Geoscience Reference
In-Depth Information
The Demand for Development of Geodetic Technology
GPS and some other analogous satellite positioning systems have pervaded or will
pervade every aspect of human society. GPS employs the geocentric coordinate
system, which requires geodetic measurement and its products to utilize its
corresponding coordinate systems.
The dynamism of precise geodetic measurements requires that it be necessary to
refer to the high-precision geocentric coordinate system with physical significance.
7.4.2 Definitions of the CTRS and the Conventional
Terrestrial Reference Frame
Earth's Polar Origin and the Origin of Longitude
The Earth's pole is the intersection of the Earth's spin axis with the crust. The
movement of the Earth's rotation axis with respect to its crust causes temporal
variations in the position of the Earth's pole on the Earth's surface, which is called
the polar motion of the Earth (shortened to polar motion). The time-varying spin
axis of the Earth is the instantaneous axis, and the corresponding pole is called the
instantaneous pole.
The movement of the instantaneous poles on the Earth's surface is contained
within a small area, so we can use a plane tangent to the Earth's surface through the
center of the pole to replace the Earth's surface within this area. Usually, a plane
rectangular coordinate system will be established on this plane to determine the
instantaneous position of the pole, so this coordinate system is called the polar
coordinate system. We take the tangent point as the origin, denoted by P 0 .This
point is the mean position of the instantaneous pole during some time period (mean
pole) and is defined as the Earth polar origin (cf. Fig. 7.6 ). Customarily, we take the
direction of the Greenwich meridian passing through P 0 to represent the positive
direction of the x-axis. The direction of the meridian 90 west of Greenwich
represents the positive direction of the y-axis. Hence, the coordinates of the
instantaneous pole P can be expressed by the rectangular coordinates (x p , y p ).
Based on the polar motion matrix A
R Y (
x p )R X (
y p ) (cf. Fig. 7.2 , y p and x p are
the Euler angles
ʵ y ), we establish the relationship between the CTRS and the
Instantaneous Terrestrial Reference System as:
ʵ x ,
0
@
1
A CTRS
0
@
1
A
X
Y
Z
X
Y
Z
A
ð
7
:
32
Þ
ITRS
The organizations involved in determining polar motions have always included
the International Latitude Service (ILS), the International Polar Motion Service
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