NATIONAL REPORT OF THE FEDERAL REPUBLIC OF ... - IAG Office

Introduction
Physical Aspects of Geodetic Modelling, Relativity
Physical models are used in geodesy for reducing unwanted
(disturbing) effects from the observation data on the one
hand, and as simplifications and generalizations of the real
Earth for geodetic parameter estimation on the other hand.
Geodetic parameters refer not directly to reality but to
physical models represented by mathematical approaches.
The correctness of the models is therefore a critical issue
in all geodetic modelling.
Modern geodetic observations are in general based on
precise time measurements. This holds for both the geometric
observations, where the travel time of electromagnetic
waves (optical or microwaves) is measured to
derive distances via speed of light, and the gravimetric
observations, where the acceleration of free falling probe
masses is determined. The dramatic advancement of geodesy
in observation and parameter estimation during the
last decades is mainly due to the continuous improvement
of the time measurements (DREWES, 2006a, b). The
progress of physical models is not always consistent with
this development, we meet the challenge of improving
them. In the following we report about German contributions
to these physical aspects of geodetic modelling in the
three fundamental fields of geodesy: geometry (positioning),
orientation (rotation), and gravity of the Earth. The
basic products of geodesy relate to these fields and have
to be generated in a uniform, consistent modelling. This
is the principal science rationale of the Global Geodetic
Observing System (GGOS) of the International Association
of Geodesy (IAG) (DREWES, 2007). The performance
enhancement of the geodetic observations based on time
measurements requires the strict consideration of relativistic
effects. Relativity theory shall therefore be treated as a
special issue in this report.
Geodetic modelling of geometry, orientation
and gravity of the Earth
Geometry modelling
Most important physical models for determining the
geometry of the Earth from geodetic positioning are those
of the atmosphere and the oceans. Concerning the vertical
component of positioning (heights), the physical ocean
surface and the gravity field serve as a reference and have
precisely to be modelled.
H. DREWES 1 , M. SOFFEL 2
1 Hermann Drewes: Deutsches Geodätisches Forschungsinstitut (DGFI), Alfons-Goppel-Str. 11, D - 80539 München, Germany,
Tel. +49 - 89 - 23031-1106, Fax +49 - 89 - 23031 1240, e-mail drewes@dgfi.badw.de
2 Michael H. Soffel, Institut für Planetare Geodäsie, Technische Universität Dresden, Helmholtzstr. 10, D - 01062 Dresden, Germany,
Tel. +49 - 351 - 4633-4200, Fax +49 - 351 - 4633-7019, e-mail michael.soffel@tu-dresden.de
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KANIUTH and HUBER (2004) use atmospheric pressure
models to estimate the loading effects causing height
variations in Europe and compare them with GPS observations.
The tidal effect of ocean loading is studied by
ZAHRAN et al. (2006). Corresponding height variations are
presented by KANIUTH and VETTER (2005) at coastal sites
from GPS measurements.
Problems related to physical models in the definition of
vertical reference frames are discussed by HECK (2004).
IHDE and SÁNCHEZ (2005) present an approach to globally
unify height systems by uniform physical models of the sea
surface and the gravity field of the Earth.
Modelling of the Earth’s orientation
Variations of the Earth’s orientation in space and Earth’s
rotation are caused by changes of the angular momentum
or torque, respectively. These are generated by mass
displacements in the solid Earth, the atmosphere and the
hydrosphere including the oceans and continental water
storage. They have to be represented by physical models
for consideration in geodetic parameter estimation.
The influences of core processes on Earth’s rotation parameters
are studied by GREINER-MAI et al. (2003). The effect
of water storage variations on polar motion is presented by
FERNANDEZ et al. (2007). STUCK et al. (2005) model the
physical mechanism of atmospheric forces in polar motion,
and THOMAS et al. (2005) concentrate on the contribution
of the oceans. Combined modelling of atmospheric and
oceanic effects from coupled physical models is published
in a series of papers by SEITZ (2004, 2005), SEITZ et al.
(2004), and SEITZ and SCHMIDT (2005). MARCHENKO and
SCHWINTZER (2003) combine Earth rotation parameters and
the Earth gravity field by a combined parameter estimation.
Gravity field modelling
The physical modelling of the gravity field parameters
concentrated in the last years on the use of observation data
from the satellite gravity field missions CHAMP and
GRACE. A number of papers deals with these issues. A
new physical approach of gravity field modelling from
these missions using the energy integral from kinematic
orbits is presented by GERLACH et al. (2003).
Physical models of the solid Earth include isostatic models
presented by KABAN et al. (2004) for the entire lithosphere
as well as by WILD and HECK (2005). WZIONTEK (2003)