Global Positioning System Reference
In-Depth Information
redundancy will help guarantee a safer position and the detection of errors. This will also
result in an improved availability as well as meet the requirements for more demanding
flight categories. Therefore, it is necessary to introduce a base-line for a combined system,
defining new parameters, a new integrity algorithm and possible ways to combine the two
independent systems.
With the term “Multisystem”, we intend the improvement of the accuracy and availability
of the navigation solution using the combined Galileo and GPS signals. In this context, it is
essential for the user to be able to take advantage of the integrity information coming from
both Galileo and GPS satellite constellations, in order to prevent users from making errors
that might represent an excessive risk. The multisystem integrity algorithm has to establish
a link between the two generations of GNSS, defining the relation for integrating different
integrity monitoring schemes (Pecchioni et al. 2007) (Ciollaro, 2009).
5.1 Definition of a new integrity algorithm (EGNOS + Galileo)
Two different approaches have been studied to define the new integrity algorithm. They
represent two opposite ways of solving the problem of how to combine different integrity
concepts: the first one has been called “One-System-Based Integrity,” and the second one
“Parallel Integrity” (Dore & Calamia, 2009).
The first approach is based on the use of only one algorithm for both systems with an a
priori definition of integrity inputs. The integrity analysis can be made either by converting
the EGNOS integrity message into an equivalent Galileo integrity message or vice versa, by
using the inverse transformation from a Galileo to an EGNOS-like message. In the first case,
known as Galileo-Based-Integrity-Algorithm (GBIA), the Galileo Integrity is used as a
baseline; in the second case, called EGNOS-Based-Integrity-Algorithm (EBIA), the One-
System Integrity is the EGNOS algorithm.
The second approach is based on the use of independent (parallel) algorithms, one for each
System, and on an a posteriori integration of the integrity results. The integrity analysis can
be performed by monitoring the values assumed by both the Integrity Risk and the
Protection Level. If the IR is used as monitored variable, the scheme will be called IR-PIA;
otherwise, if the monitored variable is the PL, the method will be called PL-PIA. It is worth
noting that the computational load for the IR/PL conversion is expected to be higher than
the PL/IR conversion, because an iterative method must be applied (Ciollaro, 2009).
5.2 Galileo based integrity algorithm
The approach chosen for this study is GBIA. The integrity data in fact arrives from the two
systems, Galileo and EGNOS, and is implemented inside the Integrity Risk equation of
Galileo, in order to estimate the HMI Probability.
Figure 5 shows the block diagram of a GBIA system. The fundamental block of this diagram
is the EGNOS/Galileo converter, which has the aim of converting the EGNOS Integrity
message into a message that can be used by the Galileo Integrity Algorithm.
The main functions implemented by the EGNOS-Galileo converter are the following
(Ciollaro, 2009):
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