GALILEO Integrity Performance Assessment Results And Recommendations (2002)

Within the frame of the European Space Agency
(ESA) project Galileo integrity performance assessment (GIPA, ESA contract no.
15393/01/NL/DS), integrity performance of the
Galileo system has been analysed in detail. The
opinions expressed in this paper are those of the
authors and do not necessarily reflect the official
views or policy of ESA.
Both, ground integrity channel (GIC) as well as user
level performances have been investigated. Several
critical issues related to Galileo system integrity
performance have been identified and a set of ten
use cases has been defined with each of the ten use
cases tackling a different issue. Within these use
cases not only the baseline Galileo integrity
architecture but also alternative approaches have
been considered. The main focus has been laid on
algorithmic issues. Operational aspects like
communication network or up-link issues have not
been considered within this project.
Among the critical issues are the signal-in-spaceaccuracy
(SISA) representation (scalar versus
vector SISA), the SISA refresh rate, SISA checking
margin assumption, integrity flag (IF) spatial
concept, integrity monitoring station (IMS)
network, IMS measurement quality, navigation
signal, satellite orbit and clock and IMS clock error
feared events as well as elevation masks at user and
IMS level. The sensitivity of the integrity
performance to these critical issues has been
investigated partially by Monte-Carlo simulation,
near end-to-end simulation, or analysis. Results with
respect to false-alarm rate and missed-detection
probabilities at GIC and user level as well as
availability at user level have been obtained for the
use cases.
The first part of the paper gives a brief introduction
to integrity and integrity performance at GIC level and user level. The basic situation is visualised and
the functionality and performance of the Galileo
integrity ground segment, the safe detection and
isolation of satellite clock and orbit errors on the
basis of the measurements taken at IMS, are
characterised.
In the second part of the paper, the use cases and
underlying assumptions are described. A userequivalent-
range-error (UERE) budget was adopted
from Galileo phase B1. Orbit and clock errors were
modelled according to the latest available early
trials study results. A priori fault probabilities have
been set to obtain statistical significant results in an
efficient way and with not too high computational
effort.
Results of the individual use cases are given in the
third part of the paper together with a discussion
and conclusions that could be drawn from the
results.
In the last part of the paper recommendations are
given with regard to different integrity architecture
options. The main conclusion that could be derived
is that under the environmental conditions that have
been assumed in the simulations the current Galileo
baseline integrity architecture seems to be well in
line with current integrity performance expectations.
Among the main recommendations are:
• Use a scalar SISA value for broadcast. A vector
SISA approach is only marginally better (in
case of global integrity approach), but
computational effort and transmission
bandwidth are higher.
• The SISA refresh rate is not critical to the
integrity checking performance at GIC level as
far as a safe checking margin is provided on top
of the real errors. However, there is a severe
impact on receiver autonomous integrity
monitoring (RAIM) availability at user level.
• As a spatial concept use a global integrity
approach. The achievable performance will
depend mainly on the density of monitoring
stations in a certain region and the quality of
the pre-processed measurements.
• Implement a depth-of-coverage (DOC) 4
network. While a DOC 3 network provides too
less redundancy, the benefit of a DOC 5
network at ground segment level is only small,
when systematic effects on measurements are
low (not considering monitoring station faults).
• Choose an IMS elevation mask of 15°. A too
low elevation mask will introduce too much
systematic effects, while a too high elevation
mask significantly reduces the number of
available measurements.
• Choose a user elevation mask of 10°. For the
user it is most important to have as much
measurements as possible.
Finally, a note on caution is given as every
simulation – as well as any experimenting with real
data – has its limitations that have to be kept in
mind to understand or extrapolate the results and
conclusions. These limitations are briefly presented.