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

1. Introduction
Airborne gravimetry developed in the past decades as to
provide a more efficient observation technique compared
to conventional ground observations and a higher spatial
resolution than satellite methods. Currently, the spatial
resolution at the level of one mGal3 is of the order of 3…8
km; the aim is 1 mGal/1 km. The gravimeter sensor utilises
a spring-mass accelerometer to sense the total acceleration
a. Gravity g is recovered by subtracting the inertial acceleration
b, derived from GNSS positioning, i.e. g = a –b. Both
observation techniques for a and b as also the problems of
data fusion, i.e. reference frame transformations and system
fit, require further developments. Classical airborne gravimeters
use one vertical sensor mounted to a gyro stabilised
platform and hence deliver scalar values; new developments
are also dealing with gravity vectors from a triad of
accelerometers.
This report will restrict to airborne gravimetry with conventional
sensors, hence we shall not be dealing with e.g. atom
interferometers for gravimetry nor with gradiometers.
Airborne gravity lends itself for data fusion with ground
gravity, satellite gravimetry and topographic-isostatic data.
The gravity (details) attenuation with height and relations
of the continuous field to discrete data require further
studies driven by applications and increasing data availability.
2. System development
The Federal Institute for Geosciences and Natural
Resources (BGR), Hannover, was using a platform gravity
meter system KSS31 of 'Bodenseewerk Geosystem' for
marine gravimetry since 1984. Modifications for airborne
gravimetry required raw data recording, improved platform
control, sensor sealing to air pressure variations, weight
reduction etc.
Four Novatel OEM4 L1/L2 GPS receivers were acquired
for kinematic positioning and inertial acceleration determination.
After dynamical ground tests of the whole system
on an airstrip, test flights out of Münster/Osnabrück
followed in November 2003, showing the need for improvements
of navigation and platform data (HEYDE, KEWITSCH
2004, 2005; MEYER, HEYDE 2004). Therefore, a Novatel
SPAN INS/GPS integrating system was added. In coopera-
1 Gerd Boedecker: Bayerische Kommission für die Internationale Erdmesung / BEK, Alfons-Goppel-Str. 11, D - 80539 München, Germany;
Tel. +49 - 89 - 23031 1212, e-mail: boe@bek.badw-muenchen.de
2 Uwe Meyer: Bundesanstalt für Geowissenschaften und Rohstoffe / Aerogeophysik, Stilleweg 1, D - 30655 Hannover, Germany,
Tel. +49 - 511 - 643-3212, e-mail: U.Meyer@bgr.de
3 1 mGal = 1 C10 -5 ms
Airborne Gravimetry
G. BOEDECKER 1 , U. MEYER 23
41
tion with BGGS (successor of 'Bodenseewerk Geosystem',
see above) a modul for platform angular error observations
was developed. Also accounting for horizontal accelerations,
the gravity effects from platform misleveling can be
corrected now.
After these system improvements, four flight profiles were
flown out of Münster/Osnabrück in May 2005, showing
very satisfactory results despite rough air conditions
(HEYDE, KEWITSCH 2006 a/b). Successful helicopter test
flights in 2006 demonstrated e.g. the benefit of a smooth
and steady flight path.
The Institute of Flight Guidance (IFF) of the Technical
University of Braunschweig had acquired a Chekan-A 2axis
platform gravimeter of Elektropribor, St. Petersburg,
Russia, several years ago (CREMER 2003, SCHÄNZER 2003).
This instrument has been upgraded in the past years. E.g.,
a ring laser azimuth gyro for an analytical 3rd axis was
added and via a Kalman filter developed for the whole
system including also GPS states the misalignment was
reduced considerably and dynamic capabilities were
improved. For altitude determination, a precision barometric
'statoscope' of small range and high resolution has
been refined by instrumental and modelling measures to
an accuracy level commensurate with GNSS but with
different characteristics which makes a fusion very attractive.
Different GNSS kinematic positioning scenarios were
studied. A patented complementary airborne gravimeter
real time feedback system warrants high accuracies. Test
flights with IFFs Dornier 128-6 demonstrated an anomaly
accuracy / resolution of 1 mGal / 5 km. See CREMER,
STELKENS (2003), STELKENS et al. (2003-2006).
The Institute of Geodesy and Navigation (EN) at the University
of Federal Armed Forces München at Neubiberg
uses a commercial strapdown inertial navigation System
(SDINS) system Sagem Sigma 30 equipped with triads of
ring laser gyros and high precision accelerometers for
airborne vector gravimetry. The focus of own activities is
on algorithms including filters both for the total acceleration
and for the kinematic acceleration signal. E.g., the aircrafts
vibrations induced on the SDINS is mitigated by customized
software instead of classical shockmounts. This
facilitates aircraft integration and avoids further transfer
function complexities. Kinematic accelerations were
determined directly from GNSS receiver output without
positioning detour. The resulting lower noise level allows