Aerospace and Electronic Systems Magazine September 2017 - 11

Layh and Gebre-Egziabher

Figure 8.

Time history of position errors for the open-loop GNSS/INS filter (green square), the DR filter (red triangle), and the position blender (blue circle).

POSITION ESTIMATION PERFORMANCE
Similar to the attitude error analysis earlier discussed, a position error analysis can be performed. Figure 8 shows the position errors
from the GNSS/INS filter, the airspeed-based DR system, and the
position blender. This figure illustrates one of the primary advantages of the decentralized architecture. When running open loop (i.e.,
no GPS aiding), the GNSS/INS filter has rather large drift errors
after only a minute of operation. This is because the well-known t2
accelerometer bias and t3 gyro bias induced position error growth

[13], [28]. The blending algorithm detects this inaccuracy indirectly
through the growth of the GNSS/INS covariance. The cascaded
AHRS-DR system has position error growth that is approximately
proportional to t1. The slower error growth of the latter system's covariance means that its blender weights are going to be larger than
that of the GNSS/INS. This process of shifting weights can be seen
in Figure 9, which shows the relative weights B1b and B3 discussed in
the Blender Equations section and Figure 4. When GPS services are
restored, the GNSS-aided INS error is reset, and the position weights
return to their preoutage levels with a minimal transient.

Figure 9.

Time history of position blender weights B1b and B3. Blender weights on the GNSS/INS solution (green square) and weights on the DR filter solution
(red triangle).

SEPTEMBER 2017

IEEE A&E SYSTEMS MAGAZINE

11



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