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the above period, which is called the Conventional International Origin (CIO). The
corresponding Earth equatorial plane is referred to as the mean equatorial plane or
conventional equatorial plane. In practice, CIO is commonly used as the Conven-
tional Terrestrial Pole (CTP) up to the present time. The TRS that refers to the CTP
is called the Conventional Terrestrial Reference System (CTRS). The TRS
corresponding to the instantaneous pole at true-of-date is called the Instantaneous
Terrestrial Reference System (see Moritz and Mueller 1987).
When being applied to the plotting of topographic maps and engineering on a
regional basis, the geocentric coordinate system is not immediately required.
However, in applications involving cross-regional surveying and mapping projects,
integration as well as studies on space technologies, geodynamics, and the gravity
field of the Earth that involve physical factors, the geocentric coordinate system
plays a critically important role as described below.
Application Demands in Geodynamics, Physical Geodesy, and Space
Technology
Following the 1960s, as an interdisciplinary subject, geodynamics has gradually
drawn the attention of geodetic scholars. People have realized that almost all
geodynamic phenomena, like crustal movement, material migration, tidal variation,
and Earth rotation, can be monitored by applying space geodetic technologies.
Therefore, geodesy has become one of the most fundamental methods for studying
the geodynamic phenomena of the Earth. Broadly speaking, to carry out systematic
studies on geodynamics, a fairly stable reference system should be established first.
Otherwise, data observation will not be homogeneous. Hence, defining and realiz-
ing a reference system that is suitable for the purpose of geodynamic study becomes
crucial.
Differing from the local coordinate system established in static geodesy,
dynamic geodesy considers the Earth as a non-rigid, deformable body. Conse-
quently, there arises the important issue of where to put the origin of the reference
system within the Earth to maintain its stability. If the Earth is considered as a
system of particles with the inside resultant force being zero, then, according to the
dynamics of a system of particles, the Earth moves in a certain fixed orbit acted
upon by the resultant external force of other celestial bodies. The law of motion of
the Earth's particles completely corresponds to the hypothesis that all the mass of
the Earth is concentrated in the center of mass. Therefore, however the mass inside
the Earth migrates or how the shape of the Earth deforms, the Earth's center of mass
moves along a fixed orbit. The significance of such an inference is that although the
position of a point on the Earth's surface is changing at any moment, the position of
the Earth's center of mass is “fixed” when observed from the outer space. Hence,
with respect to the study of geodynamics, the ideal position of the origin of the
reference system should be at the Earth's center of mass.
In physical geodesy, we need to choose a normal ellipsoid that best fits the geoid
on a global basis, and its geometric center should coincide with the Earth's mass
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