vii berichte von fachinstituten an universitäten und technischen

368 Fachinstitute an Universitäten und Hochschulen
In comparison and in extension to satellite based and
terrestrial methods the principle of airborne (vector-) gravimetry
seems to be an optimal solution to determine the
significant regional gravity changes. The fundamental equation
of airborne gravimetry is Newton’s second law of
motion. In an inertial coordinate frame the kinematical
acceleration a (= second time derivative of the position) can
be computed by the sum of specific force f and gravity g.
Therefore an integrated system for airborne gravimetry can
be implemented using GNSS observations in order to
calculate the kinematic acceleration and inertial measurements
for derivation of specific forces. Together with two
other research institutes and two private companies we are
member of the Geotechnology Program funded (jointly) by
the Federal Ministry of Education and Research (BMBF)
and the Deutsche Forschungsgemeinschaft (DFG). The
project "Development of Airborne Gravimetry using GNSS
satellite observations" belongs to the research theme 2 of
this program "Observation of the system earth from space".
The project objectives are: Determination of the regional
gravity field in the range of 1 to 50 km, development of a
GNSS based airborne vector gravimeter with an accuracy
of 1 mGal with a spatial resolution of 1 km, comparison of
different sensor systems and processing methods, opening
of new application area in earth sciences und exploration
based on increased performance and efficiency of airborne
gravimetry
GNSS/INS Coupling
Today basically three different methods are operational or
a current investigation topic: loose, tight and deep coupling.
Tightly – Coupled System (in-part)
In case of high dynamic applications or in jamming environments
the principle of tight-coupling becomes very
interesting. Information about the receiver dynamics
measured by the inertial sensor is used to support the
tracking loops of the integrated navigation system. An
extension of this concept is the deeply coupling method. In
this case the integrating navigation filter is implemented as
one element of the receiver tracking loop. Using inertial
information in combination with the in-phase and quadraphase
signals of the receiver signal processing an optimum
control input for the numerical oscillator can be computed.
Optimisation of signal tracking performance and signal
availability enhancement are the goals of this method.
A tightly coupled GNSS/INS system was bread-boarded at
the institute: In several theoretical and numerical simulations
the tracking loop performance is investigated using different
loop bandwidths and inertial data of different quality
generated by various inertial measurement units. A breadboard
system is implemented using the LITTON LN 200
IMU and the MITEL GNSS receiver development tool.
Railway Track Irregularity Measurement
Using an integrated system with double frequency carrierphase
DGPS, high precision INS and four ultra-sonic sensors
the three dimensional position of railway heads can be
derived with millimeter accuracy level. The data will be
especially used to minimize the derailment risk of high speed
trains.
Railway track surveying system
Pseudolite Research
The current pseudolite research of the institute funded by
Deutsches Zentrum für Luft- und Raumfahrt (DLR) focuses
on two main aspects:
– Evaluation of all potential error sources with respect to
the deployment of APLs and development of an error
model.
– Application and validation of the error model on real
flight data collected during two experimental flight
campaigns at the airport of Braunschweig (Germany).
Two experimental flight campaigns were carried out at the
airport of Braunschweig. An IntegriNautics IN 500 P-code
APL was used. APL signals were amplified. Configuration
and control of the APL took place by means of the IN 500
software running on a Windows laptop. Transmission of the
APL signal was realized by use of a dB Systems MLA
antenna. MLA stands for Multipath Limiting Antenna, which
provides a sharp signal power attenuation for low satellite
elevations in order to mitigate signal reflections at buildings,
etc. The DGPS (and DAPL) reference station was located
about 750 m away from the APL on plane grassland.
Reference and rover site were equipped with NovAtel
ProPak II receivers including an APL firmware. A Dornier
Do 128 aircraft served as rover.