Evolution of Integrity Concept – From Galileo to Multisystem - Global Navigation Satellite Systems: Signal,Theory and Applications - page 92

Autonomous Integrity (RAIM) technique (Weighted RAIM). One of the potential uses of this
algorithm consists in the combination of the IR algorithm with the RAIM technique. RAIM
is able to detect failures that have not been detected by the IR algorithm. In case of multiple
failures, when the WRAIM technique fails, the IR algorithm triggers an alarm.

In this Section we describe the characteristics of this innovative algorithm, pointing out the
reason for using the IR equation for the combined constellation Galileo/EGNOS and the
reason for taking advantage of a RAIM technique. The EGNOS integrity equation provides a
way to measure the integrity based on the incoming signal variances and the satellite
geometry. The same is also true for the Galileo IR equation, obviously bearing in mind
which data is the Galileo integrity data.

This algorithm is supposed to enable the user to take advantage of the data transmitted by
the Galileo and EGNOS systems: the user receiver must consider a single and large
constellation in order to strengthen the positioning algorithm and improve the accuracy.
This idea is simply in need of the definition of a new integrity concept, which would be able
to combine the techniques mentioned above.

5.3.1 IR equation
First of all, we have to explain why the protection level concept turns into the integrity risk
concept in a Galileo environment. In an EGNOS domain, IR is the probability that the
horizontal (vertical) PL exceeds the horizontal (vertical) AL without the user receiving any
alarm whatsoever. This definition requires a clear distinction between the horizontal and
vertical cases. Therefore, it is necessary to split IR into two a priori fixed quantities.

On the contrary, as far as a Galileo integrity equation is concerned, the users do not have to
evaluate the horizontal and vertical protection levels, but the global IR directly, without
making any strict allocations. In fact, different applications need distinct integrity
requirements for the horizontal and vertical situations: for example, for a ship the vertical
component of the error is not that important for a ship, but it is instead essential for a plane.
This last observation leads us to choose the Galileo integrity equation to perform the
multisystem integrity algorithm.

The first step in the definition of a new integrity algorithm concerning a combined
constellation (Galileo+GPS), is the characterization of the equivalent elements belonging to
the two navigation systems. In order to perform the test on the position solution, we opted
for the relationship between

σ of Galileo and UDR σ of EGNOS. First of all, these are
quantities defined in the same domain SIS. Secondly, they are related to the same typology
of error (clock and ephemeris).

2
SISA

The local contribution to the variance of the error in the SIS depends on the elevation angles
of the satellite belonging to the two constellations considered. As mentioned before, in order
to consider a single constellation composed by both GPS and Galileo SW, we have
considered the variance of the error in the SIS as follows:

2 2 2

σσ = + σ (34)

i SISA / RE i , u L i , ,

where the first term, in the case of an EGNOS satellite, derives from Eq. 32; the second term
instead represents the local error contribution and can be estimated via the following equation: