Global Positioning System Reference
In-Depth Information
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2.1 CONVENTIONAL TERRESTRIAL REFERENCE SYSTEM
A conventional terrestrial reference system (CTRS) must allow the products of vari-
ous geodetic space techniques, such as coordinates and orientation parameters of the
deformable earth, to be combined into a unified data set. Such a reference system
should (a) be geocentric (whole earth, including oceans and atmosphere), (b) incor-
porate corrections or procedures stemming from the relativistic theory of gravitation,
(c) maintain consistency in orientation with earlier definitions, and (d) have no resid-
ual global rotation with respect to the crust as viewed over time. This section deals
with the major phenomena such as polar motion, plate tectonic motions, solid earth
tides, and ocean loading that cause variations of coordinates in a terrestrial reference
frame. To appreciate the demand placed on a modern reference system, consider the
following statement: “GPS data are used to compute daily estimates of the earth's
center of mass and scale. Recent center of mass estimates have daily repeatability at
thelevelof1cmin
x
,1cmin
y
, and 1.5 cm in
z
. Seasonal variations in the center
of mass occur at the 3-4 mm level, due primarily to global water mass redistribution.
Recent scale estimates repeat daily at the level of 0.3 parts per billion” (Heflin, JPL,
private communication).
[13
Lin
—
0.0
——
No
PgE
2.
1.1 Polar Motion
The intersection of the earth's instantaneous rotation axis and the crust moves with
time. This motion is called polar motion. Figure 2.1 shows polar motion for the time
2001-2003. This motion is somewhat periodic. There is a major constituent of about
434 days, called the Chandler period. The amplitude varies but does not seem to
exceed 10 m. Several of the polar motion features can be explained satisfactorily
from a geophysical model of the earth; however, the fine structures in polar motion
are still subject to research.
To avoid variations in latitude and longitude of about 10 m due to polar motion,
we need to define a conventional terrestrial pole (CTP) that is fixed to the crust.
Originally, this pole was defined as the center of figure of polar motion for the
years 1900-1905. This definition required several refinements as the observation
techniques improved. The instantaneous rotation axis can be referenced to the CTP
by the polar motion coordinates
(x
p
,y
p
)
. The origin of the polar motion coordinate
system is at the CTP, the
x
axis is along the conventional zero meridian, and the
y
axis is positive along the 270° meridian. The center of figure of today's polar motion
does not contain the CTP. There appears to be “polar wander” (gradual shifting of the
center of figure away from the CTP).
The CTP represents the direction of the third axis of the conventional terrestrial
reference system. The definition of the CTRS becomes increasingly complicated be-
cause of plate tectonic motions that cause observable station drifts and other temporal
variations in the coordinates of a “crust-fixed” coordinate system. As the plates move,
the fixed station coordinates become inconsistent with each other. The solution is to
define the reference frame by a consistent set of coordinates and their velocities of
a global network of stations at a specific epoch. The center of mass of the earth is
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