GPSoft SatNav Toolbox - Fault Detection via Parity Space
Receiver autonomous integrity monitoring
(RAIM) is the satellite-based navigation
community's term for fault detection. The
main idea is to examine the consistency in a
redundant set of measurements. The SatNav
Toolbox function PARITYVEC performs the
necessary computations. The example shown
here illustrates what happens during normal
operation. The magnitude of the parity vector
(i.e., the consistency metric) is plotted over
time and demonstrates the variations typically
encountered as a result of Selective
Availability.
The second plot illustrates the influence of a
200 meter bias error injected onto a single
satellite at time t=1750. The jump is readily
apparent as is the bias in the parity vector.
The final example is similar to the previous one
except that a 1 m/s ramp error has been injected
instead of a bias error. It should be pointed out
that the behavior of the parity vector is
dependent not only on the pseudorange errors
but also on the satellite geometry. In safety
critical navigation applications, the key is in
setting thresholds such that the error is detected
before an unacceptable position error is
encountered.
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Updated: September 2003