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

38 Commission 2 – Gravity Field
authorities. The purpose of these absolute gravity determinations
is to identify systematic gravity offsets which
might have influence upon the geoid modelling. The
observations are also understood as a contribution to
validate the satellite gravity field missions CHAMP,
GRACE and GOCE by terrestrial gravity data.
The combination and evaluation works for the gravity
reference network UEGN02 (Unified European Gravity
Network 2002) was continued by the Bayerische Erdmessungskommission
(BEK). The network includes more
than 400 absolute and 33000 relative gravity observations
at 1500 stations in a common adjustment. Problems for
processing were lacking standardisation and minute data
errors which required the development of appropriate tools
for detection and which caused some delay. The progress
was reported at several meetings (BOEDECKER et al. 2004,
BOEDECKER 2006).
The two observatories Black Forest Observatory (University
Karlsruhe and University Stuttgart) and Moxa (Friedrich-
Schiller-Universität Jena) contributed high precision gravity
time series (precise spring gravimeter at BFO and superconducting
gravimeter at Moxa) which both were underlain
by repeated absolute gravity measurements of different
institutions. Moxa contributed to the Global Geodynamics
Project (GGP) the global network of superconducting
gravimeters (SG). At the Geodynamic Observatory Moxa
special care was taken upon the determination of hydrological
influences from the station environment. In 2004
a local gravity network for repeated measurements with
field gravimeters was established in the station vicinity. The
network consists of twelve observation points with six
points on an EW-running profile with distances in the range
of few to several tens of meters. The maximum elevation
difference is 24 m. The network serves two purposes: It is
used to determine what order of magnitude of gravity
variations can still be detected with field instruments under
optimal conditions for a small-scale network. The second
purpose aims at the detection of hydrology-related gravity
variations in the vicinity of the observatory, thus determining
the applicability of this type of observation for
studies of hydrological processes and the provision of
constraints for hydrological water balances in a hilly area.
By repeated measurements with high quality 3-4 LCRgravimeters
significant changes in gravity differences could
be obtained. The standard deviations obtained for the
gravity differences between two observation points are in
the order of 8 to 14 nm/s². The time-dependent variations
in the differences reach up to 140 nm/s² (± 11-20 nm/s²)
and correlate with changes in the hydrological situation.
The Institute of Physical Geodesy at the University of Technology
Darmstadt focused their works upon tectonically
active regions in of the Western part of the North Anatolian
fault in Turkey (GERSTENECKER 2003, ERGINTAV et al.
2007) and the Merapi region in Indonesia (TIEDE et al.
2005a, 2005b) to interpret the time-dependent gravity
changes.
BKG continues the observation series with the superconducting
gravimeters at Wettzell, Bad Homburg,
Medicina (Italy) and at Concepción (Chile). For Medicina
an uninterrupted time series reached a length of 10 years
in 2006. At all sites investigations have been started to
relate the observed gravity changes with hydrological,
atmospheric and height variations. BKG routinely contributes
its SG time series to the Global Geodynamic Project
(GGP).
Contribution to the International Gravity
Reference System
BKG has taken responsibility for the national gravity
reference of Germany and its integration in the international
gravity system. This is achieved on the one hand side by
the repeated four-yearly comparisons of AG within the
frame of the International Comparative Campaigns at the
Bureau International des Poids et Mesures BIPM, in Sèvres
(France). BKG participated with the absolute gravimeter
FG5-101 in 2005.
On the other hand, additional instrument comparisons were
performed on selected gravity stations as supplementary
measures and intermediate verifications for the purpose of
securing the instrumental gravity standard. In November
2003, BKG participated in the regional comparison carried
out at the station Walferdange (Luxembourg) with its
absolute gravimeter FG5-301 (FRANCIS et al. 2005).
During the report period comparative measurements of FG5
gravimeters of BKG were conducted in Metsähovi (Finland)
with the AG of the Central Research Institute for Geodesy,
Aerophotogrammetry and Cartography (SNIGAiK),
Moscow (Russia), in Strasbourg (France) with the FG5-206
of EOST and in Pecny (Czech Republic) with FG5-215 of
VÚGTK.
The concept of the BKG gravity reference station Bad
Homburg includes not only the operation of the superconducting
gravimeter SG30, but there also exists the
possibility to carry AG measurements on three different
measuring pods. These are used in particular to monitor and
compare the absolute gravimeters FG5 and A10 employed
by BKG. Within the upgrading of the station Bad Homburg
as a GREF station, and also as "ECGN Core Station", a
permanent GPS receiver was installed on the building and
tied into the local geodetic control network. Two groundwater
level gauges complement the data series and provide
information about hydrological changes at the site. The
suggestion to use Bad Homburg as a regional comparison
site for absolute gravimeters (WILMES, FALK 2006) was
seized by several groups. Hence, the FG5 gravimeters of
BKG were inter-compared with FG5-220 of IfE, Leibniz
Universität Hannover in 2003, 2005 and 2006; with FG5-
215 of the Research Institute for Geodesy, Topography and
Cartography (VÚGTK), Prague (Czech Republic) in 2003
and 2005; with the FG5-230 of the Technical University
Warsaw (Poland) in 2006 and with FG5-226 of the
Norwegian University of Life Sciences, Ås in 2006.