Global Positioning System Reference
In-Depth Information
4
Evolution of Integrity Concept
- From Galileo to Multisystem
Mario Calamia, Giovanni Dore and Alessandro Mori
University of Florence
Italy
1. Introduction
The Galileo navigation system introduced the integrity concept, intended as a continuous
control of the information broadcasted by satellites. Although the RAIM technique
represents the first example of integrity monitoring, it is able to detect only local errors
made at the receiver level. The integrity monitoring applied by EGNOS could instead be
seen as the forerunner of the Galileo system. Even if there are many differences in the
definition of integrity for the two systems, the aim is the same for both: to protect the user
against the failure of the system, warning him in the shortest time and with the greatest
precision possible.
The integrity of a navigation system can be defined as follows: “integrity relates to the trust
that can be placed in the correctness of information supplied by a navigation system.
Specifically, a navigation system is required to deliver an alarm when the error in the
derived user position solution exceeds an allowable level (alarm limit). This warning must
be issued to the user within a given period of time (time-to-alarm) and with a given
probability (integrity risk)” (Oehler et al., 2004).
In the near future a central role will be played by the integrity receiver's capability. This
service can be considered essential in the safety critical application domain, particularly in
aviation. For these applications, the system's capability of protecting the user against system
failure is of primary importance.
Integrity includes the system's ability to supply, at the right time, reliable warnings to the
user (alarm). The main problem with this service is how to determine what can be
considered safe. This depends on the requirements of the different fields of application. The
following parameters are traditionally used to define the safety of the service for a specific
application:
Alarm Limit (AL): the maximum error allowed in the position domain before an alarm
is generated.
Time To Alarm (TTA): the time that elapses between an error's overcoming of the AL
and the reception of the alarm by the user's receiver.
Integrity Risk (IR): the probability that the alarm will not be delivered within the TTA.
Allowable values of AL, TTA and IR depend on the specific application of the navigation
system. The Galileo system provides a high level of integrity of the navigation signal. The
Search WWH ::




Custom Search