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tion, tracking and positioning. It also includes a signal
monitor which analyzes the GNSS signals present in the
IF data stream. At the moment the ipexSR has been
validated for GPS C/A code signals and will be updated in
2004 to track the GPS L2 civil signal and Galileo type
signals.
2.1.4 GNSS RTK Hardware Receiver Development
In cooperation with the IfEN GmbH and the Frauenhofer
Institute for Integrated Circuits a prototype RTK receiver
is being designed and developed. Within the last years
numerous studies have been performed to assess the performance
of a future RTK system using GPS and Galileo
signals. These theoretical studies include signals acquisition,
tracking, multipath mitigation, ambiguity resolution, positioning
and integration of other sensors like INS systems
and integration with internet.
One of the most critical parts in RTK positioning is successful
carrier phase ambiguity resolution. Carrier phase
ambiguity resolution is negatively influenced by atmospheric
–especially ionospheric- delays, thermal noise and
code and carrier multipath. In one study, the effect of
multipath on the ambiguity success rate was calculated using
an end-to-end simulator. The study was carried out in
collaboration with the TU-Delft..
2.1.5 The German Galileo Test Bed – GATE
The satellite navigation system Galileo is currently being
developed in Europe. The main motivation for this development
is to provide a complement to the current GPS and
to ensure independence from GPS. The Galileo program
poses an enormous technological and financial challenge
for Europe. Therefore, associated risks have to be minimized
as far as possible. Test beds are one important
instrument to minimize such risks, as they provide the
necessary infrastructure to validate and optimize the
system’s core elements in advance.
The Institute of Geodesy and Navigation is part of a
consortium of several German companies led by IfEN
GmbH (Prime Contractor). Within the first stage of GATE
(Phase I), the Institute of Geodesy and Navigation provided
extensive scientific consultancy and was involved in the
definition of system requirements. In particular, it was
responsible to analyze to which extent the current GPS can
be used as an efficient reference for GATE. In the following,
the main fields of activity within the framework of
GATE will be presented.
According to its baseline architecture, GATE will consist
of 4 ground-based static pseudolites (pseudo satellites), 4
ground-based but mobile pseudolites, at least one additional
mobile (e.g. airborne) transmitter (in the form of aircrafts,
balloons or airships) and – in the late stage of GATE – of
Galileo (prototype) satellites. Additionally, satellite signals
from the current and future modernized GPS will also be
available. Based on this architecture (which was subject to
slight changes in the meantime), the possibility of determining
the pseudolites’ positions by means of GPS was
analyzed. Thereby, set-up of a differential GPS (DGPS)
network by means of several reference stations as well as
the use of commercial DGPS services like SAPOS have
been taken into account. By means of these approaches, the
pseudolites’ positions are determined by post processing
techniques. Alternatively, the use of real time kinematic
GPS (RTK GPS) has been discussed to provide real time
positioning of the pseudolites. Although very inefficient,
conventional terrestrial methods have been discussed as
well.
In order to make the test bed as flexible as possible, the
signal generators are to be designed to transmit both Galileo
and GPS signals (according to the baseline architecture)
resulting in special design requirements. Related issues
(modification of the pseudolites’ navigation messages,
necessity of pulsing,…) have also been addressed. Furthermore,
the use of GPS to implement a coordinate and time
frame for GATE as well as several approaches to use GPS
for validation purposes (both real time and post processing
validation) have been discussed.
2.1.6 GNSS Software Simulator
In the last few years a couple of GNSS simulation tools
were developed at the Institute of Geodesy and Navigation,
the IfEN GmbH and the Astrium GmbH. Each of the
simulation tools covered different scopes in the field of
satellite navigation. None of these simulators is capable to
cover all areas in satellite navigation or to be extended to
do this.
In Summer 2001 the BaiCES project, financed by the BFS
(Bayerische Forschungsstiftung), started, which includes
the development of complete GNSS System simulator.
The so called BaiCES simulator is based on the commercial
simulation environment, ML-Designer, from the Mission
Level Design GmbH. This simulation infrastructure enables
the user to build up a simulation application graphically,
by drag and drop models from the model library into the
simulation build up window and connecting their in- and
output ports. Each model represents a certain partition of
a GNSS System, like the satellite orbit, the satellite signal,
the receiver, the user environment (terrain data) or the
positioning algorithm.
The model library is developed by the three project partners
Astrium, IfEN and UniBW and consists mainly of the
algorithms they implemented in their existing GNSS
simulation tools in the past.
The main advantage of this approach is the enormous
flexibility and transparency to the user of this simulator.
The model library can be extended at any time to include
for example new research results in satellite navigation.
The model library, developed in the BaiCES project, will
consist of at least more than hundred models to represent
the different partitions in different detail. It is up to the user
to choose the level of detail, depending on the desired
simulation application, for example system volume or endto-end
simulation.
A quite new feature is the inclusion of real measurement
data or orbit data, to verify the simulation models.