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

Introduction
The US-German mission GRACE (Gravity Recovery and
Climate Experiment) has celebrated its fifth birthday in
March 2007. In July 2007 the German mission CHAMP
(Challenging Mini-Satellite Payload for Geophysical
Research and Application) will be in orbit for exactly 7
years. These missions have dominated the gravity field
mapping scene during the past four years. They have led
to static and time-variable gravity field models with far
reaching impacts in neighbouring geoscience disciplines.
They have also spurned a host of new methodologies for
gravity field modelling and recovery techniques, most
notably those techniques that consider the observations as
in situ gravity field functionals. At the same time the
preparations for a December 2007 launch of the European
mission GOCE (Gravity field and Ocean Circulation
Explorer) are ongoing. The German contributions over the
past four years to these and other developments will be
highlighted in the following.
From an international viewpoint the German geodetic
community is well positioned. It borrows much of its
strength from long-term involvement in the area of satellite
gravimetry. Moreover, more recently, two large research
programs came into being. The German Ministry for
Education and Research (BMBF) supports a so-called
Geotechnology Program with the title Observation of
System Earth from Space, whereas the German Research
Foundation (DFG) supports a priority program SPP1257
titled Mass transport and mass distribution in the system
Earth, both of which are devoted (to a large extent) to space
gravimetry.
Gravity field satellite missions
CHAMP has been in orbit for exactly 7 years now. The
orbital decay due to solar activity predicts a remaining life
time at least up to end of 2008. GRACE has been in orbit
for five years now. The predicted lifetime calculated from
gas consumption, thruster activations and solar activity is
at least 2011. Despite a setback during system testing,
GOCE is now ready for launch in December 2007.
The gravity field models resulting from these missions are
described in the section on Global Gravity Field Modeling.
The series of EIGEN models from the GeoForschungs-
Zentrum, Potsdam, consisting of both satellite-only and
combination models, has become a standard. The European
data center for CHAMP and GRACE products (ISDC) is
housed at GFZ Potsdam. At the same time, a number of
Overview and Highlights
N. SNEEUW 1
1 Nico Sneeuw: Geodätisches Institut, Universität Stuttgart, Geschwister-Scholl-Str. 24D, D-70174 Stuttgart, Germany, Fax +49 - 711 -
685-83285, Tel. +49 - 711 - 685-83390, e-mail sneeuw@gis.uni-stuttgart.de
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"competing" gravity models have been developed by
university groups (Bonn, Munich, Kaiserslautern), mostly
based on alternative modelling schemes.
Pre-mission GOCE activities were focused on sensor
analysis, calibration/validation schemes and, for a large
part, preparation for the actual GOCE data analysis. The
latter is done in the framework of the ESA (European Space
Agency) funded project High-level Processing Facility
(HPF) whose PI is IAPG, TU Munich, but with strong
cooperation from several other German groups. First results
are to be expected roughly one year after launch.
Through the gravity field satellite missions the goal of "10-9 geodesy", which was previously attained by geometric
geodesy, has now been achieved by physical geodesy too,
at least at the larger length scales. GOCE will further
enhance the spectral bandwidth for which "10-9 physical
geodesy" is valid.
Methodological advances
In the pre-CHAMP era conventional gravity field modelling
from satellite observables was rooted in dynamic satellite
geodesy and orbit perturbation theory. It involved largescale
computations, extensive software packages and, at
an institutional level, a certain critical mass. As a result only
a few global players were involved in global gravity field
modelling. This approach is still pursued, e.g. in the EIGEN
series of gravity models from GFZ. The observables from
CHAMP, GRACE and GOCE, on the other hand, can be
modelled as in situ observables in the theoretical framework
of classical physical geodesy. This enabled smaller, mostly
university based groups to get involved in global (but also
regional) gravity field modelling from satellite-borne
gravimetry, and to produce competitive gravity models, see
above. The methodological advances are more extensively
described in the section Gravity Field Theory.
Geoscience Applications
GRACE is geodesy’s window into neighbouring geoscience
disciplines. The monthly gravity field solutions represent
global mass redistribution between cryosphere, hydrosphere,
atmosphere and oceans. This fact was the main
driver behind the aforementioned DFG-program on Mass
transport and mass distribution in the system Earth. Some
projects in this program deal with oceanographic modelling
for and from GRACE. In other projects the continental
hydrological cycle is constrained through GRACE. The
topic of yet other projects is the monitoring of the Earth's