Global Positioning System Reference
In-Depth Information
The proper functioning of this system does require some assistance from the ground; for
example, the ISM (Integrity Support Message), which is a message developed using
reference receivers on the ground, is communicated to the aircraft. The ISM message
conveys the safety assertions associated with each of the underlying satellite systems to the
sovereign responsible for a given airspace. These messages would contain performance
estimates for each satellite to be used for navigation. ARAIM therefore uses a multiplicity of
satellites in a dual-constellation environment to take responsibility for all faults that arise
between dispatch and the completion of approach.
As described in the previous Section, one of the potential uses of the Multisystem Integrity
algorithm is represented by the combination of the IR algorithm with the RAIM technique.
ARAIM is still in a feasibility status, and a comparison—in order to test and verify the
requirements and highlight the differences between the two approaches—between its
results and Multi System Integrity cannot yet be performed. Indeed, the two systems have
the same aim: to improve the reliability of the position solution provided by the system in
particular conditions (LPV-200 for ARAIM), taking advantage of different navigation
systems.
7. Conclusions
The totality of the tests made on the implemented code has been planned with the aim of
characterizing the performances of the algorithm respectively in faulty free and in faulty
mode. The use of the Galileo and EGNOS system as a single and augmented constellation
allows us to develop the positioning algorithm and improve the position accuracy.
Furthermore, the combination of the two SVs systems enables us to obtain some benefits
from the RAIM point of view.
Our proposed solution starts from the integrity equation defined for the Galileo system and
adapts it to the combined Galileo + EGNOS system, or rather, it combines the integrity data
supplied separately by the two navigation systems, with the aim of computing the
Hazardous Misleading Information Probability. We focused our attention on the IR
equation: the implemented code reproduces the IR equation as it is presented in literature,
that is, with the SISA values relative to Galileo and GPS satellites, and SISMA relative only
to the Galileo ones, in faulty free and faulty mode, respectively. The results obtained testing
the algorithm in the presence of failure have provided positive indications on the
implemented IR equation: in these cases, the HMI probability increases with the value of the
bias.
Alhough the IR protects the user against extended failure, whose effects revert on the SISE
estimation, the RAIM technique could instead highlight instantaneous errors on the
distances measured by a Galileo or a GPS satellite. RAIM and IR compensate each other, or
rather, the RAIM indicates failure unperceived by the IR and vice versa; therefore the
combination of the RAIM technique with the integrity equation has proved to be a good
idea. This technique is based on a very different concept than protection levels and leads to
different results. However, the Galileo integrity concept is more complete than the
GPS/SBAS and RAIM integrity concepts and offers more protection from failures. However,
this concept needs to be investigated further, in particular regarding the assumptions to be
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