Global Positioning System Reference
In-Depth Information
SISA
SISMA
SISMA
SISA
0
0
SISE-predicted SISE
SISE-estimated SISE
Figure 10.15
Illustration of SISA and SISMA.
SISMA values is dependent of the geometry between the available GSSs and the SVs.
Thus, the difference between the true SISE and the estimated SISE can be described
with this Gaussian distribution with the standard deviation SISMA (see Figure
10.15) [13].
The dimensioning of the GSS network is related to the SISMA value—the design
goal is 70 cm with all GSS available and 1.3m in case of one GSS failing. At the
GCC, an integrity processing facility estimates the SISE using the measurements of
the GSSs to detect faulty satellites. If the estimated SISE for a satellite is larger than a
certain threshold, the satellite will be flagged “don't use.”
The system navigation function provides the quality of the SIS broadcast by SVs
(i.e., SISA).
The system integrity function provides integrity information to the users
containing:
Accuracy of the GSS monitoring of the SIS broadcast by the SVs (i.e., SISMA);
SIS broadcast by SVs that should not be used (i.e., integrity flag).
From this information, the user can derive his or her individual integrity risk.
This integrity risk is always calculated for a given alert limit. The alert limit is the
maximum allowed position deviation for which no alert has to be raised. Whenever
the derived integrity risk at the alert limit is larger than the allowed integrity risk, the
user equipment will raise an alert.
The dissemination of integrity information to users is driven by the following
requirements:
Three satellites in view of a user with a minimum elevation angle of 10º are
located at a so-called worst-user location, ensuring that with a failure of one
dissemination path, two independent paths are always maintained.
Time to alarm (TTA) is less than 5.2 seconds from occurrence of the event
until detection and dissemination by the GALILEO system to any user.
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