and deal with various kinds of deformation factors that influence the
stability of the surface observation sites, and establish corresponding time-
variant models to maintain stability of the CTRF.
To determine the position of the Earth's center of mass, let us first assume that
the Earth is a rigid body. We can determine the distance between n points (n
3) on
the Earth's surface and the Earth's mass center using satellite dynamic techniques
like SLR, followed by determination of the distances between these points by
measuring techniques like GPS, VLBI, etc. Thus, we can determine the position
of the geocenter through geometric constraint conditions. Nevertheless, the Earth is
not a rigid body, it is a complicated viscoelastic body. Consequently, these points
are changing all the time. The changes include both regular changes like the solid
Earth tide and irregular changes like unpredictable deformations. The precise
position of the Earth's center of mass is yet not determined and such indetermina-
tion can be improved through repeated observations over a long period. It is not
difficult to understand that the accuracy of the position of the geocenter relative to
the sites obtained using the method of least squares depends not only on the
measuring precision, but also on the number of sites and the graphic structure.
The pointing direction of the coordinate axis of the TRS is related to the spin axis
of the Earth. First, we have to clarify that the Earth's instantaneous axis of rotation
is bound to pass through the Earth's center of mass. Otherwise, the center of mass
will undoubtedly rotate around the instantaneous axis of rotation, which clashes
with the kinetics of particles. Due to polar motion, the path of instantaneous axis of
rotation forms an approximately circular conical surface, with its apex at the Earth's
center of mass. Let the mean rotation axis (i.e., the axis of symmetry of the circular
conical surface) be the Z-axis. The X-axis lies within the Greenwich Observatory's
meridian plane. Then, choose the Y-axis, making O-XYZ a right-handed Cartesian
coordinate system. In this case, we have established the geocentric reference
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