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

70
Introduction
The four years since the last IUGG General Assembly in
Sapporo have seen tremendous developments in spaceborne
gravimetry. The sections on Gravity Field Satellite Missions
and on Temporal Gravity Field Variations describe the
exciting science that results from analysis of CHAMP and
GRACE data. At the same time, these satellite missions,
including GOCE, have accelerated the development of new
methodological approaches. The sheer amount of data and
unknowns to be inverted has stimulated the advancement
of data handling strategies in several ways: both for functional
and for stochastic modelling, both in brute-force
numerical and in semi-analytical schemes, in validation
techniques, a posteriori testing, and so on.
Some of the trends and developments in Satellite Gravity
Field Theory that were identified by SNEEUW and KUSCHE
(2007):
The observables from CHAMP, GRACE and GOCE are
increasingly modelled as in situ observables in the theoretical
framework of classical physical geodesy. Combined
with semi-analytical approaches highly efficient algorithms
have been developed.
Multiresolution, space-localizing representations have
found their way from the mathematical realm into the
geodetic mainstream. Although spherical harmonic parameterization
remains the default approach for the current
missions, more and more researchers exploit the benefits
of spatio-temporal localization by multiresolution
modelling.
Despite the great successes of GRACE in monitoring the
time-variable gravity field, the Achilles’ heel of such
mission scenarios becomes obvious: separation of the
gravitational observable into its constituent mass sources.
To disentangle these individual sources, fundamentally
lumped into the gravitational observable, requires highquality
a priori models for so-called de-aliasing purposes
and a delicate characterization in space, time and spectral
domains. The separability issue will only be aggravated in
future missions of the same design with improved hardware,
e.g., a GRACE-type mission with a laser interferometry
link.
An improved understanding of the gravitational sensors on
GRACE and GOCE has motivated and necessitated more
advanced stochastic modelling.
Theoretical and computational aspects in the downward
continuation and regularization of spaceborne gravimetric
Satellite Gravity Theory
N. SNEEUW 1
data, decorrelation and outlier detection in coloured-noise
observations, full-covariance modelling, and the general
design of ‘smart’ algorithms to tackle these issues more
efficient than in the past, will continue to play a major role.
Geoscientific interpretation and application of CHAMP,
GRACE and in the near future GOCE gravity field models
requires a deeper understanding of the underlying noise
characteristics and error propagation mechanisms inherent
to these products. The combination with a priori models
and data from complementary observing systems like
satellite altimetry, GPS and INSAR requires a careful
analysis of the information content and the resolving power
of the various data sets.
Advances in gravity analysis techniques: in situ
modelling
In the pre-CHAMP era, conventional gravity field
modelling from satellite observations was rooted in
dynamic satellite geodesy and orbit perturbation theory. It
involved large-scale computations, extensive software
packages and, at an institutional level, a certain critical mass
of people and resources. As a result, only a few global
players were involved in global gravity field modelling
from satellites. The observables from CHAMP, GRACE
and GOCE, in contrast, can be modelled as in situ
observables in the theoretical framework of classical
physical geodesy. This enabled smaller, mostly universitybased,
groups to get involved in global (but also regional)
gravity field modelling from satellite-borne gravimetry, and
to produce competitive gravity models.
A point in case is the energy balance approach or Jacobi
integral approach, e.g. GERLACH et al. (2003a, 2003b) or
VISSER et al. (2003) which came to fruition at the IAPG at
TU Munich. In this approach the GPS-derived orbit positions
and velocities are converted to in situ disturbing
potential along the orbit. By careful reduction of accelerometer
outputs and of auxiliary forces this method provided
high-quality CHAMP-only gravity fields, cf. GERLACH et
al. (2003c). Despite its quality the CHAMP-only gravity
field quality proved insufficient to reveal long wavelength
time variations that remained hidden in the observation
noise and ground-track variability, cf. SNEEUW et al. (2003).
Similarly, the ITG at University of Bonn developed the
Hammerstein-Schneider approach further to a level of
sophistication that allows CHAMP and GRACE data
processing with a competitive quality. This approach is
characterized by short arcs (ILK et al., 2005), combined with
1 Nico Sneeuw: Geodätisches Institut, Universität Stuttgart, Geschwister-Scholl-Str. 24D, D - 70174 Stuttgart, Germany, Tel.
+49 - 711 - 685-83390 , Fax +49 - 711 - 685-83285, e-mail sneeuw@gis.uni-stuttgart.de