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

42 Commission 2 – Gravity Field
shorter integration times and hence a higher spatial resolution.
The system showed good resolution and accuracy (2-3
mGal / 1 km) at several test flights. See KREYE (2006),
KREYE, HEIN (2003, 2004), KREYE et al. (2003, 2004,
2006).
For an overview of GNSS use for airborne gravimetry, see
HEIN, KREYE, NIEDERMEIER, HEYEN, STELKENS-KOBSCH,
BOEDECKER (2006). Future system innovations will include
the use of Galileo.
The 'Bayerische Erdmessungskommission (BEK)' at the
Bavarian Academy of Sciences and Humanities, München,
embarked on the construction of an airborne strapdown
vector gravimeter from accelerometers, gyros, and signal
processing components. This approach offers the opportunity
for detailed optimisation and accounting for classical
gravimetric techniques such as temperature control and
optimised vibration damping (BOEDECKER, STÜRZE 2004).
The current (patented) prototype SAGS4 e.g. uses an
attitude determination by integrating fibre optical gyros and
a GPS multi antennae system for enhanced long range
stability (BOEDECKER 2005). Part of the sensor cluster are
high rate GPS receivers sampling at 50 /s for high resolution
and mm accuracy (STÜRZE, BOEDECKER 2004). The
accelerometer noise is at the level of 1 mGal. Test flights
with various aircraft provided operational experiences. A
lift constructed for dynamic calibration provides transfer
functions for the accelerometers as also for kinematic GNSS
observations and thus enables a good system fit
(BOEDECKER, STÜRZE 2006).
The aforementioned groups of IFF, EN and BEK as also
the 'Aerodata Flugmesstechnik GmbH', Braunschweig, did
some coordinated research and joint test flights in 2002-
2005 in the framework of the BMBF 'Geotechnologienprogramm'.
3. Combination techniques, upward/downward
continuation
The different altitudes of gravity observations – ground,
aircraft, satellite –, discretization and irregularity of topographic-isostatic
masses pose a number of problems:
At the 'Institut für Theoretische Geodäsie' of Bonn university,
different downward continuation methods are compared
and the impact of regularization of airborne gravimetry
data and optional postprocessing filtering is
addressed by MÜLLER, MAYER-GÜRR (2003) for simulated
and real data. The gravity field effects of the topographicisostatic
masses represent important information on the
high-frequent part of the gravity field. MAKHLOOF et al.
(2006), MAKHLOOF, ILK (2005, 2007a,b) and MAKHLOOF
(2007) address the physical-mathematical basics of the
classical topographic-isostatic models. These models are
formulated mathematically with the emphasis on a spherical
approximation from the modelling point of view and on
the observables of airborne gravimetry and modern satellite
techniques from the application point of view. Besides the
representation of the topographic-isostatic mass effects by
volume integrals, discretized by spherical volume elements,
the representations by series of spherical harmonics and
space localizing base functions are considered. Detailed
formulae are presented for the direct and secondary indirect
topographical effects as well as for the primary indirect
topographical effect in the geoid heights for the different
representations. A specific topic in some articles is the
determination of the so-called far-zones based on an
approach which goes back to a formulation by Molodenskii.
Extended test computations give an impression of the size
and distribution of the various effects for regional and
global test areas with different resolutions of the topography.
In the framework of a cooperation between the 'Lehrstuhl
Physikalische und Satellitengeodäsie' at Karlsruhe university
and the University of Calgary, NOVAK et al. (2003)
study geoid computations from airborne gravity data
combined with global gravity models and ground data; this
includes the downward continuation problem. After
numerical tests with synthetic data, the procedure is applied
to an airborne gravity data set observed in a test area (of
about 100 km x 100 km, ~1mGal / 5 km) in Canada by
Sander Geophysics Labs modern AIRGrav platform airborne
gravimeter. The fusion / comparison with global
gravity models and/or ground data demonstrates the
progress by airborne gravimetry. NOVAK et al. (2003)
evaluate the band-limited topographical effects in airborne
gravimetry: The spectrum of airborne gravity observations
(at height) is limited i) because of the attenuation of the
gravity signal higher frequencies with increasing distance
from the attracting irregular masses and ii) because of the
low pass filtering of airborne gravity observations necessary
mainly because of the aircrafts dynamics. Consequently,
the topographical effects along the flight lines are also
filtered by the same low pass filter. The resulting band
limitation permits the application of global spherical
harmonics for the topographical reduction which would not
be possible for ground gravity values. Numerical tests are
based on 3"x3" DEM in the Canadian Rockies using
Helmerts reduction.
4. Observation campaigns / commercial usage
The Federal Agency for Cartography and Geodesy (BKG)
and the Danish National Space Center (DNSC) carried out
an airborne gravimetry campaign in the Southwest Baltic
Sea and neighbouring land areas (~53.5/-55.5/ N, ~8/-15/
E), using a LaCoste & Romberg airborne gravimeter (S-38).
The observations were flown with a King Air B200 aircraft
of COWI company on 23 parallel flight tracks along and
4 across with a total of 10,000 km within 45 hours in
October 2006. Partly, a Riegl laser scanner was also used.
In summer 2007, a similar campaign is planned for the
North Sea. (Reported by U. SCHÄFER, BKG; to be published
2007).
The Alfred-Wegener-Institute (AWI), Bremerhaven, has
carried out a number of airborne gravimetry campaigns in
Antarctica for the 'Validation, Densification and Interpretation
of Satellite Data for the Determination of Magnetic
Field, Gravity Field, Ice Mass Balance and Structure of the
Earth Crust in Antarctica, Utilizing Airborne and Terrestrial
Measurements' (VISA) on the inland ice sheet of the