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

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Introduction
In the reporting period from 2003 to 2007, global gravity
field modelling work in Germany was focused on the
exploitation of data from the German CHAMP and the
US/German GRACE missions as well as on the preparations
for the data analysis of the GOCE gradiometry mission by
ESA. From the Champ mission, launched in July 2000, a
multi-year data set became available enabling fundamental
investigations on classical and new techniques for global
gravity field modelling. These techniques, as well as new
approaches specifically developed for the GRACE data
analysis, were applied to the newly available inter-satellite
tracking data from this mission. In 2005 data from the
GRACE mission (launched in 2002) were released and
since then extensively used by different groups in Germany
in order to determine the static and time variable gravity
field. For preparing the analysis of the gradiometer data
from the GOCE mission (to be launched end of 2007)
extensive work has been performed in various projects by
a number of university and research institutes all over
Germany. This preparatory work will enable these groups
to quickly use this new data type for global gravity field
modelling. In context with these gravity field satellite
missions the issue on calibrating and validating the instruments,
as well as the validation of the derived global gravity
field models becomes more and more emerging. Several
studies and attempts on this issue have been performed. The
following chapters provide short summaries on modelling
techniques, global gravity field models and model validation.
They shall provide an overview of the work performed
in this research field over the last four years. A few additional
remarks about future prospects of global gravity field
modelling are made in the final chapter. An extensive list
of references is provided at the end.
Modelling Techniques
General Aspects: With the new satellite missions, there
are available different data types to observe the gravity field
from space (e.g. high-low SST, low-low SST, gravity
gradients). A general overview about gravity field analysis
techniques based on the different observables is given in
BEUTLER et al. (2003), RUMMEL (2003a) and FLURY et al.
(2006). An overview about the application of Earth gravity
fields in different disciplines is given in RUMMEL (2005),
ILK et al. (2004) and ILK et al. (2005d). Various dedicated
geodetic techniques for gravity field determination and
analysis are addressed by several authors. See papers by
AUSTEN & KELLER (2007), BÖLLING & GRAFAREND (2005),
Global Gravity Field Modelling
T. GRUBER 1
FÖLDVARY & WERMUTH (2005b), HECK (2003a) (20034a),
HECK & SEITZ (2003b) (2007), HECK & WILD (2004b), ILK
(2003a), ILK et al. (2007), KABAN et al. (2004) (2005),
KELLER & SHARIFI (2005), KUHN & SEITZ (2004),
MARINKOVIC et al. (2003), MEYER (2006), NOVAK et al.
(2005), REUBELT et al. (2003a) (2003b), SCHÄFER et al.
(2003), SCHNEIDER (2004) (2005a) (2006) and SCHNEIDER
& CUI (2005b), SEITZ (2003), SHARIFI & KELLER (2005),
SNEEUW (2003a), SVEHLA & ROTHACHER (2005), SVEHLA
& FÖLDVARY (2006), TSCHERNING & HECK (2005), TSOULIS
(2005). Wavelet (or multiscale) techniques for gravity field
modelling are addressed by FENGLER et al. (2004c),
FREEDEN et al. (2003a, 2003b), FREEDEN & MICHEL (2004),
FREEDEN & SCHREINER (2005), FREEDEN & MAYER (2006),
KELLER (2004), SCHMIDT M. et al. (2004, 2005, 2007). A
summary on fundamental parameters and standards is given
in GROTEN (2004).
CHAMP: With the availability of a continuous multi-year
time series of GPS observations from a low Earth orbiter
to the GPS constellation dedicated analysis techniques
became possible. For a summary see MAYER-GÜRR et al.
(2005b) and WERMUTH et al. (2004). One can distinguish
between the classical orbit perturbation approach used by
REIGBER et al. (2003a) (2003b) (2003c) (2005b) (2006a),
multiscale techniques applied by FENGLER et al. (2004b,
2005), the energy balance approach used by FÖLDVARY et
al. (2005), GERLACH et al. (2003a) (2003b), SNEEUW et al.
(2003b) (2005a), VISSER et al. (2003), semi-analytical
computations performed by FÖLDVARY et al. (2003), short
arc techniques used by ILK et al. (2005a), MAYER-GÜRR et
al. (2003a), MAYER-GÜRR (2006), the accelerations
approach applied by REUBELT et al. (2006), and harmonic
splines and multipole techniques done by GLOCKNER
(2003). Several papers also deal with the determination and
application of kinematic orbits for gravity field modelling.
See GÖTZELMANN et al. (2006), SVEHLA & ROTHACHER
(2005) and SVEHLA & FÖLDVARY (2006).
GRACE: Similar as for CHAMP also GRACE data have
been analyzed intensively and gravity field solutions have
been determined by different methods. As the GRACE
observation system is much more complicated some studies
about sensor performance have been performed in parallel,
see FACKLER (2005), FROMMKNECHT et al. (2003) (2006).
Gravity field determination using GRACE data has been
done by applying the following techniques: Classical orbit
perturbation theory, see FLECHTNER (2003), FLECHTNER
et al. (2006), REIGBER et al. (2005a), SCHMIDT R. et al.
(2003, 2006), TAPLEY et al. (2007); Short arc technique,
1 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