Global Positioning System Reference
In-Depth Information
2.2 Faulty case
In case of a system failure, the range measurement will be affected by a bias that gets added
to the other errors. The aim of the system is to detect this bias. For this reason the Galileo
system consists of a Ground Segment (GSS) that is able to monitor range measurements. If
the bias exceeds an established integrity threshold, the user will become aware of this via an
alarm.
The error detected by the ground segment can be modelled using a zero-mean Gaussian
distribution with variance σ . Since the false alarm probability can be considered as the
area limited by this function between threshold and infinite, we can calculate this threshold
as follows:
2
2
TH
=⋅
k
σ
+
σ
(1)
fa
SISAu L
,
where
f
k
derives from the false alarm probability.
The alarm is notified by setting the Integrity Flag relative to the satellite with failure in the
information delivered to the users. This satellite must not then be considered by the user in
the xPL computation and in the positioning algorithm. The combination between the IF and
the PL can ensure the integrity of the information received in the position domain.
Moreover, the implementation of a RAIM algorithm comes to the aid of the integrity
monitoring, in order to face up to errors caused by local effects (i.e., multipath, interference,
jamming and ionospheric effects).
2.3 Evolution of integrity concept
The evolution of the Galileo integrity concept concerns only the verification of system
integrity. In particular, based on the above-mentioned definitions, the checking
methodology has been modified: the vertical and the horizontal protection levels have been
combined in a unique concept, and the user has to compute a probability, named Hazardous
Misleading Information Probability (P
HMI
), which will be compared to the threshold. Once
the distribution of the error in the desired reference frame is known (Gaussian
overbounding distributions with SISA and SISMA), it will be simple to derive the associated
integrity risk both in the faulty and the faulty free conditions appointed to the user
equations. Therefore, the error distributions for the vertical (one dimensional Gaussian
distribution) and horizontal (Chi Squared distribution with two degrees of freedom) cases
need to be derived, and the corresponding integrity risk can be easily computed by
analyzing the integral for both distributions with the given alert limits. The integrity risk at
the alert limits VAL and HAL are finally computed by adding the vertical and horizontal
contributions (Dore & Calamia, 2009).
2.4 Galileo integrity risk
Based on the aforementioned quantities (SISE, SISA, SISMA, IF and TH), the user receiver
can derive the integrity risk for the user position solution. This integrity risk is always
computed for a given alert limit. Whenever the derived IR at the AL is larger than the
allowed IR, the user equipment will raise an alert (Oehler et al., 2004).
Search WWH ::
Custom Search