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

Introduction
At the end of the last century temporal variations of the
Earth gravity field could only be derived for the very long
wavelengths up to degree and order 2 and for some low
degree zonal coefficients on an approximately weekly basis
primarily by analysis of Lageos Satellite Laser Ranging
(SLR) data. Since the launch of the German CHAMP
(Challenging Mini-Satellite Payload for Geophysical
Research and Application) satellite mission in July 2000
also high-low satellite-to-satellite tracking (SST) data have
been exploited to monitor annual and semi-annual gravity
variations on smaller spatial scales down to about 5000 km.
The US-German Gravity Recovery and Climate Experiment
(GRACE), launched in March 2002 and performing lowlow
SST observations with micrometer level accuracy,
revolutionized our knowledge of the Earth gravity field.
Today, not only the static gravity field can be determined
with unprecedented accuracy but, for the first time,
temporal variations at shorter wavelengths up to approximately
degree and order 35 to 40 are traceable, thus allowing
for the quantification of mass distribution and mass variations
within the Earth system with monthly and even submonthly
(e.g. 10-days) temporal resolution. Based on
currently about 5-years worth of GRACE mission data a
new era for basic studies on quantities of the Earth system
such as the continental hydrological cycle, the ice mass
balance in Greenland and Antarctica, the steric and massrelated
contribution to sea level rise or the inter-annual
ocean mass variation has been opened. This also led to the
implementation of a special priority program “Mass transport
and mass distribution in the Earth system” (ILK et al.,
2004, 2005) by the German Research Foundation.
Temporal Gravity Variations from SLR, GPS and
CHAMP
KÖNIG et al. (2005) estimated dynamic orbits and Earth
system parameters from a combined GPS (ground data) and
LEO data analysis. It could be shown that especially the
accuracy of reference frame and low degree gravity field
parameters benefits from such integrated analysis when
compared to solutions where the GPS ground data and the
LEO data is exploited in consecutive, but separate data
analysis steps. These investigations were motivated by the
work of ZHU et al. (2004) where the potential advantages
of an integrated adjustment of CHAMP, GRACE and GPS
ground data for the estimation of orbit, ground station and
Temporal Gravity Field Variations
F. FLECHTNER 1 , T. GRUBER 2 , R. SCHMIDT 1
1 Frank Flechtner / Roland Schmidt: GeoForschungsZentrum Potsdam (GFZ), c/o DLR, D-82234 Wessling, Germany, Tel. +49 - 8153 -
28-1735, Fax +49 - 8153 - 288-1297, e-mail flechtne@gfz-potsdam.de / rschmidt@gfz-potsdam.de
2 Thomas Gruber: Institut für Astronomische und Physikalische Geodäsie, Technische Universität München, Arcisstraße 21,
D-82290 München, Germany, Tel. +49 - 89 - 289-23192, Fax +49-89-289-23178, e-mail Thomas.Gruber@bv.tu-muenchen.de
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static and time variable gravity field parameters were
demonstrated for the first time.
A first insight into the annual and semi-annual variability
of the gravity field on spatial scales of some thousand km
was gained from the analysis of CHAMP data based on the
dynamic orbit determination method as described in
REIGBER et al. (2003a) and REIGBER et al. (2005). SNEEUW
et al. (2003 and 2005) analysed the time-variability of
CHAMP-derived spherical harmonic coefficients using
kinematic orbits and the energy balance approach.
Tidal and Non-tidal Atmospheric and Oceanic Short-
Term Mass Variations
Short- and long-term gravity field variations due to halfdaily,
daily and long-periodic atmospheric and oceanic tides
as well as non-tidal atmospheric and oceanic mass variations
on hourly and daily scales have to be taken into
account in the gravity recovery based on modern satellite
gravity data from CHAMP, GRACE and GOCE. For
CHAMP and GRACE such gravity variations are typically
reduced during the orbit integration as a priori information
to “de-alias” the corresponding monthly and long-term
static gravity field results (FLECHTNER, 2003). In the case
of GOCE gradiometer data the atmospheric and oceanic
signal is directly corrected at the level gradiometer measurements
to de-alias such signals in the GOCE-based estimates
of the static field (GRUBER and PETERS, 2003). WIEHL and
DIETRICH (2005) investigated the influence of the orbital
sampling and the instrument parameterization of CHAMP
and GRACE on the derived time-variable gravity field
signals.
In this context, BIANCALE and BODE (2006) developed
mean annual and seasonal atmospheric tide models based
on 3-hourly and 6-hourly ECMWF surface pressure data,
thus extending the limited set of available atmospheric tide
models. In order to study the impact of uncertainties in
ocean tidal models, one known source of spurious gravity
signals in GRACE-only gravity models (“striping”), in
WÜNSCH et al. (2005) a simulation study was carried out,
using the difference of two ocean tide models as a proxy
of such model errors. FLECHTNER et al. (2006) described
the operational GRACE Level-1B atmosphere and ocean
de-aliasing product (AOD1B) which is based on 6-hourly
operational ECMWF meteorological data and a barotropic
or a baroclinic ocean model. AOD1B is used to de-alias
short-term non-tidal mass variations in the monthly GRACE