Global Positioning System Reference
In-Depth Information
Integrity assessments are based on XPL and XAL: a new XPL is estimated for each
computed position solution. It is compared with the required XAL, and an integrity alert is
triggered if XPL > XAL.
4. RAIM integrity
The integrity of a navigation system can be checked by using external systems such as SBAS
to monitor the correctness of the signals used to calculate position. One of the main
drawbacks to this approach is the inherent delay that is introduced when an error is
detected, due to the time taken to uplink the information on errors. This section will focus
on internal monitoring, and in particular on RAIM. RAIM stands for Receiver Autonomous
Integrity Monitoring and is used to denote a monitoring algorithm that uses nothing but the
measurements of one particular navigation subsystem, usually a GPS receiver. Conventional
RAIM algorithms are designed to protect users from a single satellite failure at a time.
However, recent developments have shown that RAIM has the potential to provide integrity
even in case of multiple failures for challenging flight categories such as LPV-200 and APV-
II (Ciollaro, 2009).
Measurement information is used to compute a position. A test statistic is derived from this
position computation. It gets passed to an error detector that will warn the user whenever
something is wrong. The error detection procedure will have to obey navigation
requirements, and it is important to determine the detection power (or 'error detectability').
It depends on the measurement quality and the configuration, and it is this detection power
computation that monitors the system's integrity, determining whether the system has the
ability to provide timely warnings when the system is in error. If this is not the case, it will
inform the user that it might be unsafe to use the system. It should be noted that position
computation algorithms always assume that noise on the measurements has a zero mean.
An error or bias, as it is commonly called, is therefore defined as the non-zero mean of
measurement noise.
4.1 Satellite slope
The slope, which relates the induced position error to the test statistic, can be calculated
directly from geometry and is different for each satellite. The satellite with the largest slope
is the most difficult to detect. It produces the largest position error for a given test statistic
(Figure 4) (Ciollaro, 2009).
Max Slope
Test Statistic
Fig. 4. Satellite slope
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