Geoscience Reference
In-Depth Information
1 The Earth's Variable Rotation
The rotation of the Earth is not uniformbut subject tomanifold spatio-temporal varia-
tions. Both the direction of the spin axis as well as its absolute value, the rotation rate,
undergo significant perturbations as the Earth system responds to the action of exter-
nal and internal torques. External torques are generated by the gravitational attraction
of the Sun, the Moon and the planets, while internal torques can be assigned to large-
scale geodynamical processes prompting exchange of angular momentum between
the solid parts of the Earth and the surrounding fluid layers like the atmosphere,
the oceans and the liquid core. Highly precise monitoring of the induced rotational
fluctuations, carried out by space geodetic techniques, is an indispensable scientific
task since the interpretation of the observed variations shed light on interactions in
Earth's dynamical system and can constrain particular structural parameters of our
planet. Enhanced understanding of Earth's rheology and dynamics will in turn be
reflected in improved rotational models, which are essential to accurate positioning
and navigation on Earth and in space.
1.1 Parametrization of Earth Rotation
The full orientation of our planet in space is a complicated time-dependent function,
whose knowledge is required for the direct and precise transformation of station
coordinates from the rotating terrestrial frame to the quasi-inertial celestial frame.
While the rotational behavior of a bodywould be unambiguously determined by three
time-dependent Euler angles, astronomy traditionally uses an intermediate reference
frame for the complete transformation of terrestrial and celestial station positions.
As a result, Earth rotation is parameterized by five
Earth orientation parameters
(EOP), which can be summarized in three categories (Schuh and Böhm
2011
):
pre-
cession
and
nutation
comprise long-periodic motions of the Earth's rotation axis
in a celestial reference system.
Polar motion
denotes direction changes of the spin
axis with respect to a body-fixed reference system. Magnitude fluctuations of the
rotation vector are reckoned in changes in
length of day
(LOD) or, equivalently, in
dUT1
UTC, i.e. the deviation of UT1 (
Universal time 1
) from the uni-
form atomic time UTC (
Universal Time Coordinated
). The subset of polar motion
and dUT1 or LOD is known as
Earth rotation parameters
(ERP). A more detailed
description of these quantities is presented in the next paragraphs and largely follows
Schuh and Böhm (
2011
).
The actual definition of the EOP depends on the kind of the underlying trans-
formation method, on which the IERS (
International Earth Rotation and Reference
Systems Service
) issues regular recommendations (McCarthy and Luzum
2010
). The
current transformation procedure according to the IAU (
International Astronomical
Union
) resolutions 2000 and 2006 is defined from the body-fixed, geocentric TRS
(
Terrestrial Reference System
) to the quasi-inertial GCRS (
Geocentric Celestial Ref-
erence System
) or vice versa. Both systems are of similar orientation along the mean
=
UT1
−
