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System
Design
The observation
of gravity anomalies using the airborne gravimetry principle is already offered
by some companies. These operational systems mostly use modified Sea-Gravimeters
mounted on a stabilized platform to detect the specific forces. In order
to derive the required gravity value the kinematic accelerations of the airplane,
especially in height, must be additionally determined. This is currently
done using the GPS system in some cases combined with other sensors, e.g.
barometric sensors. So far, operational airborne gravimetry is able to achieve
resolutions of about 5 km with an accuracy of 2 mGal. Thereby the computation
of kinematic accelerations out of GNSS phase observations and the
stabilisation of the gravity sensors are the most important limitations.
But in order to fulfil the requirements for the most exploration applications,
that are very important especially in regard to the economical point of view,
the accuracy and spatial resolution of such systems
have to be increased. Another disadvantage of
the current systems are the dimensions, the weight and the acquisition
costs. Furthermore they are limited to observe only the absolute gravity
value. Information about its direction is only available if the vector gravimetry
principle is implemented.
Against this background an airborne vector
gravimetry system is in development based upon the use of a commercial high
precision strapdown inertial navigation System (INS) and a combination of
a geodetic GNSS receiver with a multi-antenna system.
The inertial
data in the instrument frame is measured by a SAGEM Sigma 30 INS. In order
to derive the kinematic acceleration on the one hand a ASHTECH L1/L2 receiver
is used. Additionally the L1-observations of four other GNSS antennas with
fixed baselines are generated by an ADU 3 multi-antenna system. The integration
of the L1/L2 observations with this data at first should provide better
performance of the phase ambiguity determination. At the same time this redundant
estimation of the airplane dynamics should increase the accuracy of acceleration
computation. GNSS reference stations on the ground guarantee differential
observations.
The system
design together with fundamental data processing aspects are demontstrated
in the picture above. It is intended to integrate GNSS observations and inertial
date on acceleration level. Additional improvements of the error budget should
be possible using advanced postprocessing algorithms.
One focal
point of the project is also the derivation of kinematical accelerations
using GNSS observations. In this case the error influences of GNSS must be
evaluated in a completely different way. It has to be taken into account,
that the process of differentiating amplifies these errors as function of
increasing frequency, causing them to be larger as the upper edge of the
bandwidth is increased. E.g. long term errors like ionosspheric influences
has only small direct effects on the acceleration solution, whereas the receiver
noise is the most dominant influence, and already a small cycle slip leads
to immense errors. It is investigated, if the classical approach of double
differencing of the position could be replaced by the direct calculation
of aircraft accelerations using GNSS phase observation. In this case the
necessity to solve the phase ambiguities can be avoided. The spectral window
of airborne gravimety could be increseasd by using the L1 frequncy data with
a lower noise level in comparison to the ionossperic free linear
combination.
 First Flight
Experiments
Further Information (Research Database)
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